# Pressure

ACTIVITY 2.1
1.

Since the area in (d) is less than that of (c), and as pressure is inversely proportional to the area, the foam is more pressed in (d) than in (c).

2.

The pressure increased in (d) rather than in (c), because the area covered in (d) is less in comparison to area covered in (c) and we know that pressure is inversely proportional to the area.
3. Since pressure is inversely proportional to the area, and in (e) as the bricks are overlapped, it covers more area in (e) than in (c), due to which pressure is more in (c) than in (e).

ACTIVITY 2.4

1. The difference in the mass of the stone after having the stone dipped into water is that the weight of the stone becomes less than its original weight. This is due to upthrust. And also the upthrust is equal to the weight of the displaced liquid.

ACTIVITY 2.7

1. The difference in the floatation of wooden cork and rubber cork is that the wooden cork floats with more of its part above the liquid whereas the rubber cork will float with less of its part above the liquid. This is because of the difference in the densities of the wooden cork and the rubber cork.

ACTIVITY 2.8

1. If the density of the liquid is more than the density of ice, plastic and iron, then they float.

2. Kerosene floats on water whereas mercury does not float.

EXERCISE

1.
Pressure is defined as the force acting per unit area. Its S.I. unit is Pascal (N/m2)

The factors affecting pressure are (i) force and (ii) area.

2.
Pressure of a liquid depends on (i) height, (ii) density and (iii) acceleration due to gravity

We know, Pressure (P) = Force (F)/Area(A)
Or, P= (Mass (m) x acceleration due to gravity (g))/Area (A)
Or, P=(Density (d) x Volume (v) x g)/Area (A)
Or, P= (d x g x Area (A) x Height (h))/Area (A)
Or, P= hdg.Area (A)

3.
Pascal's Law states that," When pressure is applied on liquid contained in an enclosed vessel, the pressure is transmitted equally and perpendicularly in all directions."
For example, when we fill a plastic bag with liquid and the opening is closed and holes are made in it and pressure is exerted, then the liquid comes out from the holes in equal pressure.

4.
Archimedes Principle states that."When an object is wholly or partially immersed into a liquid, the weight of the liquid displaced is equal to the upthrust of the liquid on the object." 5.
Law of floatation states that, "The weight of the floating body is equal to the weight of the liquid displaced by the body."
The density of liquid is directly proportional to the floatation of the object. It means that if the density of liquid is more than the object, then the object floats whereas if the density of liquid is less than that of the object, then the object will sink.

6.

a.

Force

Pressure

It is an external energy that changes or tries to change the state of motion to rest or rest to motion.

It is defined as the force acting per unit area.

It is a vector quantity.

It is scalar quantity

Its S.I. unit is Newton(N)

Its S.I. unit is Pascal (Pa)

It is measured by spring balance.

It is measured by manometer, pressure gauge etc.

b.

Archimedes Principle

Law of floatation

Archimedes Principle states that."When an object is wholly or partially immersed into a liquid, the weight of the liquid displaced is equal to the upthrust of the liquid on the object."

Law of floatation states that, "The weight of the floating body is equal to the weight of the liquid displaced."

It is used to calculate the upthrust or apparent weight of an object.

It is used for the construction of hydrometer, ships etc.

7.

a.
The studs have smaller area due to which pressure is more and it help players to run easily without slipping. Thus the studs helps player to run without losing their balance, so they are used in the shoes.

b.
As we know that, liquid pressure is directly proportional to the depth of the liquid. So, the bucket at more depth fills faster than the bucket at less depth. Thus, the bucket in lower floor gets filled faster than that on the top floor.

c.
Inside the water, the upthrust helps in lifting the bucket but when it is outside the water, the upthrust is absent due to which it becomes heavier and difficult to lift it. So, it is easier to lift the bucket inside the water than outside the water.

d.
The nail made of iron sinks in water as its weight is more than the weight of water displaced by it or upthrust applied on it. But ship made up of iron is hollow and thus its density is less than that of water. Because of this, it can displace water equal to its own weight which causes ship to float.

8.
In order to float on water, we need to displace the water equal to our weight.

9.
Solution,
Density of the block (d1) = 800 kg/m3
Density of water (d2) = 1000 kg/m3
Volume of ice (V1) = 4 x 5 x 8 = 160 cm3
Now, Mass = d1 x V1
= (800 x 160)/(100 x 100 x 100) = 0.128 kg
Mass of water to be displaced = Mass of ice = 0.128 kg
Then, Volume of displaced water = Mass / Density = 0.128/1000 = 0.000128 m3 = 128cm3
The part of ice immersed in water = Volume of water displaced/Volume of ice = 128/160 = 4/5 parts.
Thus, parts immersed in water is 4/5 parts.

10.
Solution,
Density of ice (d1) = 0.92 gm/cm3
Volume of ice (v1) = 50 x 30 x 20 = 30000 cm3 Now,
Mass of ice (m1) = d1 x v1 = 0.92 x 30000 = 27600 gm
We know,
Amount of water displaced = mass of ice = 27600 gm

11.
Solution,
Here,
Pressure = height x density x acceleration due to gravity
= 1 x 1000 x 9.8 [density of water = 1000 kg/m3]
= 9.8 x 103 Pa
Thus, the pressure exerted is 9.8 x 103Pa.

12.
Solution,
Here,
Pressure = height x density x acceleration due to gravity
=1.5 x 1000 x 9.8 [density of water = 1000 kg/m3]
= 1.47x 104 Pa
Thus, the pressure exerted is 1.47x 104 Pa

13.
Solution,
Height = 76 cm = 0.76 m
Density = 13.6 gm/cm3 = 13.6 x 10-3 x 106 =13600 kg/m3
Acceleration due to gravity = 9.8 m/s2
Now,
Pressure = height x density x acceleration due to gravity
= 0.76 x 13600 x 9.8
= 1.013 x 105 Pa
Thus, the pressure exerted is 1.013 x 105 Pa

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