## ICSE Living Science Physics for Class 8 Solutions Chapter 3 Force and Pressure

Check Your Progress

A. Answer the following.

Question 1.

Define moment of a force. Write its SI unit.

Answer:

The turning effect of a force about a fixed point (pivot) is called the moment of the force. Its SI unit is N m

(newton metre).

Question 2.

It is easier to open a door by applying the force at its handle than at the hinged end. Why?

Answer:

It is easier to open a door by applying the force at its handle than at the hinged end because the moment of force is large when the force is applied at the handle as the moment arm is larger as compared to when the force is applied at the hinged end.

Question 3.

Write any two applications of moment of force.

Answer:

The twð applications of moment of force are :

a. The hand flour grinder is provided with a handle near its rim to increase the moment arm so that even a small force applied can produce a. large turning effect.

b. A long spanner is used to loosen a tight nut. This is because by increasing the length, of the spanner, the moment arm gets increased and even a small force can produce a large turning effect.

B. Solve the following numericals.

Question 1.

A nut is opened by a spanner of length 30 cm. If the applied force is 20 N, find the moment of force needed to loosen the nut.

Answer:

Here,

Force (F) = 120 N

Moment arm (d) = 30 cm = \(\frac {30}{100}\) = 0.3 m

Moment of force = F X d

= (120 N) x (0.3 m) = 36 N m

So, the moment of force needed to loosen the nut is 36N m.

Question 2.

A force of 250 N produces a moment of force of 500 N m. Calculate the moment arm.

Answer:

Here,

Force (F) = 250 N

Moment of force 500 N m

Moment arm (d) = ?

By using the formula,

Moment of force = F X d

we get,

500 N m = 250 N x d

d = \(\frac {500Nm}{250N}\) = 2m

Question 3.

Calculate the force which will produce a moment of 60 N m when the perpendicular distance between the point of application of force and the fixed point is 60 cm.

Answer:

Here,

Moment of force = 60 N m

Moment arm (a) = 60 cm = \(\frac {60}{100}\)m = 0.6 m

By using the formula,

Moment of force F x d

we get,

60Nm = F x 0.6m

F= \(\frac {60 Nm}{0.6 m}\) = 100N

Check Your Progress

A. Fill in the blanks.

- Smaller the area on which a force acts, …………. is the effect of force.
- Pressure exerted on a surface depends upon ………… and ………….
- The SI unit of pressure is …………
- If the thrust acting on a surface decreases, then the pressure …………

Answer:

- Smaller the area on which a force acts, greater is the effect of force.
- Pressure exerted on a surface depends upon thrust and area of contact.
- The SI unit of pressure is pascal (Pa).
- If the thrust acting on a surface decreases, then the pressure decreases.

B. Answer the following.

Question 1.

If the magnitude of force acting on a surface is doubled and area over which it acts is halved, then

what will be the effect on pressure?

Answer:

When,

magnitude-of force = F, and

area over which the force acts = A

then, the pressure (P) is given by

P = \(\frac {F}{A}\)

Now, if the force is doubled (2F) and area over is halved (A/2), then the pressure is given by –

Pressue =\(\frac {2F}{A/2}\)= \(\frac {4F}{A}\) = AP

So, when the magnitude of force acting on a surface is doubled and area over which it acts ¡s halved, then the pressure becomes 4 times.

Question 2.

Why do school bags have broad straps?

Answer:

School bags have broad straps to reduce the pressure on our shoulders.

A. Tick the most appropriate answer.

Question 1.

The turning effect of a force is known as

a. work.

b. energy.

c. moment.

c. pressure.

Answer:

c. moment

Question 2.

What is the pressure exerted by a body of weight 200 N acting on an area of 5 m^{2}?

a. 50 Pa

b. 200 Pa

c. 40 Pa

d. 0.02 Pa

Answer:

c. 40 Pa

Question 3.

What is the value of one pascal?

a. 1 N^{2}/m^{2}

b. 1 N/m^{2}

c. 1 N^{2}/m

d. 1 N/rn

Answer:

b. 1 N/m^{2}

Question 4.

Army tanks are provided with caterpillar tracks under their wheels in order to

a. decrease pressure on the ground

b. increase pressure on the ground

c. make them look different

d. prevent them from damage.

Answer:

a. decrease pressure on the ground

Question 5.

Pressure ¡n liquid’s is measured by an instrument called

a. barometer

b. hydrometer

c. odometer

d. manometer

Answer:

d. manometer

Question 6.

The value of atmospheric pressure at sea level in SI system is approximately equal to

a. 1000 Pa.

b. 10000 Pa.

c. 100000 Pa.

d. 10000000 Pa.

Answer:

c. 100000 Pa

Question 7.

