## FORCE & PRESSURE CLASS VIII ICSE & CBSE

__Force and Pressure__

(A) __Definition:__

1) What is turning force?

Ans. The force which can rotate a body about the axis passing through a fixed point is called theturning force.

2) What is torque?

Ans. The turning effect of a force acting on a bodyabout an axis passing through a fixed point iscalled the moment of force or torque.

3) What is pressure?

Ans. The force acting on a body per unit area is called pressure.

4) What is atmospheric pressure?

Ans. The pressure exerted by the atmospheric air at any point on the earth is called atmospheric pressure at that point.

5) What is Pascal’s law?

Ans. When pressure is applied on a liquid, it get transmitted throughout and equally in all

directions. This statement is called Pascal’s law.

(B) __Relation and unit__

1) State the relation between Moment of force and Force.

Ans. Moment of force about the point =

Force x[Perpendicular distance between the line ofaction of force and the axis of rotation].

2) State the proper units of moment of force.

Ans. S.I. unit of the moment of force =S.I. unit of force x S.I. unit of distance

=newton x metre

= N x m

= newton-metre

3) State the relation between Pressure and Force.

Ans. Pressure =

4) State the unit of pressure.

Ans. S.I. unit of pressure =

= _{2}

= Nm^{-2}

5) State another name of newton per sq. metre.

Ans. S.I. unit of pressure is newton per square metre which also called pascal (Pa).

1 Pa = 1N/m^{2}

__Short type Question:__

1) What are the effects of pressure on liquid?

Ans. Effects of pressure on liquid are: –

- a) Pressure at a point in a liquid increase with depth.
- b) Pressure of a liquid is the same in all the directions at the same depth.
- c) Pressure of a liquid does not depend upon the size and shape of the container.

2) What is the effect of pressure on the volume of a gas?

Ans. When the volume of a gas is reduced the pressureexerted by the gas increase. When the volume of a gas is increased, the pressure exerted by the gasdecreases.

3) What is barometer? Who made it?

Ans. Our present day knowledge about atmospheric pressure is based on the experimentsconducted by E. Torricelli (1608 – 1647). Torricellimade a device to measure atmospheric pressure which is known as simple barometer or mercurybarometer.

4) What is one atmospheric pressure?

Ans. Pressure exerted by 76cm high column of mercuryis called one atmospheric pressure.

5) What is the meaning of 760mm?

Ans. A simple barometer measures atmospheric pressure in terms of height of mercury column. At

sea level, the height of mercury column in a simple barometer is 760mm or 76cm.

__GIVE REASON__

1) School bags has wide straps made of thick canvas –justify

Ans. Because to distribute the entire weight of the bag over a larger area. This in turn produces a small pressure.

2) Knives are ground to a thin edge – justify

Ans. Because thin edges decrease the area and pressure will be increased when a force is applied on it.

3) The tip of a needle is made sharp – justify

Ans. The tip of a needle is made sharp so that the force applied on its acts on a small area. This increases the pressure on it and hence it can easily pierce the cloth.

4) Wooden or concrete sleepers are placed below the rail lines – justify

Ans. Because this is done to distribute the load over a large area thereby avoiding the sinking of the rail lines.

5) A truck or a motor bus has wide tyres – justify.

Ans. Because the tyres of vehicles designed to carry heavy load are wide as compared to those of cycles and bikes. To reduce pressure on the ground. Hence, heavy vehicles can move easily.

__NUMARICAL ____PROBLEMS:__

1) Which will exert more pressure- 100kg mass on 10m^{2 }or 50kg mass on 4m^{2}? Give reason.

Ans. According to the Question: [g – 10m/8^{2} taken]

P_{1} = (F_{1} = m_{1} X g = 100X10N; A_{1}= 10m^{2})

So P_{1 }= = 100 pa.

P_{2} = (F_{2}= m_{2}Xg = 50X10N ; A_{2} = 4m^{2})

So P_{2} = = 125 Pa

P_{2 }> P_{1} so 50 kg mass on 4m^{2 }exerts more pressure.

2) Two iron nails are used to penetrate the skin. One nail is having 5m^{2} area with 20N Force. Another iron nail is 10m^{2} area with 30N Force. Calculate the pressure with which iron nails are penetrating in the skin.

Ans. According to the Question

P_{1}= = [F_{1} = 20 N ; A_{1} = 5m^{2}] = = 4 Pa

P_{2 = } = [F_{2} = 30N ; A_{2}=10m^{2}] = = 3 Pa

P_{1}> P_{2 }so iron nail with pointed end having 5m^{2} area gives more pressure in the skin.

3) The door of a building is 1m wide. It can be opened by applying a force of 10N at the middle of the door. Calculate the least force which can open the door.

__Ans.__ As the door is opened by applying force at its middle so 1 moment of the force = 10N x m = 5Nm.

The least force which can open the door is applied at the open end of the door.

If F is the force needed here, then F x 1m = 5Nm

So F = = 5N.

Thus, a force of 5 N is the least force required to open the door.

4) Moment of force of 20N about a point x is 2Nm. Calculate the distance of point of application of theforce from the point x.

Ans. Moment of the force = force x Distance of point of application

= 2 Nm

Force= 20N

So, Distance of point of application =

= = 0.1m.

So the Distance of point of application of the force from the point x is 0.1 m.

5) A force of 50N acts on a body of 10cm^{2}Cross-Section. Calculate the pressure exerted by the body.

Ans. Force (F) = 50N

Area of Cross-Section = 10cm^{2}

= 10 x )^{2 }m^{2 }

^{ }= 10^{-3}m^{2}

Thus, Pressure exerted by the body =

= = (50 x 1000) N/m^{2}

= 50,000 Pa

So the pressure exerted by the body = 50,000 Pa.