
Chapter 13 – Fluid Mechanics
Page No 270:
Question 1:
Is it always true that the molecules of a dense liquid are heavier than the molecules of a lighter liquid?
Answer:
Density is defined as mass per unit volume. It tells how closely packed are the molecules inside an object. When molecules are tightly packed, we say that the object is dense; when molecules are far apart, we say that the object is light.
The density of a liquid depends on the mass of the molecules that make up the liquid and the closeness of the molecules of the liquid. It is not always true that the molecules of a dense liquid are heavier than the molecules of a lighter liquid.
For example, alcohol is less dense than oil. Alcohol molecules are mostly carbon and hydrogen atoms, so they are similar to oil. But they also contain an oxygen atom, which makes them a little heavy. For this reason, you might think that alcohol is denser than oil. But because of their shape and size, alcohol molecules do not pack as efficiently as oil molecules. This property of molecules makes alcohol less dense than oil.
Question 2:
If someone presses a pointed needle against your skin, you are hurt. But if someone presses a rod against your skin with the same force, you easily tolerate. Explain.
Answer:
We know that pressure is the application of force over a particular area. In mathematical terms, pressure is equal to force divided by area; that is, with more force comes more pressure and with more area comes less pressure. An iron rod has more surface area than the pointed tip of a needle. That is why the pointed needle exerts more pressure than the iron rod (with the same force) when pressed against our skin.
Question 3:
In the derivation of P_{1} − P_{2} = ρgz, it was assumed that the liquid is incompressible. Why will this equation not be strictly valid for a compressible liquid?
Answer:
In case of an incompressible liquid, the density is independent of the variations in pressure and always remains constant. But it is not so in case of a compressible liquid. Thus, the given equation will not be strictly valid for a compressible liquid.
Question 4:
Suppose the density of air at Madras is ρ_{o} and atmospheric pressure is P_{0}. If we go up, the density and the pressure both decrease. Suppose we wish to calculate the pressure at a height 10 km above Madras. If we use the equation P_{o} − P = ρ_{o}gz, will we get a pressure more than the actual or less than the actual? Neglect the variation in g. Does your answer change if you also consider the variation in g?
Answer:
Using the equation P_{o} − P = ρ_{o}gz, we get:
P = P_{o} − ρ_{o}gz
The pressure calculated by using this equation will be more than the actual pressure because density at a height of 10 km above Madras will be less than ρ_{o}.
Yes, the answer will change if we also consider the variation in g. Because g decreases with height, it will have the same effect on pressure as that of density.
Question 5:
The free surface of a liquid resting in an inertial frame is horizontal. Does the normal to the free surface pass through the centre of the earth? Think separately if the liquid is (a) at the equator (b) at a pole (c) somewhere else.
Answer:
Yes, the normal to the free surface of the liquid passes through the centre of the Earth. Because of the gravitational force, the free surface of the liquid takes the shape of the surface of the Earth. Also, because the gravitational force is directed towards the centre of the Earth, the normal to the free surface also passes through the centre of the Earth (in all cases).
Question 6:
A barometer tube reads 76 cm of mercury. If the tube is gradually inclined keeping the open end immersed in the mercury reservoir, will the length of mercury column be 76 cm, more than 76 cm or less than 76 cm?
Answer:
The length of the mercury column will be more than 76 cm. The pressure depends on the height of the highest point of the mercury from the ground and not on the length of the liquid column.
Let:l=Length of the mercury columnθ=Angle at which the tube is inclined with the verticalGiven:h=76 cmlcosθ=hor, l=hcosθ∴l>hOr,
l > 76 cm
Question 7:
A one meter long glass tube is open at both ends. One end of the tube is dipped into a mercury cup, the tube is kept vertical and the air is pumped out of the tube by connecting the upper end to a suction pump. Can mercury be pulled up into the pump by this process?
Answer:
No, mercury cannot be pulled up into the pump by this process. The level up to which mercury can rise is 76 cm (to maintain equal pressure at points A and B).
Question 8:
A satellite revolves round the earth. Air pressure inside the satellite is maintained at 76 cm of mercury. What will be the height of mercury column in a barometer tube 1 m long placed in the satellite?
Answer:
Pressure at point A is 76 cm of mercury. Therefore, mercury will rise to full length of the tube, i.e., 1 m, to maintain equal pressure at points A and B. Inside the satellite,
geffective=0, so the pressure due to height of the mercury column will be zero.
Question 9:
Consider the barometer shown in figure. If a small hole is made at a point P in the barometer tube, will the mercury come out from this hole?
figure
Answer:
Two pressures are acting upon point P:
(1) Pressure due to mercury level above point P equals to P_{1} (say)
(2) Atmospheric pressure = P_{0} (inwards)
And,
P_{0} > P_{1}
As the inward pressure is more, the mercury will not come out of the hole.
Page No 271:
Question 10:
Is Archimedes’ principle valid in an elevator accelerating up ? In a car accelerating on a level road?
Answer:
Archimedes’ principle is not valid in case of an elevator accelerating upwards, but it is valid for a car accelerating on a level road.
According to Archimedes’ principle,
Buoyant force, B = Weight of the substituted liquid
Or,
B = mg
The above principle is satisfied in case of a car accelerating on a level road.
In case of an elevator, the buoyant force will be as below:
B = mg + ma (If the elevator is going upwards with an acceleration a)
Thus, Archimedes’ principle is not valid in this case.
Question 11:
Why is it easier to swim in sea water than in fresh water?
Answer:
Whether an object sinks or floats in a liquid depends upon the density of the two. We know that sea water has dissolved salts in it, which increase its density. So, sea water exerts more buoyancy force (in the upward direction) on the swimmer than that exerted by fresh water. This helps the person to swim easily in sea water compared to fresh water.
Question 12:
A glass of water has an ice cube floating in water. The water level just touches the rim of the glass. Will the water overflow when the ice melts?
Answer:
The water of mass equal to the mass of the ice cube will take less volume compared to the ice cube. The water will not overflow when the ice melts because the ice will displace the space it would take if it were in a liquid state.
Question 13:
A ferry boat loaded with rocks has to pass under a bridge. The maximum height of the rocks is slightly more than the height of the bridge so that the boat just fails to pass under the bridge. Should some of the rocks be removed or some more rocks be added?
Answer:
Some rocks should be added to increase the force acting in the downward direction. It will help the boat to pass under the bridge. If some rocks are removed, the upthrust of water on the boat will be greater than the weight of the boat. So, the boat will rise in water and will fail to pass under the bridge.
Question 14:
Water is slowly coming out from a vertical pipe. As the water descends after coming out, its area of cross section reduces. Explain this on the basis of the equation of continuity.
Answer:
Let a be the area of cross section and v be the velocity of water.
According to the equation of continuity,
av = Constant
or,
v α 1aIt means the larger the area of cross section, the smaller will be the velocity of liquid and vice versa.
Thus, as the water comes out of the vertical pipe, its velocity increases and area of cross section decreases.
Question 15:
While watering a distant plant, a gardener partially closes the exit hole of the pipe by putting his finger on it. Explain why this results in the water stream going to a larger distance.
Answer:
According to the equation of continuity, if the exit hole of the pipe is partially closed, the water stream comes out with more velocity due to decrease in area. This results in the water stream going to a larger distance.
Question 16:
A Gipsy car has a canvass top. When the car runs at high speed, the top bulges out. Explain.
Answer:
This can be explained through Bernoulli’s principle, which states that the higher the air speed, the lower the pressure in that area. Because the air inside the car does not move, the pressure in the car is atmospheric. Because air moves outside the car (directly above it), the pressure is low. The canvas top of the Gipsy car is pushed upwards because the pressure inside the car is greater than the pressure directly above the car.
