SEE Science (Pressure) Notes
Pressure
1. “Only the object having the density less than water floats.” Is the statement true? Why or why not? Give reasons.
Ans: “Only the object having the density less than water floats.” This statement is false because object having more density can also float in water if it is provided with appropriate shape. For Instant: Iron has more density than water but the sheep made from iron does not sink and float easily in water.
Ans: “Only the object having the density less than water floats.” This statement is false because object having more density can also float in water if it is provided with appropriate shape. For Instant: Iron has more density than water but the sheep made from iron does not sink and float easily in water.
2. What is difference between the immersion of hydrometer in pure water and salty water?
Ans: When hydrometer is immersed in pure water and salty water, more portion of hydrometer immerses in pure water whereas less portion of hydrometer immerses in salty water because the density of salty water is greater than pure water as a result it experience more upthrust.
3. State law of flotation and name the instrument based on it.
Ans: It state that, "For a body to float, the weight of the floating object should be equal to the weight of the liquid by it."
Hydrometer is the instrument based on it.
4. Define pressure.
Ans: The force acting normally per unit area of the surface is called pressure. Its S.I. unit is N/m2 or Pascal (Pa).
5. Prove the relation of pressure exerted by an object with its surface area the applied force.
Ans: Let us suppose, force 'F' is applied normally to an area 'A' and pressure 'P' is produced which is shown in the figure.
We know that, By the definition of pressure 
Pressure is directly proportional to force applied.
i.e. P α F................. (i)
Pressure is inversely proportional to the area on which force acts.
i.e. P α 1/A .............. (ii)
Combining (i) and (ii), we get
P α F/A
Or, P = kF/A (where k is constant)
If 1 N force is applied in 1 m2 then pressure will be 1 N/m2and thus k = 1
Or, p = 1*F/A
... p = F/A proved
6. By which two properties of liquid is it used in hydraulic break?
Ans: The two properties of liquid that is used in hydraulic break are as follow:-
(i) Liquid transmit pressure equally in all the directions.
(ii) Liquid cannot be compressed.
7. What is hydrometer?
Ans: A device which is used to measure the specific gravity and density of liquids is called hydrometer.
8. Name the device which works on the basis of Pascal’s Law.
Ans: The device which works on the basis of Pascal’s Law are:
a) Hydraulic Brakes
b) Hydraulic Press
c) Hydraulic Garage Lift
9. "The weight of liquid displaced is equal to the weight of floating object." Which principle does this statement belong to?
Ans: The above statement belongs to 'Law of Flotation'.
10. What difference will you get from the flying of air filled balloon and hydrogen filled balloon?
Ans: The density of hydrogen is less than that of the air. So, the balloon filled with hydrogen rises up faster than the balloon filled with air.
11. One feels difficult to breathe in the deep well while cleaning it, why?
Ans: Carbondioxiode is the densest gas. It is heavier than air. In the well, Carbondioxiode displaces the air present inside the well and only Carbondioxiode gas is collected at the bottom. So, there is lack of oxygen. Hence the person feels difficult to breathe while cleaning the deep well.
12. Write any two differences between density and relative density.
S. N
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Density
|
S.N
|
Relative density
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a)
|
It is the mass per volume of the substance.
|
a)
|
It is the ratio of the density of the substance to the density of water at 40C.
|
b)
|
Its S.I. unit is Kg/m3.
|
b)
|
It has no unit.
|
13. Why a stone does sinks in water?
Ans: The density of stone is more than that of water. So, the stone cannot displace the water equal to its own weight. As a result, a stone sinks in water.
14. What is the relation between the density of liquid and the upthrust exerted by the liquid?
Ans: Upthrust exerted by the liquid is directly proportional to the density of liquid.
i.e. U = vdg
Or, U α d
Hence, upthrust increase with increase in density and vice versa.
15. Write the relation between upthrust and volume of displaced liquid.
Ans: Upthrust is directly proportional to the volume of displaced liquid.
i.e. U = vdg
Or, U α v
Hence, upthrust increase with increase in volume and vice versa.
