ICSE Class 8 Physics Chapter 03 Force and Pressure

3 Force And Pressure

Theme: A force is a push or pull upon an object resulting from the object's interaction with another object. Turning effect of a force is more if the distance between the point of application of force and the hinge of a door is more. It is given a special name - Moment of force. Pressure is defined as force per unit area. Solids, liquids and gases all exert pressure. Atmosphere also exerts pressure, activities are carried out to demonstrate that solid, liquid and gases exert pressure.

In This Chapter You Will Learn To

Explain the turning effect of a force, with examples from daily life

Define moment of force

Express moment of force in proper units

Solve simple numerical problems based on moment of force

Define pressure

Express pressure in proper units

Solve simple numerical problems based on formula for pressure

Describe pressure exerted by a liquid

Demonstrate that liquids exert pressure

Describe pressure exerted by a gas

Describe atmospheric pressure

Express thoughts that reveal originality, speculation, imagination, a personal perspective, flexibility in thinking, invention or creativity

Present ideas clearly and in logical order.

Learning Objectives

Revising previous concepts learnt by children

Building on children's previous learning

Demonstration of turning effect of force

Explanation of turning effect and factors on which it depends

Engaging children in task for calculation of turning effect

Demonstration of pressure exerted by a force on an object

Explanation; pressure depend on the area of surface on which the force acts

Demonstration of pressure exerted by a liquid

Demonstration of pressure exerted by a gas

Explanation of pressure exerted by atmosphere

Engaging children in tasks to show that:

(a) Pressure depends on area

(b) Liquids exert pressure

(c) Gases exert pressure

Observation/Experimentation/Analysis

Student led experiments (reasoning to be given by children individually)

Investigate the effect on pressure when walking on flat shoes and pointed heels on our body support system

For example: Children reasoning as to why is it easier to hammer a sharp pin respective to a blunt pin?

Knowing Concepts

Turning effect of force (moment of force): concept, definition and calculation

Pressure

Definition

Unit

Calculation of pressure in simple cases

Pressure exerted by liquids (Qualitative only)

Pressure exerted by gases - Atmospheric pressure (qualitative only)

Force

We have read that a body which does not change its position with respect to its surroundings is said to be at rest or stationary, whereas a body which changes its position with respect to its surroundings is said to be in motion or a moving body.

A force is a cause (push or pull) which tends to result in movement or change in size or shape of the body. A force when applied as push or pull on a stationary body which is free to move, can produce motion in it and if applied on a moving body, it can change the speed of motion of body (i.e. can speed up or slow down the moving body) or it can change both the speed and direction of motion.

Examples:

A grass roller initially at rest when pulled, begins to move.

A fielder when catches a ball, stops the moving ball.

A moving car slows down on applying brakes on it.

A push on a swinging girl speeds up her swing.

A player when applies force by his hockey stick on the ball, the direction of motion of ball changes.

When force is applied as stretch or squeeze on a body which is not free to move, it changes the size or shape of the body.

Examples:

On stretching a rubber string, its length increases.

On squeezing a tube of gum, its shape changes.

Thus, we define force as below:

Force is that cause which changes the state of a body (either the state of rest or of motion) or it changes the size or shape of the body.

Do You Know?

The speed of a body is defined as the distance travelled by it in one second.

Speed up means more distance travelled in one second and slow down means less distance travelled in one second.

Note:

A force does not change the mass of the body on which it is applied.

We cannot see a force. However, we can see or feel the effect of a force.

A force is expressed by stating both its magnitude and direction.

Unit Of Force

The S.I. unit of force is newton. The symbol for newton is N. This unit is named after the English scientist Sir Isaac Newton who did a lot of research work on force.

One newton is defined as the force which when applied on a moving body of mass 1 kg in the direction of its motion, increases its speed by 1 m s-1 in one second.

We have read that our earth attracts each body towards it. The force of attraction exerted on a body by earth is called the weight of the body or the force of gravity that acts on the body.

