Unit :- 2
Pressure
#. Answer the following Questions
1). What is pressure ? Write its formula and SI unit .
Ans:- The force acting per unit area of the surface is called pressure.
Formula: Pressure (P) = Force (F) / Area (A)
SI unit of pressure is Nm-2 or Pascal (Pa) .
2). Name the factors on which pressure depends .
Ans:- Pressure depends on the following two factors,
(i). Force (ii) Area
3). Prove that P = F/A.
Ans:- Let ‘F’ be the force acting perpendicularly on the surface area ‘A’ and ‘P’ be the pressure then, Then, the pressure is directly proportional to the force acting perpendicularly.
P axF . . . . . . . .(i)
And pressure is inversely proportional to surface area.
P ax1/A . . . . . . . . . (ii)
Now, combining equation (i) and (ii) we get.
:. P axF/A
Or, P = KxF/A
Where ‘k’ is a proportional constant.
If F is IN, A is 1m2 and P is 1 Pascal then equation (ii) becomes,
1 = Kx1/1
K = 1
Now, putting the value of K in equation (ii) we get,
P = 1xF/A
| P = F/A |
:.
Proved.
4). What is one Pascal pressure ?
Ans:- If IN force is acting perpendicularly on 1 m2 surface are then it is called one Pascal pressure .
5). What is liquid pressure ?
Ans:- The force exerted by the liquid per unit area of the surface is called liquid pressure.
6). Differentiate between pressure and force (thrust) .
Ans:- The differentiate between force and pressure are as follows :-
| Force | Pressure |
| i). It is an external agency which changes or tends to change the motion of a body. | i). The force per unit area of a surface is called pressure . |
| ii). Its SI unit is Newton . | ii). Its SI unit is Pascal. |
| iii). It is measured by spring balance . | iii). It is measured by manometer, barometer pressure guage . |
7). Write any four properties of liquid pressure .
Ans:- The properties of liquid pressure are as follows :-
i). The pressure at the same depth in a liquid is the same in all the directions.
ii). A liquid determines its own level.
iii). The liquid pressure is directly proportional to the density of the liquid.
iv). The pressure of a liquid is independent of the shape of the container in which it is kept.
8). Prove that, p = hdg . or, pah
OR, Prove the relation of pressure exerted by the liquid column with its height, density and acceleration due to gravity of that place .
Ans:- Let us consider a liquid of density ‘d’ is kept in a beaker of cross-sectional area ‘A’. Let ‘h’ be the Now, from the definition of pressure,
P = F/A
or, P = mg/A [:. F= mg]
Or, P = dvg/A [ d = m/v ⇒ m = dv]
or, P = dxAxhxg/A [v = 1 x b x h] [⇒ v = A x h]
or, P = dxhxg [v = 1 x b x h] [⇒ v = A x h]
P = dhg
The density of the liquid and the value of ‘g’ at a certain place remains constant.
| P = hgd |
Proved.
9). What are the factors affecting the pressure exerted by the liquid ?
Ans:- The factors affecting pressure exerted by the liquid are:-
i). Density of the liquid. (:. P a d)
ii). Acceleration due to gravity. ( :. P a g)
iii). Depth/ height of liquid column. (:. P a h)
10). The bottom of the vessel is made thicker, why ?
Ans:- The pressure exerted by liquid is directly proportional to the depth of the liquid column. Water exerts more pressure at the bottom of the vessel as liquid pressure increases with increases in depth. So, the bottom of the vessel is made thicker to bear the great pressure .
11). Walls of a dam are made thicker at the bottom, why ?
Ans:- Water exerts more pressure near or at the bottom of the wall of a dam as liquid pressure is directly proportional to the depth. So, walls of the dam are made thicker at the bottom to bear the greatest pressure .
12). The bucket is filled slower in the tap of the top floor than the down floor, why ?
Ans:- The height of the water column in the tap of top floor is less than that of the down floor. As the liquid pressure is directly proportional to the depth of the liquid, the speed of the flow of the water is less in the tap of top floor. So the bucket is filled slower in the tap of the top floor than the down floor.
13). The size of the water bubbles becomes bigger as it rises up in the water, why ?