At sea level, the height of the mercury column in a barometer is

a. 72 cm.

b. 76 cm.

c. 82 cm.

d. 98 cm.

Answer:

b. 76 cm.

B. Fill in the blanks.

- The SI unit of moment of force is ………..
- Pressure is directly proportional to …………
- If the force acting on an area ¡s increased, pressure will ………
- The straps and handles of travel bags are made broad to ………. the pressure on our shoulders.
- An object exerts a pressure of 2 Pa on the ground. If another object of the same weight is kept above it, then the net pressure will become …………..
- The pressure exerted by a liquid at a given depth of the liquid …………. is in all directions.
- If the height above the sea level increases, atmospheric pressure will …………

Answer:

- The SI unit of moment of force is N m.
- Pressure is directly proportional to thrust.
- If the force acting on an area is increased, pressure will increase.
- The straps and handles of travel bags are made broad to reduce the pressure on our shoulders.
- An object exerts a pressure of 2 Pa on the ground. If another object of the same weight is kept above it, then the net pressure will become 4 Pa.
- The pressure exerted by a liquid at a given depth of the liquid same is in all directions.
- If the height above the sea level increases, atmospheric pressure will decrease.

C. Write true or false. Correct the false statements.

Question 1.

The perpendicular distance from the pivot of a rotating body to the line of action of the force is known as the moment arm.

Answer:

True.

Question 2.

Pressure is inversely proportional to the force applied.

Answer:

False.

Correct : Pressure is directly proportional to the force applied.

Question 3.

Wooden sleepers are placed under railway tracks in order to increase pressure on the ground.

Answer:

False.

Correct : Wooden sleepers are placed under railway tracks to reduce the pressure on the ground.

Question 4.

Pressure is independent of the roughness of the surfaces in contact.

Answer:

True.

Question 5.

Skis have a large surface in order to reduce the pressure on snow.

Answer:

True.

Question 6.

Liquids seek their own level regardless of the shape and cross sectional area of the vessels they are kept

in.

Answer:

True.

Question 7.

The standard value of atmospheric pressure is about 1,50,000 Pa.

Answer:

False.

Correct : The standard value of atmospheric pressure is about 101325 Pa.

D. Answer the following in short.

Question 1.

Name the factors affecting the moment of a force.

Answer:

The factors affecting the moment of force are magnitude of the force applied and the moment arm.

Question 2.

The hand flour grinder ¡s provided with a handle near its rim. Give reasons why.

Answer:

The hand flour grinder is provided with a handle near its rim to increase the moment arm so that even a small force applied can produce a large turning effect.

Question 3.

How are force, area and pressure related to each other?

Answer:

Force, area and pressure are related by the following expression.

If the force applied is doubled for the same area, then the pressure will also double.

Question 4.

Why do safety pins and drawing pins have narrow and sharp tips?

Answer:

Safety pins and drawing pins have narrow and sharp tips because the smaller surface area of the tip causes a high pressure and, therefore, makes it easier for these objects to pierce through a given material.

Question 5.

What do you understand by atmospheric pressure? How does it change with altitude?

Answer:

The force exerted by the atmosphere per unit area of the surface of earth is called atmospheric pressure. It decreases with increase in altitude.

Question 6.

Why is water not used as a barometric liquid?

Answer:

Water is not used as a barometric fluid as it has less density due to which it will have to rise to a height of 10.5 m to balance the atmospheric pressure. Moreover, it vaporizes in vacuum and thus, never records the true atmospheric pressure.

E. Answer the following in detail.

Question 1.

Differentiate between the clockwise and anticlockwlse moments.

Answer;

If the force has a tendency to turn or rotate a body in clockwise direction, the moment of force is called clockwise moment. If the force has a tendency to turn or rotate a body in anticlockwise direction, the moment of force is called an anticlockwise moment.

Question 2.

Define pressure. Write the factors on which the pressure exerted by a body depends.

Answer:

Pressure is defined as the amount of force acting per unit area. It can be mathematically expressed as

It depends on the following two factors – magnitude of force and the area of contact.

Pressure increases if the magnitude of force (thrust) increases, or the area of contact decreases.

Pressure decreases if the magnitude of force (thrust) decreases, or the area of contact increases,

Question 3.

Where is pressure of the sea water greater : 5 m below the surface of the sea or 10 m below the surface of the sea? Why?

Answer:

Pressure ¡s greater at 10 m below the surface of the sea as compared to that at 5 m below the surface of the sea. This is because pressure exerted by a liquid

increases with depth.

Question 4.

Why do the pillars of a bridge have a broader base than the top?

Answer:

Pillars of a bridge have a broader base than the top. This is done to reduce the pressure exerted on the ground by the enormous weight of the bridge. As pressure is inversely proportional to the area of contact, so if the base is broader, the area of the contact increases, reducing the pressure on the ground due to the weight of

the bridge.