Question 1:
A liquid can easily change its shape but a solid can not because
(a) the density of a liquid is smaller than that of a solid
(b) the forces between the molecules is stronger in solid than in liquids
(c) the atoms combine to form bigger molecules in a solid
(d) the average separation between the molecules is larger in solids
Answer:
(b) the forces between the molecules is stronger in solids than in liquids
The forces between the particles of a solid are stronger than those between the particles of a liquid, so the particles cannot move freely but can only vibrate. Thus, a solid has stable, definite shape and volume. A solid can only change its shape by force (when broken or cut), whereas a liquid can easily change its shape because of weak interparticle forces.
Question 2:
Consider the equations
P=lim∆s→0F∆S and P1P2=ρgz.In an elevator accelerating upward
(a) both the equations are valid
(b) the first is valid but not the second
(c) the second is valid but not the first
(d) both are invalid
Answer:
(b) the first is valid but not the second
For a point inside the elevator, pressure can be defined as
P=lim∆s→0F∆S. It is independent of the acceleration of the elevator.
The modified form of the second equation, which will be valid in the given case, is given by
P1P2=ρ(g+a0) z
Here, acceleration a_{0} (say) due to elevator accelerating upwards is also taken into account.
Question 3:
The three vessels shown in figure have same base area. Equal volumes of a liquid are poured in the three vessels. The force on the base will be
(a) maximum in vessel A
(b) maximum in vessel B
(c) maximum in vessel C
(d) equal in all the vessels
Figure
Answer:
(c) maximum in vessel C
Here, the height of the liquid column is maximum in vessel C. Thus, the force on the base of vessel C, i.e.,
F=P0+hρgwhere P_{0} is atmospheric pressure, is maximum.
Question 4:
Equal mass of three liquids are kept in three identical cylindrical vessels A, B and C. The densities are ρ_{A}, ρ_{B}, ρ_{C} with ρ_{A} < ρ_{B} < ρ_{C}. The force on the base will be
(a) maximum in vessel A
(b) maximum in vessel B
(c) maximum in vessel C
(d) equal in all the vessels
Answer:
(d) equal in all the vessels
The force on the base is given by
F=hρg×A⇒F=(hAρ)g⇒F=(Volume×Density)×g⇒F=mg
In the question, the masses are equal. So, the force on the base is the same in all cases.
Question 5:
Figure shows a siphon. The liquid shown is water. The pressure difference P_{B} − P_{A} between the points A and B is
(a) 400 Nm^{−2}
(b) 3000 Nm^{−2}
(c) 1000 Nm^{−2}
(d) zero
figure
Answer:
(d) zero
At both points A and B, pressure is equal to atmospheric pressure.
Thus, we have:PA=PB=Patm⇒PBPA=0
Question 6:
A beaker containing a liquid is kept inside a big closed jar. If the air inside the jar is continuously pumped out, the pressure in the liquid near the bottom of the liquid will
(a) increase
(b) decrease
(c) remain constant
(d) first decrease and then increase
Answer:
(b) decrease
As the air inside the jar is pumped out, the air pressure decreases. Thus, the pressure in the liquid near the bottom of the beaker decreases.
Question 7:
The pressure in a liquid at two points in the same horizontal plane are equal. Consider an elevator accelerating upward and a car accelerating on a horizontal road. The above statement is correct in
(a) the car only
(b) the elevator only
(c) both of them
(d) neither of them
Answer:
(b) the elevator only
The two points in the same horizontal line will not have equal pressure if the liquid is accelerated horizontally. There should be vertical acceleration.
Question 8:
Suppose the pressure at the surface of mercury in a barometer tube is P_{1} and the pressure at the surface of mercury in the cup is P_{2}.
(a) P_{1} = 0, P_{2} = atmospheric pressure
(b) P_{1} = atmospheric pressure P_{2} = 0
(c) P_{1} = P_{2} = atmospheric pressure
(d) P_{1} = P_{2} = 0
Answer:
(a) P_{1} = 0, P_{2} = atmospheric pressure
The upper part of the tube contains vacuum as the mercury goes down and no air is allowed in. Thus, the pressure at the upper end, i.e., at the surface of mercury in a barometer tube is zero (P_{1} = 0). However, the pressure at the surface of mercury in the cup or any another point at the same horizontal plane is equal to the atmospheric pressure.
Question 9:
A barometer kept in an elevator reads 76 cm when it is at rest. If the elevator goes up with increasing speed, the reading will be
(a) zero
(b) 76 cm
(c) < 76 cm
(d) > 76 cm
Answer:
(c) < 76 cm
If the elevator goes up at an increasing speed, then the effective value of g increases.
We know:
P=ρghSo, h will have a lesser value for the same value of P if g increases.
Page No 272:
Question 10:
A barometer kept in an elevator accelerating upward reads 76 cm. The air pressure in the elevator is
(a) 76 cm
(b) < 76 cm
(c) > 76 cm
(d) zero
Answer:
(c) > 76 cm
When the elevator is going upwards with acceleration a, the effective acceleration is a’ = (g + a).
Thus, pressure is given by
P=hρ(g+a)Air pressure in the elevator =
P=h’ρgBecause the pressure is the same, h’ > h.
∴ Air pressure > 76 cm
Question 11:
To construct a barometer, a tube of length 1 m is filled completely with mercury and is inverted in a mercury cup. The barometer reading on a particular day is 76 cm. Suppose a 1 m tube is filled with mercury up to 76 cm and then closed by a cork. It is inverted in a mercury cup and the cork is removed. The height of mercury column in the tube over the surface in the cup will be
(a) zero
(b) 76 cm
(c) > 76 cm
(d) < 76 cm
Answer:
(d) < 76 cm
Because of the trapped air, the pressure at the upper end of the mercury column inside the tube is not zero.
In other words,
P0>0.
Using this relation, we get:
Patm=P0+ρghHere,ρ=Density of mercuryh=Height of the mercury column∵P0>0And,Patm>ρgh∴76 cm of Hg>ρghor, h<76 cm
Question 12:
A 20 N metal block is suspended by a spring balance. A beaker containing some water is placed on a weighing machine which reads 40 N. The spring balance is now lowered so that the block gets immersed in the water. The spring balance now reads 16 N. The reading of the weighing machine will be
(a) 36 N
(b) 60 N
(c) 44 N
(d) 56 N
Answer:
(c) 44 N
Upthrust exerted by the water on the block = Change in the reading of the spring balance
= (20 − 16) N = 4 N
Downthrust = 4 N
Actual weight of the beaker containing water = 40 N
∴ Effective weight = (40 + 4) N = 44 N
Question 13:
A piece of wood is floating in water kept in a bottle. The bottle is connected to an air pump. Neglect the compressibility of water. When more air is pushed into the bottle from the pump, the piece of wood will float with
(a) larger part in the water
(b) lesser part in the water
(c) same part in the water
(d) it will sink
Answer:
(c) same part in the water
When more air is pushed into the bottle from the pump, the pressure of air increases on the wood as well as on the water surface with the same amount. So, the level of water and wood does not change. Thus, the piece of wood floats with the same part in the water.
Question 14:
A metal cube is placed in an empty vessel. When water is filled in the vessel so that the cube is completely immersed in the water, the force on the bottom of the vessel in contact with the cube
(a) will increase
(b) will decrease
(c) will remain the same
(d) will become zero
Answer:
(c) will remain the same
In the absence of water, the force acting on the bottom of the vessel is due to the air and the cube. Now, when water is filled in the vessel, the force due to the water and the cube is greater. The extra force is balanced by the buoyant force acting on the cube in the upward direction.
Question 15:
A wooden object floats in water kept in a beaker. The object is near a side of the beaker. Let P_{1}, P_{2}, P_{3} be the pressures at the three points A, B and C of bottom as shown in the figure.
Figure
(a) P_{1} = P_{2} = P_{3}
(b) P_{1} < P_{2} < P_{3}
(c) P_{1} > P_{2} > P_{3}
(d) P_{2} = P_{3} ≠ P_{1}
Answer:
(a) P_{1} = P_{2} = P_{3}
If the fluid is in equilibrium, then the pressure is the same at all points in the same horizontal level.