Ans:- Pressure exerted by water to the air bubble decreases when the bubble rises up in water. So, the size of the water bubbles becomes bigger as it rises up in the water.
14). The shape of a vessel does not affect the liquid pressure . Why ?
Ans:- The pressure exerted by the liquid is depend upon the depth of the liquid as liquid pressure increases with increases in depth but not the shape of the vessel. So the shape of vessel does not affect the liquid pressure .
15). The blood pressure in human body is greater at the feet than at the brain, Why ?
Ans:- The liquid pressure is directly proportional to the depth of the liquid and the depth of the feet from the heart is more than the height of the brain. So the blood pressure in human body is greater at the feet than at the brain.
16). Answer the following questions on the basis of given figure .
i). Is there any difference between the pressure of water at the middle and at the bottom? Why ?
Ans:- The pressure of water is more at the bottom than the middle because the liquid pressure is directly proportional to the depth of the liquid.
ii). Why is the wall made thicker at the bottom ?
Ans:- The water exerts more pressure at the bottom of the wall as liquid pressure increases with increase in depth. So to bear the great pressure the bottom of the wall is made thicker.
17). What change occurs in pressure when a drum completely filled with water is taken from earth to lunar surface ?
Ans:- The liquid pressure is directly proportional to the acceleration due to gravity. The value of acceleration due to gravity on the lunar surface is less than that on the earth surface. So the pressure exerted by the drum decreases when a drum completely filled with water is taken from earth to lunar surface.
18). Deep sea driver need special protective wear, why ?
Ans:- Liquid pressure increases with increase in depth of the liquid, so the large liquid pressure is exerted on the driver. So, to bear this large liquid pressure, the sea driver requires special protective wear.
19). The water tank is kept at some height, why ?
Ans:- Liquid pressure is directly proportional to the height of the liquid, so the water tank is kept at some height to supply water to all houses or building with great pressure.
20). In the given figure, on which surface more pressure is exerted and why ?
Ans:- In the given figure, both surfaces ‘A’ and ‘B’ experiences equal pressure. Because the liquid pressure is directly
proportional to the height of the liquid. Here the height of the liquid is equal in both containers.
21). Study the given figure and answer the following question .
i). Which tank experience less pressure at the bottom when both the tanks are completely filled with water ? 
Ans:- Both tanks experiences equal pressure at the bottom when both the tanks are completely filled with water.
ii). Which tank experiences grater pressure as its bottom if each of tank is filled with 1000 liter, why ?
Ans:- Tank ‘B’ experiences grater pressure at the bottom if each of tank is filled with 1000 liter because the height of water in tank ‘B’ will be more than in tank ‘A’ .
22). In the given diagram an object ABCD is shown dipped in water . On which surface out of AB and CD of the object will the water exert more pressure ? Why ?
Ans:- In the diagram water exerts more pressure on the surface ‘CD’ of an object. Because the depth of the CD is more than that of AB. As we know liquid pressure is directly proportional to the depth of the liquid.
23). In the diagram, the spouts A, B and C having equal diameters are connected in the water – filled tank . From which spout does the water spout does the water flow maximum in per second and why ?
Ans:- From the spout ‘C’ more water flows per second because the spout ‘C’ has more depth than the spouts ‘A’ and ‘B’. And the pressure exerted by a liquid is directly proportional to the depth of the liquid .
24). What is hydrostatics ?
Ans:- The branch of physics which deals with the properties of fluids (liquid and gas ) at rest is called hydrostatics .
25). State Pascal’s law of liquid pressure .
Ans:- Pascal’s law of liquid pressure state that ”when pressure is applied on a liquid enclosed in a vessel, it is transmitted equally in all the directions” .
26). Write the formula based on Pascal’s law .
Ans:- The formula based on Pascal’s law is,
| F1/ A1 = F2/ A2 |
27). Name the equipment which are based on Pascal’s law of liquid pressure .
Ans:- The equipment which are based on Pascal’s law of liquid pressure are as follows :-
i). hydraulic brake
ii). hydraulic press
iii). hydraulic garage lift
28). Which law is explained by the given figure ? State the law . 
Ans:- It explain Pascal’s law. Pascal’s law of liquid pressure state that ”when pressure is applied on a liquid enclosed in a vessel, it is transmitted equally in all the directions” .