16. Lifting a stone in water is felt to be comparatively lighter, why?
Ans: Lifting a stone in water is felt to be comparatively lighter because when a stone immersed in water, the water exerts an upward force called upthrust on a stone. Due to this upthrust the downward force on a stone reduced. Thus, apparent weight of the stone is far less than its real weight.
17. What is hydraulic press?
Ans: A hydraulic press is a simple machine which convert small force into large force and vice versa.
18. Prove: P = hdg
Solution:
Let, p = pressure of liquid 
d = density of liquid
h = depth of liquid column
g = acceleration due to gravity
And A = cross sectional area of container
We know that, By the definition of pressure
p = F/A
or, p = mg/A (F = mg)
or, p = d*v*g/A (m = d×v)
or, p = d*A*h*g/A (v = A×h)
p = hdg proved
19. What is relative density?
Ans: The ratio of the density of the substance to the density of water at 40C is called relative density.
Ans: The ratio of the density of the substance to the density of water at 40C is called relative density.
20. What number will be shown by hydrometer if sunk in pure water?
Ans: 1000kg/m3number will be shown by hydrometer if sunk in pure water.
21. On what factors liquid pressure depends?
Ans: The factor on which liquid pressure depends is:
a) The density (d) of liquid
b) The depth (h) of liquid column
c) The value of acceleration due to gravity (g).
22. When a ball is pressed in water, why does it try to come up?
Ans: When a ball is pressed in water, the water exerts an upward force called upthrust on it. Due to this upthrust the downward force on the ball is reduced. Therefore, a ball tries to come up when it is pressed in water.
23. If a hydrometer is immersed in sugar solution and in pure water, in which it immersed more, why?
Ans: The hydrometer immersed more in pure water than in sugar solution because as we know that upthrust is directly proportional to the density of the liquid (i.e. U d). The density of sugar solution is more than that of pure water.
24. What is density?
Ans: The mass per volume of the substance is called density.
25. Write the formula based on Pascal’s Law.
Ans: The formula based on Pascal’s Law is F1 / A1 = F2 / A2
26. An iron ball sinks in water but ship made up of iron floats on water, why?
Ans: If the weight of the water displaced by a sinking body is greater than the weight of the body, the body floats on water. Otherwise, it sinks. In case of iron piece, the weight of the displaced liquid by it is less than its weight. Hence, it sinks in water. But in the case of ship made of iron, it is designed in such a way that the weight of liquid displaced by it is greater than its weight. Hence it floats on water. Therefore, an iron ball sinks in water but ship made up of iron floats on water.
27. When a bucket full of water is pulled up from a well, in which condition either inside or outside the water it is difficult to lift up and why?
Ans: When a bucket full of water is pulled up from a well, it become difficult to lift up when the bucket is outside the water because the upthrust provided by air is very less than the upthrust provided by water in the well.
28. Write any two differences between force and pressure.
S.N.
|
Force
|
S.N.
|
Pressure
|
a)
|
Its S.I. unit is Newton (N).
|
a)
|
Its S.I. unit is Pascal or Nm-2.
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b)
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It is vector quantity.
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b)
|
It is scalar quantity.
|
29. State Archimedes principle.
Ans: It states that, "When a body is partially or wholly immersed in a liquid, it experiences an upthrust which is equal to the weight of the liquid displaced by it."
30. At what condition a body floats on liquid?
Ans: A body floats on a liquid when the weight of liquid displaced by it is equal to its own weight.
31. It is easier to swim on the sea than on the pond, why?
Ans: As we know that upthrust is directly proportional to the density of the liquid (i.e. U d). The density of sea water is more than that of pond water. Due to this, when a man swims, the upthrust acting on him due to sea water is greater than the upthrust acting on him due to the pond water. Because of this reason, it is easier to swim on the sea than on the pond.
32. State Pascal's law.
Ans: It states that, "When pressure is applied on a liquid enclosed in a vessel, it is transmitted equally in all the directions."
33. Weight of an object is decrease when immersed in water, which law does this statement represent? How much weight will an object lose in water?
Ans: Weight of an object is decrease when immersed in water. The above statement belongs to Archimedes' Principle. This is because of upthrust given by water to that object.
The weight lost by an object in water is equal to the upthrust given by water or the weight of the water displaced by that object.