Question 16:
A closed cubical box is completely filled with water and is accelerated horizontally towards right with an acceleration α. The resultant normal force by the water on the top of the box
(a) passes through the centre of the top
(b) passes through a point to the right of the centre
(c) passes through a point to the left of the centre
(d) becomes zeros
Answer:
(c) passes through a point to the left of the centre
When the box is accelerated towards right, the water in the box experiences a pseudo force (ma) towards left, where m is the mass of water. So, the resultant normal force by the water on the top of the box passes through a point to the left of the centre.
Question 17:
Consider the situation of the previous problem. Let the water push the left wall by a force F_{1} and the right wall by a force F_{2}.
(a) F_{1} = F_{2}
(b) F_{1} > F_{2}
(c) F_{1} < F_{2}
(d) the information is insufficient to know the relation between F_{1} and F_{2}
Answer:
(b) F_{1} > F_{2}
When the box is accelerated towards right, the water in the box experiences a pseudo force (ma) towards left, where m is the mass of water. So, the force F_{1} exerted by the water on the the left wall of the box is greater.
Question 18:
Water enters through end A with a speed v_{1} and leaves through end B with a speed v_{2} of a cylindrical tube AB. The tube is always completely filled with water. In case I the tube is horizontal, in case II it is vertical with the end A upward and in case III it is vertical with the end B upward. We have v_{1} = v_{2} for
(a) case I
(b) case II
(c) case III
(d) each case
Answer:
(d) each case
This happens in accordance with the equation of continuity.
As the area of the cross section of cylindrical tube AB is constant, the velocity of water will also be the same. The equation is derived from the principle of conservation of mass and it is true for every case, i.e., when the tube is either horizontal or vertical.
Question 19:
Bernoulli theorem is based on conservation of
(a) momentum
(b) mass
(c) energy
(d) angular momentum
Answer:
(c) energy
The principle behind the Bernoulli theorem is the law of conservation of energy. It states that energy can be neither created nor destroyed; it merely changes from one form to another.
Question 20:
Water is flowing through a long horizontal tube. Let P_{A} and P_{B} be the pressures at two points A and B of the tube.
(a) P_{A} must be equal to P_{B}.
(b) P_{A} must be greater than P_{B}.
(c) P_{A} must be smaller than P_{B}.
(d) P_{A} = P_{B} only if the crosssectional area at A and B are equal.
Answer:
(d) P_{A} = P_{B} because the crosssectional areas at A and B are equal.
According to Bernoulli’s theorem, pressures at points A and B of the horizontal tube will be equal if water has the same velocity at these points.
Also, according to the equation of continuity, velocity at points A and B will be equal only if the crosssectional areas at A and B are equal.
So, P_{A} = P_{B} only if the crosssectional areas at A and B are equal.
Question 21:
Water and mercury are filled in two cylindrical vessels up to same height. Both vessels have a hole in the wall near the bottom. The velocity of water and mercury coming out of the holes are v_{1} and v_{2} respectively.
(a) v_{1} = v_{2}
(b) v_{1} = 13.6 v_{2}
(c) v_{1} = v_{2}/13.6
(d)
v1=13.6 v2
Answer:
(a) v_{1} = v_{2}
The velocity of efflux does not depend on the density of the liquid. It only depends on the height h (given same in the question) and acceleration due to gravity g (constant value here).
v1=v2=v=2gh
Question 22:
A large cylindrical tank has a hole of area A at its bottom. Water is poured in the tank by a tube of equal crosssectional area A ejecting water at the speed v.
(a) The water level in the tank will keep on rising
(b) No water can be stored in the tank
(c) The water level will rise to a height v^{2}/2g and then stop
(d) The water level will oscillate
Answer:
(c) The water level will rise to a height v^{2}/2g and then stop.
From the principle of continuity and Bernoulli’s equation, â€‹we have:
v2=2gh⇒h=v22gSo, h is the maximum height up to which the water level will rise if the water is ejected at a speed v.
Question 1:
A solid floats in a liquid in a partially dipped position.
(a) The solid exerts a force equal to its weight on the liquid.
(b) The liquid exerts a force of buoyancy on the solid which is equal to the weight of the solid.
(c) The weight of the displaced liquid equals the weight of the solid.
(d) The weight of the dipped part of the solid is equal to the weight of the displaced liquid.
Answer:
(a) The solid exerts a force equal to its weight on the liquid.
(b) The liquid exerts a force of buoyancy on the solid which is equal to the weight of the solid.
(c) The weight of the displaced liquid equals the weight of the solid.
Force exerted by any solid on a liquid = F = mg = W = Weight of the solid
According to Archimedes’ principle, any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
Also, any floating object displaces its own weight of fluid. Thus, we can say that the weight of the object is equal to the weight of the fluid displaced.
Page No 273:
Question 1:
The surface of water in a water tank on the top of a house is 4 m above the tap level. Find the pressure of water at the tap when the tap is closed. Is it necessary to specify that the tap is closed?
Answer:
Given:
Height of the water tank above the tap level, h = 4 m
Acceleration due to gravity, g = 10 m/s^{2}
Density of water, ρ = 10^{3} kg/m^{3}
When the tap is closed, the pressure of the water in the tap is
P = hρg
On substituting the respective values in the formula, we get:
P = 4 × 10^{3} × 10
= 40,000 N/m^{2}
It is necessary to specify that the tap is closed because if the tap is open, then the pressure gradually decreases as h decreases and also because the pressure in the tap is atmospheric.
Question 2:
The heights of mercury surfaces in the two arms of the manometer shown in figure are 2 cm and 8 cm.
Atmospheric pressure = 1.01 × 10^{5} N^{−2}. Find (a) the pressure of the gas in the cylinder and (b) the pressure of mercury at the bottom of the U tube.
Figure
Answer:
(a) Given:
Height of the first arm, h_{1} = 8 cm
Height of the second arm, h_{2} = 2 cm
Density of mercury, ρ_{Hg} = 13.6 gm/cc
Atmospheric pressure, p_{a} = 1.01 × 10^{5} N/m^{2} = 1.01 × 10^{6} dyn/cm^{2}
Now,
Let p_{g} be the pressure of the gas.
If we consider both limbs, then the pressure at the bottom of the tube will be the same.
According to the figure, we have:
pg+ρHg×h2×g=pa+ρHg×h1×g⇒pg=pa+ρHg×g(h1h2) =1.01×106+13.6×980×(82) dyn/cm2 =(1.01×106+13.6×980×6) dyn/cm2 =1.09×105 N/m2(b) Pressure of the mercury at the bottom of the Utube:
p_{Hg} = pa + ρ_{Hg}× h_{1} × g
=1.01×106+13.6×8×980 =1.12×105 N/m2
Question 3:
The area of cross section of the wider tube shown in figure is 900 cm^{2}. If the boy standing on the piston weighs 45 kg, find the difference in the levels of water in the two tubes.
Figure
Answer:
Given:
Area of the wider tube, A = 900 cm^{2}
Weight of the boy, m = 45 kg
Density of water,
ρ = 10^{3 }kgm^{−3}
Let h be the difference in the levels of water in the tubes and p_{a} be the atmospheric pressure.
As per the figure, we have:
pa+hρg=pa+mgA
⇒hρg=mgA⇒h=mρA⇒h=m1000×A =451000×900×104=12 m=50 cm
Question 4:
A glass full of water has a bottom of area 20 cm^{2}, top of area 20 cm^{2}, height 20 cm and volume half a litre.
(a) Find the force exerted by the water on the bottom.
(b) Considering the equilibrium of the water, find the resultant force exerted by the sides of the glass on the water. Atmospheric pressure = 1.0 × 10^{5} N m^{−2}. Density of water 1000 kg m^{−2} and g = 10 m s^{−2}.