29). State principle of hydraulic machines .
Ans:- Principle of hydraulic machines state that ”a large force is developed on the bigger piston, when a small force applied on a smaller piston. ”
30). Prove that hydraulic machine is a force multiplyer .
Ans:- Let us considered a hydraulic press having two pistons of cross – sectional area A1 and A2. When an effort is applied to the small piston, large force is developed in a large piston .
Now, according to the Pascal’s law
Pressure on small piston ( P1) = Pressure on large piston ( P2) 
:. P1 = P2
Or, F1 / A1 = F2/A2
Or, F1A2 = F2A1
Or, F2A1 = F1A2
:. F2 = F1xA2/A1
Since, A2 > A1 ⇒ F2 > F1
:. Hence, hydraulic machine is force multiplier.
31). What is hydraulic garage lift ?
Ans:- Hydraulic garage lift is a machine used to uplift light vehicles like car, jeep, vans etc in service stations during their servicing .
32). What is hydraulic brake ?
Ans:- A hydraulic break is a braking system that used in four wheelers and other heavy automobiles.
33). What is the role of water in hydraulic press to multiply small force into large one ?
Ans:- Water transmits pressure perpendicularly in all directions and it cannot be compressed . So water plays an important role to multiply the small force into large one .
34). Define hydraulic press .
Ans:- Hydraulic press is equipment which is based on Pascal’s law and used to press the things like cotton, paper etc .
35). By which tow properties of liquid is used in hydraulic brake ?
Ans:- Due to the following two properties of liquid it is used in hydraulic brake .
i). Liquid transmits pressure perpendicularly in all directions .
ii). Liquid cannot be compressed .
36). Answer the following questions after observing the given diagram of hydraulic press .
i). On what principle does this machine work ? State the principle also .
Ans:- It works on the basis of Pascal’s law of liquid pressure . Pascal’s law of liquid pressure state that ”when pressure is applied on a liquid enclosed in a vessel, it is transmitted equally in all the directions”.
ii). What role is played by water to multiply the force in this device ?
Ans:- Liquid transmits pressure perpendicularly in all directions and it cannot be compressed. So water plays a great role to multiply the force in all directions.
37). What is density ? write its formula and SI unit .
Ans:- The density of a substance is defined as the mass per unit volume of that substances.
Formula, Density ( d ) = Mass ( M )/ Volume ( V )
The SI unit of density is kg/ m3 .
38). What is relative density ? Write its formula .
Ans:- The ratio of the density of a substance to the density of pure water at 4 ֗C is called relative density of that substance . It is also known as specific gravity .
Relative Density = Density of substance / Density of water at 4 ֗C
39). The relative density has no unit, why ?
Ans:- The relative density has no unit it is the ratio of two physical quantities .
40). Differentiate between density and relative density .
Ans:- The differentiate between density and relative density are as follows :-
| Density | Relative Density |
| i). The density of a substance is defined as the mass per unit volume of that substance. | i). The ratio of the density of a substance to the density of pure water at 4 ֗ C is the relative density of that substance . |
| ii). The formula to calculate the density is, Density (d ) = Mass (M )/ Volume(V) | ii). The formula to calculate the relative density is, Relative density = Density of the substance / Density of pure water at 4 ֗ C |
| iii). Its SI unit is kg / m3 . | iii). It has no unit . |
41). Ice burg made up of water float in water, why ?
Ans:- Density of ice is less than that of water, so the ice burg made up of water floats in water.
42). What difference will you get from the air – filled balloon and the hydrogen filled balloon.
Ans: –The density of hydrogen is less than that of air. Therefore the balloon filled with hydrogen rises up and the balloon filled with air falls down.
43). One feels difficult to breathe in deep well while cleaning it, why ?
Ans:- Carbon dioxide is found in the deep well as it is heavier than oxygen. So due to the absence of oxygen one feels difficult to breathe in the deep well.
44). What is up thrust ? Write its SI unit .
Ans:- When a body is a partially or wholly immersed in a fluid, the resultant force acted on it by the fluid is called up thrust. Its SI unit is Newton ( N ).
45). Prove that upthrust is equal to the weight of liquid displaced. OR, Prove that, up thrust = vdg.