34. Lower portion of dam is made stronger than upper portion, why?
Ans: As we know that, pressure is directly proportional to the depth of liquid (i.e. p α h). If the bases of dame are made wide, the pressure exerted by the dam on the ground will be small. As a result, the dam is avoided from descending into the ground. In addition to this, the dame has to withstand high pressure due to the stored water in the pond. For these reason, lower portion of dam is made stronger than upper portion.
35. Prove: p α h.
Solution:
Let, p = pressure of liquid
d = density of liquid
h = depth of liquid column
g = acceleration due to gravity
And A = cross sectional area of container
We know that, by the definition of pressure
p = F/A
or, p = mg/A (F = mg)
or, p = d*v*g/A (m = d×v)
or, p = d*A*h*g/A (v = A×h)
p = hdg
For a given liquid at a given place density of the liquid and acceleration due to gravity are constant.
... P α h
36. What is the role of water in hydraulic press to multiply small force into large one?
Ans: Water plays a great role to transmit the pressure equally, exerted by the small piston, perpendicularly to all sides of the closed container. As a result, force is multiplied in the larger piston.
37. Define one Pascal pressure.
Ans: If 1N force is applied in 1m2 area then the pressure is said to be one Pascal.
Ans: If 1N force is applied in 1m2 area then the pressure is said to be one Pascal.
38. Mention the use of hydrometer.
Ans: The uses of hydrometer are:
a) It is used to measure the specific gravity and density of liquids.
b) Lactometer is a type of hydrometer used to measure the purity of milk.
Ans: The uses of hydrometer are:
a) It is used to measure the specific gravity and density of liquids.
b) Lactometer is a type of hydrometer used to measure the purity of milk.
39. Why does hydrometer have heavy bulb and narrow stem?
Ans: Hydrometer has heavy bulb and narrow stem because heavy bulb helps hydrometer to float in a liquid vertically and narrow stem provide greatest sensitive on reading.
40. An egg sinks in pure water but floats on salty water, why?
Ans: As we know that, upthrust is directly proportional to the density of the liquid (i.e. U d). The density of salty water is more than that of pure water. So, the weight of the pure water displaced by the egg is less than the weight of the egg. Hence, the egg sinks in pure water. But the weight of the salt water displaced by the egg is greater than the weight of the egg. Hence, the egg floats in salty water. Therefore, an egg sinks in pure water but floats on salty water.
41. Why it is easier to pull the bucket of water from the well until it is inside the water but difficult out of water?
Ans: When a bucket remains inside the water, it experiences an upthrust due to water. So, it is easier to pull the bucket of water from the well until it is inside the water. But it difficult to lift up when the bucket is outside the water because the upthrust provided by air is very less than the upthrust provided by water in the well.
42. The weight of any object decreases inside water. Give reason.
Ans: When an object immersed in water, the water exert an upward force called upthrust on the body. Due to this upthrust the downward force on the body reduced. Therefore, the weights of any object decreases inside water.
43. A ship coming from the sea enters the river, will it hull sinks more or less in river water? Give reason.
Ans: The hull of the ship sinks more in river water. As we know that, upthrust is directly proportional to the density of the liquid (i.e. U d). The density of river water is less than that of sea water. So, the upthrust provided by the river water is less than the upthrust provided by the sea water.
44. A hole at the bottom of the boat is more dangerous than that at the side, why?
Ans: The bottom of the boat exert more pressure, as pressure is directly proportional to the depth of liquid (i.e. p h), than that at the sides. If there is hole at the bottom of the boat then liquid enters with large pressure inside the ship but at the top it enters slowly. Therefore, a hole at the bottom of the boat is more dangerous than that at the side.
45. Name the device and their uses which work on the basis of Pascal’s Law.
Ans: The device which works on the basis of Pascal’s Law are:
(a) Hydraulic Brakes
A hydraulic break is a breaking system which is used in stopping four wheelers and other heavy automobiles.
Uses:
(1) It is used to stop heavy automobiles like trucks, buses, aeroplanes etc. by applying a small force.
(b) Hydraulic Press
A hydraulic press is a simple machine which convert small force into large force and vice versa.
Uses:
(1) It is used for compressing cotton goods, paper, etc. in industries.