Figure
Answer:
Given:
Atmospheric pressure, pa=1.0×105 N/m2Density of water, ρW=103 kg/m3Acceleration due to gravity, g=10 m/s2Volume of water, V=500 mL≈500 g≈0.5 kgArea of the top of the glass, A = 20 m^{2}
Height of the glass, h = 20 cmâ€‹
(a) Force exerted on the bottom of the glass = Atmospheric force + Force due to cylindrical water column or glass
=pa×A+A×h×ρw×g=Aρa+hρwg=20×104105+20×102×103×10=204 N(b) Let F_{â€‹s }be the force exerted by the sides of the glass. Now, from the free body diagram of water inside the glass, we can find out the resultant force exerted by the sides of the glass.
Thus, we have:
pa×A+mg=A×h×ρw×g+Fs+pa×A⇒mg=A×h×ρw×g+Fs⇒0.5×1=20×104×20×102×103×10+Fs⇒Fs=54=1 N (upward)
Question 5:
Suppose the glass of the previous problem is covered by a jar and the air inside the jar is completely pumped out. (a) What will be the answers to the problem? (b) Show that the answers do not change if a glass of different shape is used provided the height, the bottom area and the volume are unchanged.
Answer:
When the glass is covered by a jar and the air is pumped out of the jar, atmospheric pressure has no effect on the glass.
(a) Force exerted on the bottom:
(hρwg)×A
=20×102×103×1020×104=4 Nb mg=h×ρw×g×A+Fs⇒Fs=54=1 N(c) If we use a glass of different shape with same volume, height and area, then there will not be any change in the answer.
Question 6:
If water be used to construct a barometer, what would be the height of water column at standard atmospheric pressure (76 cm of mercury)?
Answer:
Case: When water is used in a barometer instead of mercury
Pressure exerted by 76 cm of mercury column, P = 76 × 13.6 × g dyn/cm^{2}
Density of water,
ρw = 10^{3} kg/m^{3}
Let h be the height of the water column
∵P=hρwg ∴76×13.6×g= h×ρw×g⇒h=76×13.61⇒h=1033.6 cm
Question 2:
The weight of an empty balloon on a spring balance is W_{1}. The weight becomes W_{2} when the balloon is filled with air. Let the weight of the air itself be w. Neglect the thickness of the balloon when it is filled with air. Also neglect the difference in the densities of air inside and outside the balloon.
(a) W_{2} = W_{1}
(b) W_{2} = W_{1} + w
(c) W_{2} < W_{1} + w
(d) W_{2} > W_{1}
Answer:
(a) W_{2} = W_{1}
(c) W_{2} < W_{1} + w
According to the question, the density of air inside and outside the balloon is the same. So, the weight w of air inside the balloon is equal to the weight of displaced air. Thus, the spring balance will not register any difference because the balloon will experience buoyant force equal to w that cancels out the weight of the added air.
Question 3:
A solid is completely immersed in a liquid. The force exerted by the liquid on the solid will
(a) increase if it is pushed deeper inside the liquid
(b) change if its orientation is changed
(c) decrease if it is taken partially out of the liquid
(d) be in the vertically upward direction.
Answer:
(c) decrease if it is taken partially out of the liquid
(d) be in the vertically upward direction.
The force exerted by the liquid on the solid is the vertically upward force (buoyant force) that opposes the weight of the immersed solid.â€‹ As more and more volume of the solid is immersed in the liquid, the buoyant force increases.
Buoyant force depends on the weight of the displaced liquid. So, maximum upward buoyant force acts on the solid when it is completely immersed in the liquid. It decreases if the solid is taken partially out of the liquid. Once the object is immersed in the liquid, then pushing it further in the liquid does not increase the buoyant force.
Question 4:
A closed vessel is half filled with water. There is a hole near the top of the vessel and air is pumped out from this hole.
(a) The water level will rise up in the vessel.
(b) The pressure at the surface of the water will decrease
(c) The force by the water on the bottom of the vessel will decrease
(d) The density of the liquid will decrease
Answer:
(b) The pressure at the surface of the water will decrease.
(c) The force by the water on the bottom of the vessel will decrease.
As air is pumped out of the hole, there is a decrease in the atmospheric pressure above the water surface in the vessel. Due to this, pressure at the surface of the water decreases. Thus, the force exerted by the water on the bottom of the vessel also decreases.
Question 5:
In a streamline flow,
(a) the speed of a particle always remains same
(b) the velocity of a particle always remains same
(c) the kinetic energies of all the particles arriving at a given point are the same
(d) the momenta of all the particles arriving at a given point are the same
Answer:
(c) the kinetic energies of all the particles arriving at a given point are the same
(d) the momenta of all the particles arriving at a given point are the same
In a streamline flow, every fluid particle arriving at a given point has the same velocity v. Thus, the kinetic energies (
12mv2) and momenta (mv) of all particles arriving at a given point are the same, as the mass of a particle is constant.
Question 6:
Water flows through two identical tubes A and B. A volume V_{0} of water passes through the tube A and 2 V_{0} through B in a given time. Which of the following may be correct?
(a) Flow in both the tubes are steady.
(b) Flow in both the tubes are turbulent.
(c) Flow is steady in A but turbulent in B.
(d) Flow is steady in B but turbulent in A.
Answer:
(a) Flow in both the tubes are steady.
(b) Flow in both the tubes are turbulent.
(c) Flow is steady in A but turbulent in B.
In a steady flow, the velocity of liquid particles reaching a particular point is the same at all times, but if the liquid is pushed in the tube at a rapid rate, i.e., if the flow rate increases, then the flow may become turbulent. Here, the flow rate is the volume of fluid per unit time per unit area flowing past a point.
Large volume of water passes through tube B compared to tube A. Thus, the flow rate is greater in tube B than in tube A. So, if the flow is turbulent in A, then the flow in B cannot be steady. Therefore, the first three options are possible.
Question 7:
Water is flowing in streamline motion through a tube with its axis horizontal. Consider two points A and B in the tube at the same horizontal level.
(a) The pressures at A and B are equal for any shape of the tube.
(b) The pressures are never equal.
(c) The pressures are equal if the tube has a uniform cross section.
(d) The pressures may be equal even if the tube has a nonuniform cross section.
Answer:
(c) The pressures are equal if the tube has a uniform cross section.
(d) The pressures may be equal even if the tube has a nonuniform cross section.
In streamline flow in a tube, every particle of the liquid follows the path of its preceding particle and the velocity of all particles crossing a particular point is the same. However, the velocity of the particles at different points in their path may not necessarily be the same. Thus, by applying Bernoulli’s theorem and equation of continuity, we can say that if the tube has a uniform cross section, the pressures will be equal; and if the tube has a nonuniform cross section, the pressures may or may not be equal.
Question 8:
There is a small hole near the bottom of an open tank filled with a liquid. The speed of the water ejected does not depend on
(a) area of the hole
(b) density of the liquid
(c) height of the liquid from the hole
(d) acceleration due to gravity
Answer:
(a) area of the hole
(b) density of the liquid
The emergent speed v of the liquid flowing from the hole in the bottom of the tank is given by
v=2ghHere, g is acceleration due to gravity and h is height of the liquid from the hole.
Thus, it is clear from the above relation that the speed of the liquid depends on the height of the liquid from the hole and on the acceleration due to gravity. It does not depend on the area of the hole and the density of the liquid.
Page No 274:
Question 7:
Find the force exerted by the water on a 2 m^{2} plane surface of a large stone placed at the bottom of a sea 500 m deep. Does the force depend on the orientation of the surface?
Answer:
Given:
Depth of the stone from the water surface, h = 500 m
Area of the plane surface of the large stone, A = 2 m^{2}
Density of water, ρ_{w} = 10^{3 }kgm^{−3}
^{â€‹}Force (F) is given by
F=P×A=hρw×gA P=Pressure⇒F=500×103×10×2 =107 N/m2The force does not depend on the orientation of the rock when the surface area of the stone remains the same.