Ans:- Let us consider a body ABCD whose height is h and cross-sectional area ‘A’ is completely immersed in a liquid of density ‘d’. Let ‘h1’ be the height of liquid above the surface AB and h2 be the height of liquid above the surface CD as shown in figure. Now, the thrust on the upper surface of AB (F1 ) = P1x A
And, the thrust on the lower surface of CD (F1) = h1dg x A [:. P = dgh
= P2 x A
= h2dg x A
:. Resultant upward trust = F2 – F1
or, Upthrust = h2dgA – h1dga
or, Upthrust = dgA (h2 – h1)
or, Upthrust = dgA x h [:. h = h2 – h1]
| Upthrust = dgv |
:.
or, Upthrust = d.v.g
or, Upthrust = m.g [ :. m = v x d]
or, Upthrust = weight of liquid displaced [:. w = mg]
| Upthrust = weight of liquid displaced |
:.
Proved.
46). Name the factors affecting upthrust.
Ans:- The factors affecting upthrust are :-
i). Volume of submerged part of the body.
ii). Density of the liquid.
iii). Acceleration due to gravity.
47). What is the relation between upthrust and density of the liquid ?
Ans:- Upthrust is directly proportional to density of liquid.
48). What is the relation between upthrust and volume of displaced liquid ?
Ans:- Upthrust is directly proportional to the volume of displaced liquid.
49). It is easier to lift a heavy stone inside the water, why ?
Ans:- The weight of stone inside the water decreases due to upthrust given by water, so it is easier to lift a heavy stone inside the water.
50). In the given figure which is sea water and river water ? Why ?
Ans:- In the given figure, water kept in figure (2) is sea water and (1) is river water, because an egg is floating just below the surface of water in figure (2), as sea water provides more upthrust and egg is near the bottom in figure (1) as river water provides less upthrust.
51). State Archimede’s principle .
Ans:- Archimede’s principle states that ”when a body is partially or wholly impressed in a liquid, it experiences an upthrust which is equal to the weight of the liquid displaced by it. ”
52). Which one of the figures is salt solution and why ?
Ans:- In the given figure, figure ‘B’ is salt solution. Because in figure ‘B’ an egg is floating just below the surface water as salt water provides more upthrust than that of river water.
53). Study the given figure and answer the following question.
i). What is the mass and weight of stone in air ? 
Ans:- Here
Weight of the stone in air = 15 N
Weight of the displaced water = 8 N
Weight of the stone in air = ?
Now, we know that,
Weight of the stone in air (W) = Weight of the stone in water + Weight of the displaced water
=15N+8N
:. Weight of the stone in air (W) = 23N
Again, we know that,
W = mg
or, 23 = m x 10
or, m = 23/10
:. m = 2.3kg
Therefore, the mass and weight of stone in air are 2.3kg and 23N respectively.
ii). How much upthrust is exert on the stone ?
Ans:- Upthrust = Weight of displaced water
Upthrust = 8N
iii). In which principle is this experiment based on ? State the principle.
Ans:- It is based on Archimed’s principle.
Archimede’s principle states that ”When a body is partially or wholly impressed in a liquid, it experiences an upthrust which is equal to the weight of the liquid displaced by it. ”
54). Study the given diagram and answer the following questions .
i). In which principle is the experiment based ? State this principle too.
Ans:- It is based on Archimedes’s principle.
Archimede’s principle states that ”when a body is partially or wholly impressed in a liquid, it experiences an upthrust which is equal to the weight of the liquid displaced by it. ”
ii). What is the weight of displaced water ?
Ans: Here,
Weight of the stone in air = 10N
Weight of the stone in water = 8N
Weight of the displaced water = ?
:. Weight of the displaced water = Weight of the stone in air – Weight of the stone in water
= 10N – 8N
:. Weight of the displaced water = 2N
iii). What is the reason that the weight of object is less in water than in air?
Ans: The upthrust given by water is more than the upthrust given by air. So, the weight of object is less in water than in air.
55. State law’s of floatation.
Ans: Law of flotation states that ” weight of the floating body is equal to weight of the fluid displaced by the body.”