(2) It is used for extracting oil from oil seeds.
(c) Hydraulic Garage Lift
Hydraulic Garage Lift is a machine used to lift light vehicles in service stations during their services,
Uses:
(1) It is used for lifting automobiles like car, bike, bus, etc. in service station.
46. If an object immersed in water at what condition it can sinks on water and at what condition it can floats on water?
Ans: If the density of an object is greater than the density of the liquid, the body completely sinks in the liquid and lies on the bottom of the container.
If the density of an object is equal to the density of the liquid, the body completely immersed but remains floating in the body of the liquid.
If the density of an object is less than the density of the liquid, only a part of the body remains inside the liquid. The
portion of the body which lies inside the liquid displaces the liquid equal to the weight of the body.
If the density of an object is less than the density of the liquid, only a part of the body remains inside the liquid. The
portion of the body which lies inside the liquid displaces the liquid equal to the weight of the body.
47. The ice made of water floats on water, why?
Ans: The sinking and floating of a substance depends on density of a substance and density of a liquid. If the density of substance is less than the density of liquid then substance floats on liquid. The density of ice is slightly less than the density of water. Therefore the ice made of water floats on water.
48. The different weight of a piece of stone weighing in three different media air, water and solution of common salt with water are given in the table below. Answer the following question.
Medium
A
B
C
|
Weight
15N
20N
17N |
a) Which should be water and which one the solution of common salt with water out of three medium A, B and C.
Ans: Medium C should be water and medium A should be the solution of common salt with water out of three medium A, B and C.
b) If the weight of 1Kg of mass in air is 10N, find out the mass of the piece of stone?
Ans: Here given,
Weight of stone in air (Wa) = 20N
Stone of weight 10N has mass = 10kg
Stone of weight 1N has mass = (1/10) kg
Stone of weight 20N has mass = (1/10) 20 = 2kg
c) Find out the mass of water displace by the piece of stone?
Ans: Here given,
Weight of stone in air (Wa) = 20N
Weight of stone in water (Ww) = 17N
Mass of water displaced by stone =?
Now,
Weight of water displaced by stone = Wa - Ww
= 20-17
= 3N
... Mass of water displaced by stone = weight of displaced water/acceleration due to gravity
= 3/10
= 0.3 kg = 300gm
49. If two water tanks, one (A) having larger base area than the other (B), carry equal volume of water, explain whose bottom experience more pressure.
Ans: In the given diagrams, both tanks 'A' and 'B' contain equal volume of water. The height of liquid column in tank 'B' is more than that of tank 'A' because of less cross-sectional area. Since the liquid pressure is directly proportional to the height of liquid column, the base of tank 'B' experiences more pressure than the base of tank 'A'.
50. In the given figure, on which surface more pressure is exerted and why?
Ans: In the given diagram, both surfaces A and B experience same pressure because the height of the liquid is equal in both container.
51. The speed of flow of water out of a tap of upper floor is less than that of the down floor. Why?
Ans: We know that the liquid pressure is directly proportional to the height of the liquid column. The height of the liquid column on the down floor is more than that on the upper floor. So, the speed of flow of water out of a tap of upper floor is less due to less pressure and that in the lower floor is more due to more pressure of water.
52. The gravity bulb of hydrometer is made heavier.
Ans: The gravity bulb of hydrometer is made heavier so that its center of gravity remains very low, when it is floating in denser liquids.It helps the hydrometer to float upright in stable equilibrium.
53. A hydrometer is once put into fresh water and then it is put into salt water. Which one is fresh water in between 'A' and 'B' ? Justify your answer.
Ans: Out of two container A and B, container A contains fresh water because more portion of hydrometer has immersed in the water of the container A. As we know that, upthrust is directly proportional to density of liquid and fresh water has less density than that of salt water. So, it will experienced less upthrust in fresh water.
54. The size of air bubble becomes bigger as it rises up in the water, why?
Ans: We know that the liquid pressure is directly proportional to the depth of the liquid from the free surface. The liquid pressure is more at the bottom of the water than its surface. Due to the more pressure at the bottom of the water, the size of air bubbles is small than that in the surface of the water. So, the size of air bubble becomes bigger as it rises up in the water due to decrease in water pressure.
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