Question 8:
Water is filled in a rectangular tank of size 3 m × 2 m × 1 m. (a) Find the total force exerted by the water on the bottom surface on the tank. (b) Consider a vertical side of area 2 m × 1 m. Take a horizontal strip of width δx metre in this side, situated at a depth of x metre from the surface of water. Find the force by the water on this strip. (c) Find the torque of the force calculate in part (b) about the bottom edge of this side.
(d) Find the total force by the water on this side.
(e) Find the total torque by the water on the side about the bottom edge. Neglect the atmospheric pressure and take g = 10 ms^{−2}.
Answer:
Dimensions of the rectangular tank:
Length, l = 3 m
Breadth, b = 2 m
Height, h = 1 m
Area of the bottom surface of the tank, A =
2×3=6 m2Density of water, ρ_{w} = 1000 kgm^{−3}
(a) Total force exerted by water on the bottom surface of the tank:
f=Ahρwg =6×1×103×10 =6×104=60,000 N(b) Force exerted by water on the strip of width δx:
df=p×A=xρwg×A =x×103×10×2×δx =20,000xδx N(c) Inside the tank, the water force acts in every direction due to adhesion. Therefore, torque is given by
di=F×r=20,000×δx(1x) N(d) Total force exerted by water on the side about the bottom edge
F:
F=∫0120,000 xδx⇒ F=20,000×2201 =10,000 N(e) Torque by the water on the side
τ:
τ=20,000×∫01xδx1x=20,000×22x3301=20,000×1213=20,0006 Nm =100003 Nm
Question 9:
An ornament weighing 36 g in air, weighs only 34 g in water. Assuming that some copper is mixed with gold to prepare the ornament, find the amount of copper in it. Specific gravity of gold is 19.3 and that of copper is 8.9.
Answer:
Given:
Weight of the ornament in air, m_{1}a = 36 gm
Weight of the ornament in water, m_{2}w = 34 gm
Specific gravity (density) of gold, ρ_{Au} = 19.3 gm/cc
Specific gravity (density) of copper, ρ_{Cu} = 8.9 gm/cc
Using Archimedes’ principle, we get:
Loss of weight = Weight of displaced water
= 36 − 34
= 2 gm
Let m_{Au} and m_{Cu} be the masses of gold and copper, respectively.
Now, the mass of the ornament
mc will be
m_{c} = m_{Au} + m_{Cu}= 36 gm …(i)
Now, let the volume of the ornament in cm be V.
Thus, we have:V×ρw×g=2×g⇒(vAu+vCu)×ρw×g=2×g ρw=Density of water⇒mAuρAu+mCuρCuρw×g=2×g⇒mAu19.3+mCu8.9×1=2⇒ 8.9 mAu+19.3 mCu=2×19.3×8.9 =343.54 …(ii)From (i) and (ii), we get:8.9 (mAu+mCu)=8.9×36⇒8.9 mAu+8.9mCu=320.40 …(iii)From (ii) & (iii), we get:10.4 mCu=23.14⇒mCu=2.225 gmTherefore, the amount of copper present in the ornament is 2.2 gm.
Question 10:
Refer to the previous problem. Suppose, the goldsmith argues that he has not mixed copper or any other material with gold, rather some cavities might have been left inside the ornament. Calculate the volume of the cavities left that will allow the weights given in that problem.
Answer:
Given:
Mass of copper, m_{Au} = 36 gm
Now,mAuρAu+Vcρw×g=2×gHere, Vc=Volume of cavityρAu= Density of goldρw=Density of water On substituting the respective values, we get:3619.3+Vc×1=2⇒Vc=23619.3 =36619.3 =0.112 cm3
Question 11:
A metal piece of mass 160 g lies in equilibrium inside a glass of water. The piece touches the bottom of the glass at a small number of points. If the density of the metal is 8000 kg m^{−3}, find the normal force exerted by the bottom of the glass on the metal piece.
Figure
Answer:
Given:
Mass of the metal piece, m =160 gm = 160 × 10^{−3} kg
Density of the metal piece, ρ_{m} = 8000 kg/m^{3}
Density of the water, ρ_{w} = 1000 kg/m^{3}
^{â€‹}Let R be the normal reaction and U be the upward thrust.
From the diagram, we have:
mg = U + R
⇒R = mg −Vρ_{w}g [U = Vρ_{w}g]
⇒R=mgmρm×ρw×g
=160×103×10103×108000=160×103×10118=1.4 N
Question 12:
A ferry boat has internal volume 1 m^{3} and weight 50 kg.(a) Neglecting the thickness of the wood, find the fraction of the volume of the boat immersed in water.(b) If a leak develops in the bottom and water starts coming in, what fraction of the boat’s volume will be filled with water before water starts coming in from the sides?
Answer:
Mass of the ferryboat, m = 50 kg
Internal volume, V = 1 m^{3} = External volume of the ferryboat
Density of water,
ρw = 10^{3} kg/m^{â€‹3}
(a) Let V_{1} be the volume of the boat inside the water. It is equal to the volume of the water displaced in m^{3}.
As the weight of the boat is balanced by the buoyant force, we have:
mg=V1×ρw×g⇒50=V1×103⇒V1=5100=0.05 m3(b) Let V_{2} be the volume of the boat filled with water before water starts coming in from the side.
∴ mg+V2ρw×g=V×ρw×g [V is the volume of the water displaced by the boat.]⇒50+V2×103=1×103⇒V2=10350103 =9501000=0.95 m3Fraction of the boat’s volume filled with water
=1920
Question 13:
A cubical block of ice floating in water has to support a metal piece weighing 0.5 kg. Water can be the minimum edge of the block so that it does not sink in water? Specific gravity of ice = 0.9.
Answer:
Given:
Specific gravity of ice, ρ_{ice} = 0.9 gm/cc
Weight of the metal piece, m = 5 kg
Density of water,
ρw = 10^{3} kg/m^{3}^{â€‹}
Let x be the minimum edge of the ice block in cm.
We have:
mg + W_{ice} = U
Here,
U = Upward thrust
W_{ice} = Weight of the ice
Thus, we have:0.5×g+x3×ρice×g=x3×ρw×g Volume of the liquid displaced=x3⇒0.5×103+x3×(0.9)=x3×1⇒x3×(0.1)=(0.5)×103⇒x3=5×103⇒x=17.09 cm⇒x=17 cm
Question 14:
A cube of ice floats partly in water and partly in K.oil. Find the ratio of the volume of ice immersed in water to that in K.oil. Specific gravity of K.oil is 0.8 and that of ice is 0.9.
Figure
Answer:
Given:
Specific gravity of water,
ρW= 1 gm/cc
Specific gravity of ice, ρ_{ice} = 0.9 gm/cc
Specific gravity of kerosene oil, ρ_{k} = 0.8 gm/cc
Now,
V_{â€‹}_{ice }= V_{k} + V_{w}
Here,
V_{k} = Volume of ice inside kerosene oil
V_{w} = Volume of ice inside water
V_{ice}_{ }= Volume of ice
Thus, we have:
Vice×ρice×g=Vk×ρk×g+Vw×ρw×g⇒Vk+Vw× ρice=Vk×ρk+Vw×ρw⇒(0.9) Vk+(0.9)Vw=(0.8)Vk+1×Vw⇒(0.1)Vw=0.1 Vk⇒VwVk=1.⇒Vw:Vk=1:1
Question 15:
A cubical box is to be constructed with iron sheets 1 mm in thickness. What can be the minimum value of the external edge so that the cube does not sink in water? Density of iron = 8000 kg m^{−3} and density of water = 1000 kg m^{−3}.
Answer:
Given:
Density of iron, ρ_{I} = 8000 kg/m^{3} = 8 gm/u
Density of water, ρ_{w} = 1000 kg/m^{3} = 1 gm/u
Let x be the external edge of iron.