Question 5.

List any three characteristics of liquid pressure.

Answer:

The characteristics of liquid pressure are given as follows.

- Liquids exert pressure on the walls of the container.
- The pressure exerted by a liquid is directly proportional to its density. More the density of a liquid, greater is the pressure exerted by it.
- Pressure exerted by a liquid increases with depth.

Question 6.

Describe the function of a simple mercury barometer. State its disadvantages.

Answer:

A simple mercury barometer is used to measure atmospheric pressure. Some of its disadvantages are listed below.

- The barometer is not compact, and thus, not portable.
- The level of mercury in the open trough increases or decreases with change in atmospheric pressure. Hence, a
- permanent scale cannot be marked on the glass tube.
- The trough is open and there are chances of impurities getting mixed with the mercury in the trough.

F. Solve the following numerical problems.

Question 1.

A force of 200 N acts on a rigid body such that the perpendicular distance between the force and the turning point is 50 cm. Calculate the moment of force.

Answer:

Force (F) = 200 N

Moment arm (d) = 50 cm = \(\frac {50}{100}\)= 0.5 m

Moment of force = F x d

= (200 N) x (0.5 m) = l00 N m

Question 2.

Calculate the force applied on a body producing a moment of force of 16 N m such that the perpendicular distance between the point of application of force and the turning point is 80 cm.

Answer:

Moment of force = 16 N m

Moment arm (d) = 80 cm = \(\frac {80}{100}\)m = 0.8 m

Force (F) = ?

From the formula,

Moment of force = F x d

we get,

l6 N m = F x 0.8 m

F= \(\frac {16Nm}{0.8m}\) = 20N

Question 3.

A 40 kg boy sits 1.2 m from the fulcrum of a see saw. Where should another boy of 30 kg sit on the other side ¡n order to balance it ?

Answer:

The see-saw will be balanced when the moment of force of both the boys is equal.

For 40 kg boy,

Weight, F = 40 kg x 10 m/s^{2} = 400 N

Moment arm, d = 1.2 m

Moment of force = F X d = 400 N x 1.2 m = 480 N m

For 30 kg boy,

Weight,F = 30kg x 10m/s^{2}= 300N

Moment of force = 480 N m

Moment arm, 1.6 m

The 30-kg boy should sit at a distance of 1.6 m from the fulcrum of the see-saw.

Question 4.

The tip of a drawing pin has an area of 0.01 mm^{2} Find the pressure exerted by it if a force of 20 N is

applied by it.

Answer:

Force exerted by the tip of a drawing pin, F = 20 N

Area of the tip of the drawing pin, A = 0.01 mm^{2}

\(\frac{0.01}{1000 \times 1000}\)m^{2}

Pressure exerted by the tip of the pin,

P = \(\frac {F}{A}\) = \(\frac{20 \mathrm{~N}}{0.01 /(1000 \times 1000) \mathrm{m}^{2}}\)

= 2,00,00,00,000 Pa

Question 5.

A motorcycle weighs 1200 N. If the pressure exerted by one of its types is 120 Pa, then find the area of the type in contact with the road.

Answer:

Weight of the motorcycle = 1200 N

Force exerted by the motorcycle on the road, F = Weight

of the motorcycle = 1200 N

Pressure exerted by one of its tyres, P = 120 Pa

Area of the tyre in contact with the road, A = ?

From the formula, P = \(\frac {F}{A}\)

weget, 120Pa = \(\frac {1200 N}{A}\)

or A = \(\frac {1200 N}{120Pa}\) = 10 m^{2}

Think your answer

Question 1.

Explain in brief why birds that walk on the leaves floating on the surface of water have wide feet.

Answer:

Birds that walk on leaves floating on the surface of the water have wide feet. This is because wide feet increases the area of contact with the leaves. So, they exert less pressure on the leaves, allowing them to walk on these leaves.

Question 2.

Explain why the wall of a dam is made thicker at the bottom.

Answer:

The wall of a dam is made thicker at the bottom because of the following reasons.

1. A dam has a massive weight and thus, applies a large force on the ground. In order to reduce the pressure exerted by it on the ground, its area of contact with the ground is increased by making its base broad. So, a broad base supports the weight of the dam.

2. Moreover, the pressure of water increases with depth. So, it is greater at the base than at the top. So, the lase is broad to withstand the huge pressure of the water.

Question 3.

When the nozzle of a syringe is dipped in a liquid and its piston is withdrawn, the liquid pushes up into the syringe. Give reason.

Answer:

When the nozzle of a syringe Is dipped in a liquid and its piston is withdrawn, a region of low pressure is created in the syringe. The atmospheric pressure acting on the surface of the liquid is greater and hence forces the liquid

to rise in the syringe.