56. Why does a stone sink in water?
Ans: The density of the stone is greater than the density of water. So a stone sink in water.
| Note:
1. A body sinks in liquid if the density of a body (dj) is greater than the density of liquid (d2). 2. A body just floats on a liquid if the density of a body is equals to the density of liquid. 3. A body floats keeping some parts of it out if its density is lesser than the liquid. |
57. Three objects with different densities A, B and C are in water as shown in figure. Now answer the following questions.
(i) Which object has higher density than water? 
Ans: Object ‘C’ has higher density than water.
(ii) If the mass of object A is 1kg, how much water is displaced by the object? Ans: If the mass of object ‘A’ is 1kg, then from the law of floatation the object displaced 1kg water.
(iii) Which law is applicable for object B?
Ans: Law of floatation is applicable for the object B.
58. Why is it easier to pull a bucket of water from the well until it is inside the water but difficult out of water?
Ans: When the bucket of water is inside the well, the well water provides more upthrust so it is easier to pull but when the bucket of water is out of well water the upthrust provides by air is loss than water so it is difficult to pull the bucket of water.
59. “Only the objects having density less than water float”. Is the statement true, give reason.
Ans: The statement is not 100% true because the substance can also float on water if the substance displaces the water equal to its own weight.
60. At what condition does a body float on liquid?
Ans: When the weight of liquid displaced by a body is equal to its own weight then the body floats on liquid.
61. “The weight of displaced liquid is equal to the weight of floating object”, which principle does this statement belong to?
Ans: “The weight of displaced liquid is equal to the weight of floating object”. This principle belong to law of floatation.
62. Differentiate between Archimedes principal and law of flotation.
Ans: The differences between Archimedes principal and law of flotation are as follows:
| Archimedes Principal | Law of flotation |
| 1. It states that when a body if partially or wholly immersed into a liquid it experiences an upthrust which is equal to the weight of the liquid displaced by it. | 1. It states that weight of the floating body is equal to the fluid displaced by it. |
| 2. It is applicable for both sinking and floating objects | 2. It is applicable for floating objects only.
objects. |
63. Name any two instruments that are based on the law of floatation.
Ans: Any two instruments that are based on the law of floatation are as follows:
i) Hydrometer.
ii) Lactometer
64. Write any two conditions for a substance to float on liquid.
Ans: Any two conditions for a substance to float in liquid are as follows:
(i) When the density of a substance is less than the density of a liquid.
(ii) When the substance displaces the liquid equal to its own weight.
65. What is hydrometer? Write its types.
Ans: The device which is used to measure the specific gravity and density of liquid is called hydrometer. The types of hydrometer are as follows:
(i) Constant immersion or Nicholson hydrometer.
(ii) Constant weight hydrometer.
66. What is lactometer?
Ans: A specially designed constant weight hydrometer used to test the purity (density of milk) is called lactometer.
67. A ship coming from sea inters in the river, will it hull sink more or less in river water, why?
Ans: The hull of a ship sink more in river water because the upthrust given by river water is less than that of sea water.
68. A iron ball (nail) sink in water but a ship made of up iron floats why?
Ans: Although the iron nail has more density than that of water, it displaces the water less than its own weight, so it sinks in water. But the ship made of iron is designed in such a way that it can displace the water equal to its own weight, so it floats in water.
69. What is the difference between the floating of an empty ship and a cargo-loaded ship?
Ans: The weight of an empty ship is less than that of an cargo-loaded ship. So less part of empty ship immerses in water than that of a cargo-loaded ship while floating in water.
70. What is atmospheric pressure? Write any four importance of it.
Ans: The pressure exerted by the atmosphere air is called atmospheric pressure.
The importance of atmospheric pressure are as follows:
(i) It balances the pressure in and out of our body.
(ii) Change in atmospheric pressure causes wind and it helps to rain.
(iii) Due to atmospheric pressure a syringe can work and ink can filled in pens.
(iv) Due to atmospheric pressure air pumps and water pumps can work.
71. What is meant by standard atmospheric pressure? Write its value.
Ans: The pressure exerted by atmosphere at the sea level is called standard atmospheric pressure or normal pressure. OR,
The pressure exerted by 760mm long column of mercury at the temperature of 0°C at sea level is called standard atmospheric pressure.