According to Archemedes’ principle,
Weight displaced = Upward thrust
∴ w = u
For the given condition, we have:
Weight of the box = Buoyant force
⇒ V_{1}
ρIg = vρ_{w}g
⇒ (x^{2} × (0.1) × 6) × 8 = x^{3} × 1 [Volume of iron = v_{1} = 6 times the volume of each sheet]
⇒ x = 4.8 cm
Question 16:
A cubical block of wood weighing 200 g has a lead piece fastened underneath. Find the mass of the lead piece which will just allow the block to float in water. Specific gravity of wood is 0.8 and that of lead is 11.3.
Answer:
Given:
Density of wood, ρ_{w} = 0.8 gm/cc
Density of lead, ρ_{pb} = 11.3 gm/cc
Weight of the cubical wood block, m_{w} = 200 g
The cubical block floats in water.
Now,
(m_{w}+ m_{pb}) × g = (V_{w} + V_{pb})ρ × g
Here,
ρ = Density of water
V_{w} = Volume of wood
V_{pb} = Volume of lead
⇒(mw+mpb)=mwρw+mpbρpbρ⇒(200+mpb)=2000.8+mpb11.3×1⇒mpbmpb11.3=250200⇒10.3 mpb11.3=50⇒mpb=50×11.310.3=54.8 gm
Question 17:
Solve the previous problem if the lead piece is fastened on the top surface of the block and the block is to float with its upper surface just dipping into water.
Answer:
Given:
Mass of wood, m_{w} = 200 g
Specific gravity of wood,
ρW= 0.8 gm/cc
Specific gravity of lead,
ρPB= 11.3 gm/cc
We know:Mg=wThus, we have:(mw+mpb)g=Vw×ρ×g [ρ=Density of water]⇒200+mpb=2000.8×1⇒mpb=250200=50 gm
Question 18:
A cubical metal block of edge 12 cm floats in mercury with one fifth of the height inside the mercury. Water in it. Find the height of the water column to be poured.
Specific gravity of mercury = 13.6.
Answer:
Given:
Length of the edge of the metal block, x = 12 cm
Specific gravity of mercury,
ρHg= 13.6 gm/cc
It is given that
15th of the cubical block is inside mercury initially.
Let
ρbbe the density of the block in gm/cc.
∴(x)3×ρb×g=(x)2×x5×ρHg×g⇒(12)3×ρb×g=(12)2×125×13.6⇒ρb=13.65 gm/ccLet y be the height of the water column after the water is poured.
∴ V_{b} = V_{Hg} + V_{w} = (12)^{3}
Here,
V_{Hg} = Volume of the block inside mercury
V_{w} = Volume of the block inside water
∴(Vb×ρb×g)=(VHg×ρHg×g)+(Vw×ρw×g)⇒(VHg+Vw)×13.65=VHg×13.6+Vw×1⇒(12)3×13.65=(12y)×(12)2×13.6+(y)×(12)2×1⇒12×13.65=(12y)×13.6+(y)⇒12.6y=13.612125=(13.6)×(9.6)⇒y=(9.6)×(13.6)(12.6)=10.4 cm
Question 19:
A hollow spherical body of inner and outer radii 6 cm and 8 cm respectively floats halfsubmerged in water. Find the density of the material of the sphere.
Answer:
Given:
Inner radius of the hollow spherical body, r_{1} = 6 cm
Outer radius of the hollow spherical body, r_{2} = 8 cm
Let the density of the material of the sphere be
ρ and the volume of the water displaced by the hollow sphere be V.
If
ρw is the density of water, then:
Weight of the liquid displaced=V2(ρw)×gWe know:Upward thrust=Weight of the liquid displaced∴43πr3243πr13ρ=1243πr23×ρw⇒r23r13×ρ=12r23×1⇒(8)3(6)3×ρ=12(8)3×1⇒ρ=5122×(512216) = 5122×296= 0.865 gm/cc =865 kg/m3^{3}
Question 20:
A solid sphere of radius 5 cm floats in water. If a maximum load of 0.1 kg can be put on it without wetting the load, find the specific gravity of the material of the sphere.
Answer:
Given:
Radius of the sphere, r = 5 cm
Mass of the maximum load, m = 0.1 kg
Let the weight of the sphere be W_{1} and the weight of the load be W_{2}.
Now,
W_{1} + W_{2} = U
Here, U is the upward thrust.
Let V be the volume of the sphere.
Thus, we have:mg+V×ρs×g=v×ρw×gHere, ρs=Density of the sphere in gm/ccρw=Density of waterOn substituting the respective values in the above equation, we get: (0.1)×103+43×π×(5)3×ρs=43×π×(5)3×1⇒100=43×π×125×(1ρs)⇒1ρs=3×1004×π×125=0.19⇒ρs=1(0.19) =0.81 gm/cc=0.8 gm/cc
Question 21:
Find the ratio of the weights, as measured by a spring balance, of a 1 kg block of iron and a 1 kg block of wood. Density of iron = 7800 kg m^{−3}, density of wood = 800 kg m^{−3} and density of air = 1.293 kg m^{−3}.
Answer:
Given:
Density of iron, ρ_{I}= 7800 kgm^{−3}
Density of wood, ρ_{w} = 800 kgm^{−3}
Density of air, ρ_{air} = 1.293 kgm^{−3}
Net weight of Iron W1:WI=mIgVIρair×gHere, mI and VI are the mass and volume of the iron, respectively.Now,mImIρIρairg=117800×1.293×(9.8)Net weight of wood=Ww=mwgVw . ρair gHere, mw and Vw are the mass and volume of the wood, respectively.Now,mmρwρairg=11800×1.2939.8∴WIWw=9.878001.29378009.88001.293800=78001.2938001.293×878=1.0015
Question 22:
A cylindrical object of outer diameter 20 cm and mass 2 kg floats in water with its axis vertical. If it is slightly depressed and then released, find the time period of the resulting simple harmonic motion of the object.
Answer:
Given:
Outer diameter of the cylindrical object, d = 20 cm
∴ Radius
, r=d2=10 cmMass of the object, m = 2 kg
When the object is depressed into water, the net unbalanced force causes simple harmonic motion.
Let x be the displacement of the block from the equilibrium position.
Now,
Driving force=U=VρwgHere,U is the upward thrust.V is the volume.ρw is the density of water. Thus, we have:ma=πr2(x)×ρwga=πr2ρw9x2×103T=2πDisplacementAcceleration =2π(x)×2×103πr2ρwg(x) =2π2×103π×(10)2×1×10 =2π2π×10=0.5 sTherefore, the required time period is 0.5 second.
Question 23:
A cylindrical object of outer diameter 10 cm, height 20 cm and density 8000 kg m^{−3} is supported by a vertical spring and is half dipped in water as shown in figure. (a) Find the elongation of the spring in equilibrium condition. (b) If the object is slightly depressed and released, find the time period of resulting oscillations of the object. The spring constant = 500 N m^{−1}.
Figure
Answer:
Given:
Diameter of the cylindrical object, d = 10 cm
∴ Radius, r = 5 cm
Height of the object, h = 20 cm
Density of the object, ρ_{b} = 8000 kg/m^{3} = 8 gm/cc
Density of water, ρ_{w} = 1000 kg/m^{3}
Spring constant, k = 500 N/m = 500 × 10^{3} dyn/cm
(a) Now,
F + U = mg [F = kx]
Here, U is the upward thrust and x is the small displacement from the equilibrium position.
kx + Vρ_{w}g = mg
⇒500×103×(x)+πr2×h2×1×1000=πr2×h×ρb×1000⇒500×103×(x)= πr2×h×1000ρb12=π×(5)2×20×1000ρb12⇒50x=π×25×2×ρb12x=π(80.5)or, x=π×7.5=23.5 cm(b) We know that X is the displacement of the block from the equilibrium position.