The force of gravity (or weight) of a body is different at different places on earth. At a place, the force of gravity on a body of mass 1 kg is called 1 kgf or 10 N. In other words, 1 N is the force of gravity at a place on 0-1 kg (100 g) mass. Thus, the unit of force kgf and N are related as:

\[1 \text{ kgf} = 10 \text{ N (nearly)}\]

In other words, one newton is the force that we have to exert to hold a mass of 100 g on our palm (Fig. 3.1).

A force is represented by an arrow. The length of arrow is a measure of its magnitude and the arrow head shows the direction.

Do You Know?

A body in which the inter-spacing between its constituent particles do not change when a force is applied on it, is called a rigid body and if it changes, the body is called a non-rigid body.

A force when applied on a rigid body can cause only the change in motion of the body. But a force when applied on a non-rigid body can cause both the change in its size or shape and the motion in it.

Turning Effect Of A Force

We have read above that if a force is applied on a stationary rigid body, it starts moving in a straight line in the direction of force as shown in Fig. 3.2. In Fig. 3.2, a ball moves on pushing.

Now if the body is not free to move, but it is pivoted at a point O and a force F is applied at a suitable point A, it begins to turn about the point O. (Fig. 3.3). The vertical axis passing through the point O about which the body turns, is called the axis of rotation. In Fig. 3.3, on pushing, the wheel begins to turn about its pivoted point O.

Similarly, when the handle of a door is either pushed or pulled, the door begins to turn about the hinges which hold the door at rest.

Thus, a force (push or pull) has a turning effect on a body which is not free to move in a straight line, but is pivoted at a point about which it can turn.

Factors Affecting The Turning Of A Body

The turning effect of a force on a body depends on the following two factors:

1. The magnitude of the force applied. Larger the magnitude of force applied, more is the turning effect on the body.

2. The perpendicular distance of the force from the pivoted point. Larger the perpendicular distance of point at which the force is applied, from the pivoted point, more is the turning effect on the body.

Some Examples In Daily Life

1. To open or shut a door, we apply a force (push or pull) F normal to the door at its handle P which is provided at the maximum distance from the hinges as shown in Fig. 3.4. We can notice that if we apply the force at a point Q (near the hinge R), much greater force is required to open the door and if the force is applied at the hinge R, we will not be able to open the door howsoever large the force may be. Thus, the handle is provided near the free end of the door so that a smaller force at a larger perpendicular distance, produces the required turning effect of force to open or shut the door.

2. The upper circular stone A of a hand flour grinder is provided with a handle H near its rim (i.e. at the maximum distance from centre) so that it can easily be rotated about the iron pivot P at its centre by applying a small force at the handle H (Fig. 3.5).

3. A potter's wheel has a wheel pivoted at the centre. The potter turns the wheel by means of a stick at the rim of the wheel as shown in Fig 3.6.

4. A carpenter uses a drill machine which is provided with a handle so that by applying a less force at the end of handle, the drill can be turned easily (Fig. 3.7).

5. To turn a steering wheel in a car or truck, the driver applies force at a point on the rim of the wheel as shown in Fig. 3.8.

6. In a bicycle to turn the wheel, the force is applied on the pedal so that distance of force from the axle of wheel is increased (Fig. 3.9).

7. A spanner used to tighten or loosen a nut, has a long handle to produce a large turning effect by a small force applied at the end of its handle as shown in Fig. 3.10.

Conclusion

From the above examples, we conclude that the turning of a body about the pivot depends not only on the magnitude of the force, but it also depends on the perpendicular distance of the force from the point of rotation. Larger the perpendicular distance, less is the force needed to turn the body. Actually the turning effect on a body depends on the product of both the magnitude of force and the perpendicular distance of force from the pivoted point. This product is called the moment of force (or torque). In other words, a body turns (or rotates) about the pivoted point due to the moment of force.

Moment Of Force

The moment of a force is equal to the product of the magnitude of the force and the perpendicular distance of the force from the pivoted point.

Consider a body which is pivoted at a point O. If a force F is applied on the body in the direction FP as shown in Fig 3.11, the force is unable to produce linear motion of the body in its direction because the body is not free to move, but this force turns (or rotates) the body about the point O, in the direction shown by the arrow in Fig. 3.11.