The value of standard atmospheric pressure is 760 mm Hg (1.01 x 105Nm-2).
72. Write any the three units in which atmospheric pressure is expressed.
Ans: Any three units in which atmospheric pressure expressed are,
(i) Atmosphere
(ii) Torr
(iii) Bar
| Note:
i. 1 atmosphere = 1.01 x 105pa ii. 1 torr = 132.322 pa iii. 1 bar = 105 pa |
73. Which instrument is used to measure the atmospheric pressure?
Ans: Barometer is used to measure the atmospheric pressure.
74. What is barometer? Write its types.
Ans: The scientific instrument which is used to measure the atmospheric pressure is called barometer. There are two types of barometer. They are,
(i) Mercury barometer
(ii) Aneroid barometer
75. We do not feel atmospheric pressure, why?
Ans: We do not feel atmospheric pressure because the atmospheric pressure and the blood pressure inside our body are equal.
76. Nose bleeding occurs at high altitude, why?
Ans: Nose bleeding occurs at high altitude due low atmospheric pressure.
77. How does mercury barometer works?
Ans: when the atmospheric pressure increases, the mercury level rises up in the tube and When the atmospheric pressure decreases, the mercury level falls down. In this way the mercury barometer works.
78. What is torricellian vaccum? Who invented it?
Ans: The empty space above the mercury level in the mercury barometer is called torricellian vaccum. It was invented by an Italian physicist Evangelista Torricelli.
79. Name any three instruments which are based on atmospheric pressure.
Ans: Any three instruments which are based on atmospheric pressure are as follows:
(i) Syringe
(ii) Air pump
(iii) Water pump
80. What is meant by syringe?
Ans: The device which is used in medical field to inject medicine in patients body is called syringe.
81. Name the main two parts of syringe?
Ans: The main two parts of syringe are as follows:
(i) Cyllinder . (ii) Piston.
82. Name any two repetition process in which syringe works.
Ans: Syringe works by the following two repetition process.
(i) Pulling the piston
(ii). Pushing the piston
83. What is air pump? Name any three parts of it.
Ans: The device which is use to pump air in stoves, balls, toys, balloons, bicycle’s etc is called air pump. The main three parts of air pump are as follows:
(i) Piston
(ii) Cylinder
(iii) Nozzle
84. The washer in air pump is made soft by applying oil (grease) on it, why? Give any two reasons.
Ans: The washer in air pump is made soft by applying oil (grease) on it due to the following reasons:
(i) It works as the valve for the pump.
(ii) It reduces the friction while the piston slides in the cylinder.
86. What do you mean by down stroke?
Ans: When the handle of the water pump is lifted up, the piston is pushed down, it is called down stroke of the piston.
87. What do you mean by upstroke?
Ans: When the handle of the water pump is pushed down, the piston is lifted up, it is called upstroke.
88. In which principle are syringe, air pump and water pump based on?
Ans: Syringe, air pump and water pump are based on the principle of atmospheric pressure.
89. Why water pump cannot lift water higher than 10 meter from the earth surface?
Ans: A water pump cannot lift water higher than 10 meter from the earth surface because a water pump is based on the principle of atmospheric pressure. Atmospheric pressure cannot push up water up to 10 meter or higher.
90. Washer of an air pump is used with lubricants, why?
Ans: The washer works as a piston as well as valve. In flexible condition only, it can expand and contract to work properly. Also smooth lubricant also reduces friction between the piston and cylinder. So washer of an air pump is used with lubricants.
91. Differentiate between air pump and water pump.
Ans: The differences between air pump and water pump are as follows:
| Air pump | Water pump |
| It is a devices used to pump air in stoves, balloons, balls, bicycles etc. | i. It is device used to pump the water from a well and underground water. |
| ii. It has only one valve. | ii.It has two valve. |
92. Differentiate between upstroke and down-stroke.
Ans: The differences be upstroke and down stroke are as follows:
| Upstroke | Downstroke |
| i. When the handle of a water pump is pushed down, it lifts the piston up, it is called upstroke. | i. When the handle of a water pump in lifter up, the piston is pushed down, it is called down stroke. |
| ii. In this condition, water comes out from the spout. | ii. In this condition water does not came out from the spout. |
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