Now,
Driving force:
F=kX+Vρw×g⇒ma=kx+πr2×(X)×ρw×g=(k+πr2×ρw×g)x⇒ω2×(X)=k+πr2×ρw×gm×(X)In SHM a=ω2X, time period T is T=2πmk+πr2×ρw×g =2ππ×25×20×8500+103+π×25×1×1000 =0.935 s
Page No 275:
Question 24:
A wooden block of mass 0.5 kg and density 800 kg m^{−3} is fastened to the free end of a vertical spring of spring constant 50 N m^{−1} fixed at the bottom. If the entire system is completely immersed in water, find (a) the elongation (or compression) of the spring in equilibrium and (b) the timeperiod of vertical oscillations of the block when it is slightly depressed and released.
Answer:
Given:
Mass of the wooden block, m = 5 kg
Density of the block, ρ = 800 kg/m^{3}
Spring constant, k = 50 N/m
Density of water, ρ_{w} = 1000 kg/m^{3}
(a) Using the free body diagram, we get:
mg = kx + Vρ_{w}g (Here, Vρ_{w}g > mg. So, there will be some elongation.]
⇒(0.5)×(10+50×(x))= 0.5800×103×(10)⇒50x=0.5×10×1081⇒50x=54⇒x=2.5 cm
(b) As the system is inside the water, the unbalance force will be the driving force, which is kx for SHM.
Hence, there will be no change in the buoyant force.
∴ma=kx⇒a=kx/m⇒w2x=kx/m⇒2pT2=k/m⇒T=2πmk=2π×0.550=π5 s
Question 25:
A cube of ice of edge 4 cm is placed in an empty cylindrical glass of inner diameter 6 cm. Assume that the ice melts uniformly from each side so that it always retains its cubical shape. Remembering that ice is lighter than water, find the length of the edge of the ice cube at the instant it just leaves contact with the bottom of the glass.
Answer:
Let the length of the edge of the ice block when it just leaves contact with the bottom of the glass be x and the height of water after melting be h.
Given:
Inner diameter of the cylindrical glass = 6 cm
∴ Inner radius, r = 3 cm
Edge of the ice cube = 4 cm
Weight = Upward thurst⇒(x)3×ρice×g=(x)2×h×ρw×g⇒h=(0.9)xAgain, volume of the water left from the melting of the ice is given by
(4)3(x)3=π×(r)2×hx2h [Amount of water=ρ(r2x2)h]⇒ (4)3(x)3=π×(3)2×hx2hPutting h=0.9x, we get:(4)3(x)3=π×(3)2×(0.9)xOn solving the above equation, we get:
x = 2.26 cm
Question 26:
A Utube containing a liquid is accelerated horizontally with a constant acceleration a_{0}. If the separation between the vertical limbs is l, find the difference in the heights of the liquid in the two arms.
Answer:
Let a_{0} be the acceleration with which Utube is accelerated horizontally. So, the horizontal part will experience some inertial force.
Also,
p_{a} = Atmospheric pressure
A = Area of cross section
h = Increase in height of the liquid
According to the question,
paA+A×l×ρ×a0=paA+hρg×A⇒hg=a0l⇒h=a0lg
Question 27:
At Deoprayag (Garwal, UP) river Alaknande mixes with the river Bhagirathi and becomes river Ganga. Suppose Alaknanda has a width of 12 m, Bhagirathi has a width of 8 m and Ganga has a width of 16 m. Assume that the depth of water is same in the three rivers, Let the average speed of water in Alaknanda be 20 km h^{−1} and in Bhagirathi be 16 km h^{−1}. Find the average speed of water in the river Ganga.
Answer:
Given:
Average speed of water in Alaknanda = 20 kmh^{−1}
Average speed of water in Bhagirathi = 16 kmh^{−1}
Width of Bhagirathi = 8 m
Width of Alaknanda = 12 m
Width of Ganga = 16 m
Now,
Volume of water discharged from Alaknanda + Volume of water discharged from Bhagirathi = Volume of water flow in Ganga
Let:
d = Depth of the three rivers
⇒V_{A} × d × 12 + V_{B} × d × 8 = V_{G} × d × 16
Here, V_{A}, V_{B} and V_{G} be the average speeds of water in Alaknanda, Bhagirathi and Ganga, respectively.
Using the equation of continuity, we get:
20×12+16×8=VG×16⇒VG × 16= 368⇒VG=36816=23 km/h
Question 28:
Water flows through a horizontal tube of variable cross section. The area of cross section at A and B are 4 mm^{2} and 2 mm^{2} respectively. If 1 cc of water enters per second through A, find (a) the speed of water at A, (b) the speed of water at B and (c) the pressure difference P_{A} − P_{B}.
Figure
Answer:
(a) Given:
The areas of crosssections of the tube at A and B are 4 mm^{2} and 2 mm^{2}, respectively.
1 cc of water enters per second through A.
Let:
ρ= Density of the liquid
P_{A} = Pressure of liquid at A
P_{B} = Pressure of liquid at B
(a)
QAt=1as=Discharge⇒aA×VA=QAGiven: aA=4 mm2=4×102 cm2∴ 4×102×VA=1cc/s ⇒VA=25 m/s(b) aA×VA=aB×VB⇒4×102×25=2×102×VB⇒VB=50 cm/s(c) By Bernoulli’s equation, we have:
12ρvA2+PA=12ρvB2+PB⇒(PAPB)=12ρvB2vA2=12×1×(2500625)=18752=937.5 dyn/cm2=93.75 N/m2
Question 29:
Suppose the tube in the previous problem is kept vertical with A upward but the other conditions remain the same. the separation between the cross sections at A and B is 15/16 cm. Repeat parts (a), (b) and (c) of the previous problem.
Answer:
Given:
Separation between the cross sections at A and B = 15/16 cm
Speed of water at A, v_{A} = 25 cm/s
Speed of water at B, v_{B} = 50 cm/s
(c) By Bernoulli’s equation, we get:
12pvA2+ρghA+PA=12pvB2+ρghB+PB⇒PAPB=12pvB2vA2+ρghBhA [hAhB=1516cm]⇒PAPB=12(2500625)1000×1516 =0
Question 30:
Suppose the tube in the previous problem is kept vertical with B upward. Water enters through B at the rate of 1 cm^{3} s^{−1}. Repeat parts (a), (b) and (c). Note that the speed decreases as the water falls down.
Answer:
Water enters through B at the rate of 1 cm^{3}s^{−1}.
(a) Speed of water at A,
V→A=25 cm/s(b) Speed of water at B,
V→B=50 cm/s(c) By Bernoulli’s equation, we have:
12pvA2+ρghA+PA=12ρvB2+ρghB+PB⇒ PAPB=12×1×1875+12×1000×1516 =1875 dyn/cm2=188 N/m2
Question 31:
Water flows through a tube shown in figure. The area of cross section at A and B are 1 cm^{2} and 0.5 cm^{2} respectively. The height difference between A and B is 5 cm. If the speed of water at A is 10 cm s^{−1} find (a) the speed at B and (b) the difference in pressures at A and B.
Figure
Answer:
Given:
Difference in the heights of A and B = 5 cm
Area of cross section at A, a_{a} = 1 cm^{2}
Area of cross section at B, a_{b} = 0.5 cm^{2}
Speed of water at A,
vA= 10 cms^{−1}
(a) From the equation of continuity, we have:V→A×aA=V→B×aB⇒10×1=V→B×0.5⇒V→B=20 cm/sThe required speed of water at cross section B is 20 cms^{−1}
(b) From Bernoulli’s equation, we get: 12ρvA2+ρghA+PA=12ρvB2+ρghA+PB⇒PBPA=12ρvA2vB2+ρghAhBHere,
P_{A} and P_{B} are the pressures at A and B, respectively.
h_{A} and h_{B} are the heights of points A and B, respectively.
ρ is the density of the liquid.
On substituting the values, we have:
PBPA=12×1(100400)+1×1000(5.0) =150+5000=4850 Dyne/cm2 =485 N/m2Therefore, the required pressure difference at A and B is 485 N/m^{2}.
Question 32:
Water flows through a horizontal tube as shown in figure. If the difference of heights of water column in the vertical tubes is 2 cm, and the areas of cross section at A and B are 4 cm^{2} and 2 cm^{2} respectively, find the rate of flow of water across any section.
Figure
Answer:
Given:
Difference in the heights of water columns in vertical tubes = 2 cm
Area of cross section at A, a_{A} = 4 cm^{2}
Area of cross section at B, a_{B} = 2 cm^{2}
Now, let v_{A} and v_{B} be the speeds of water at A and B, respectively.
From the equation of continuity, we have:
vAaA=vB×aB⇒ vA×4=vB×2⇒ vB=2vA …(i)From Bernoulli’s equation, we have:
12ρvA2+ρghA+pA=12ρvB2+ρghB+pB⇒12ρvA2+pA=12ρvB2+pB⇒pApB=12ρvB2vA2Here,
p_{A} and p_{B} are the pressures at A and B, respectively.
h_{A} and h_{B} are the heights of water columns at point A and B, respectively.
ρ is the density of the liquid.
Thus, we have:
12×1×4vA2vA2⇒2×1×1000=12×1×3vA2[pApB=2 cm=2×1×1000 dyne/cm2 (water column)]⇒ vA2=40003=36.51 cm/s∴Rate of flow=vAaA=36.51×4=146 cm3/sHence, the required rate of flow of water across any section is 146 cm^{3}/s.
Question 33:
Water flows through the tube shown in figure. The areas of cross section of the wide and the narrow portions of the tube are 5 cm^{2} and 2 cm^{2} respectively. The rate of flow of water through the tube is 500 cm^{3} s^{−1}. Find the difference of mercury levels in the Utube.
Figure
Answer:
Given:
Area of cross section of the wide portions of the tube, a_{a} = 5 cm^{2}
Area of cross section of the narrow portions of the tube, a_{b}= 2 cm^{2}
Now, let v_{a} and v_{b} be the speeds of water at A and B, respectively.
Rate of flow of water through the tube = 500 cm^{3}/s
⇒vA=5005=100 cm/sFrom the equation of continuity, we have:vAaA=vBaB⇒vAvB=aBaA=25⇒5vA=2vB⇒vB=52vA …(i)From the Bernoulli’s equation, we have:12ρvA2+ρghA+pA=12ρvB2+ρghB+pB⇒ pApB=12pvB2vA2Here,
ρ is the density of the fluid.
p_{A} and p_{B} are the pressures at A and B.
h is the difference of the mercury levels in the Utube.
Now,h×13.6×980=12×1×214(100)2 [Using (i)]⇒ h=21×(100)22×13.6×980×4 [∵pApB]=1.969 cmTherefore, the required difference is 1.969 cm.
Question 34:
Water leaks out from an open tank through a hole of area 2 mm^{2} in the bottom. Suppose water is filled up to a height of 80 cm and the area of cross section of the tanks is 0.4 m^{2}. The pressure at the open surface and at the hole are equal to the atmospheric pressure. Neglect the small velocity of the water near the open surface in the tank. (a) Find the initial speed of water coming out of the hole. (b) Find the speed of water coming out when half of water has leaked out. (c) Find the volume of eater leaked out using a time interval dt after the height remained is h. Thus find the decrease in height dh in terms of h and dt.
(d) From the result of park (c) find the time required for half of the water to leak out.
Answer:
Given:
Area of the hole of the water tank = 2 mm^{2}
Height of filled water, h = 80 cm
Area of the crosssection of the tanks, A = 0.4 m^{2}
Acceleration due to gravity, g = 9.8 ms
2
Pressure at the open surface and at the hole is equal to atmospheric pressure.
(a) Velocity of water:
v
=2gh
= 2×10×0.80= 4 m/sec(b) Velocity of water when the tank is halffilled and h
is 802, i.e., 40 cm:
= 2×10×0.40= 8 m/sec(c) Volume:
Volume=Ah=A v dt=A×2gh dt=2 mm22ghdtVolume of the tank = Ah = V (say)
i.e., dVdt=Adhdt
a1v1=Adhdt⇒2×1062gh=0.4dhdt⇒dh=5×1062ghdt
(d) ∵dh=5×106 2gh dt∴dh2gh=5×106 dtOn integrating, we get:
5×106∫0tdt=128∫0.80.4dhh=5×106×t=128×2h120.80.4=5×106×t=128×2h120.80.4⇒t=120×2×(0.4)1/2(0.8)1/4×15×106⇒t=14.47×2×23.16×15×106×13600 h = 6.51 hThus, the time required to leak half of the water out is 6.51 hours.
Question 35:
Water level is maintained in a cylindrical vessel up to a fixed height H. The vessel is kept on a horizontal plane. At what height above the bottom should a hole be made in the vessel so that the water stream coming out of the hole strikes the horizontal plane at the greatest distance from the vessel.
Figure
Answer:
It is given that H is the height of the cylindrical vessel.
Now, let h be the height of the hole from the surface of the tank.
The velocity of water
vis given by
v=2g(Hh)Also, let t be the time of flight.Now,t=2hgLet x be the maximum horizontal distance.∴ x=v×t =2g(Hh)×2hg =4(Hhh2)For x to be maximum,
ddhHhh2=0⇒0=H2h⇒h=H2
HC Verma Solutions for Class 11 Physics – Part 1
 Chapter 1 – Introduction to Physics
 Chapter 2 – Physics and Mathematics
 Chapter 3 – Rest and Motion: Kinematics
 Chapter 4 – The Forces
 Chapter 5 – Newton’s Laws of Motion
 Chapter 6 – Friction
 Chapter 7 – Circular Motion
 Chapter 8 – Work and Energy
 Chapter 9 – Center of Mass, Linear Momentum, Collision
 Chapter 10 – Rotational Mechanics
 Chapter 11 – Gravitation
 Chapter 12 – Simple Harmonic Motion
 Chapter 13 – Fluid Mechanics
 Chapter 14 – Some Mechanical Properties of Matter
 Chapter 15 – Wave Motion and Wave on a String
 Chapter 16 – Sound Wave
 Chapter 17 – Light Waves
 Chapter 18 – Geometrical Optics
 Chapter 19 – Optical Instruments
 Chapter 20 – Dispersion and Spectra
 Chapter 21 – Speed of Light
 Chapter 22 – Photometry
HC Verma Solutions for Class 12 Physics – Part 2
 Chapter 23 – Heat and Temperature
 Chapter 24 – Kinetic Theory of Gases
 Chapter 25 – Calorimetry
 Chapter 26 – Laws of Thermodynamics
 Chapter 27 – Specific Heat Capacities of Gases
 Chapter 28 – Heat Transfer
 Chapter 29 – Electric Field and Potential
 Chapter 30 – Gauss’s Law
 Chapter 31 – Capacitors
 Chapter 32 – Electric Current in Conductors
 Chapter 33 – Thermal and Chemical Effects of Electric Current
 Chapter 34 – Magnetic Field
 Chapter 35 – Magnetic Field due to a Current
 Chapter 36 – Permanent Magnets
 Chapter 37 – Magnetic Properties of Matter
 Chapter 38 – Electromagnetic Induction
 Chapter 39 – Alternating Current
 Chapter 40 – Electromagnetic Waves
 Chapter 41 – Electric Current through Gases
 Chapter 42 – Photoelectric Effect and Wave Particle Duality
 Chapter 43 – Bohr’s Model and Physics of the Atom
 Chapter 44 – Xrays
 Chapter 45 – Semiconductors and Semiconductor Devices
 Chapter 46 – The Nucleus
 Chapter 47 – The Special Theory of Relativity
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