In Fig. 3.11, the perpendicular distance of the force F from the pivoted point O is OP. Therefore,

Moment of force about the point O = Force × perpendicular distance of force from the point O = F × OP

Note:

For producing maximum turning effect on a body by a given force, the force is applied on the body at a point for which the perpendicular distance of the force from the pivoted point is maximum so that the given force may provide the maximum torque to turn the body.

Unit Of Moment Of Force

Unit of moment of force = unit of force × unit of distance

The S.I. unit of force is newton and that of distance is metre, so the S.I. unit of moment of force is newton × metre. This is written in short form as N m.

Note: The unit newton × metre (N m) of moment of force or torque is not written as joule (J).

The C.G.S unit of moment of force is dyne × cm.

But if force is measured in gravitational unit, then the unit of moment of force in S.I. system is kgf m and in C.G.S. system, the unit is gf cm.

These units are related as follows:

\[1 \text{ N m} = 10^5 \text{ dyne} \times 10^2 \text{ cm} = 10^7 \text{ dyne cm}\]

\[1 \text{ kgf m} = 10 \text{ N m (nearly)}\]

and \[1 \text{ gf cm} = 1000 \text{ dyne cm (nearly)}\]

For example, Reena has to apply a minimum force of 1.5 N on the handle of the door of width 1.2 m to open it. This means that the minimum moment of force required to open the door is 1.5 N × 1.2 m = 1.8 N m. Now if she wants to open it by applying the force at the mid point between the handle and hinges, (i.e. at distance 0.6 m from the hinges), she will have to apply a force F such that

F × 0.6 m = 1.8 N m

or

F = 1.8 N m / 0.6 m = 3.0 N

Thus, on decreasing the distance of the applied force from the point of rotation, the magnitude of force increases.

Do You Know?

Conventionally, if the effect on the body is to turn it anticlockwise, moment of force is called anticlockwise moment and it is taken positive. If the effect on the body is to turn it clockwise, the moment of force is called clockwise moment and taken in negative.

The direction of rotation of a body can be changed either by changing the point at which the force is applied or by changing the direction of force applied as shown in Fig. 3.12.

Pressure

Thrust: A force can be applied on a surface in any direction. If the force is applied on a surface in a direction normal (perpendicular) to the surface, the force is called thrust. Thus, the force acting normally on a surface is called thrust. A body, when placed on a surface, exerts a thrust on the surface equal to its own weight.

The unit of thrust is same as that of the weight or force. Thus, the units of thrust are kilogram force (kgf), gram force (gf) and newton (N). These units are related as:

1 kgf = 1000 gf

1 kgf = 10 N (nearly)

1 N = 100 gf (nearly).

The Effect Of Thrust

The effect of thrust depends on the area of the surface on which it acts. Smaller the area of the surface on which a thrust acts, larger is its effect. But the effect of a thrust is less on a larger area.

Examples:

(1) If you stand on loose sand, your feet sink deeply into the sand. But when you lie on sand, your body does not sink much into the sand. In both the cases, the thrust exerted on the sand is same. The reason is that, when you stand, the thrust acts on a smaller area, so you sink more in the sand, and when you lie down, the same thrust acts on a larger area so you sink less in the sand. (Fig. 3.13).

(2) If you hammer a nail holding it with its flattened end resting on a wooden block [Fig. 3.14 (a)], you find it difficult to get the nail into the block. But if you hammer the nail holding its sharp end resting on the block [Fig. 3.14 (b)], the nail penetrates into the block easily. The reason is that when thrust acts on the flattened end, the effect of thrust is small but when the same thrust acts on the sharp end the effect of thrust is more.

The effect of thrust is expressed in terms of a quantity called pressure. More the effect of a given thrust on a surface, we say that the thrust exerts more pressure on the surface and if less is the effect of thrust on a surface, we say that the thrust exerts a less pressure on the surface.

Definition Of Pressure

Pressure is defined as the thrust per unit area. Thus,

Pressure = Thrust / Area

It is denoted by the letter P.

If a thrust F acts on an area A, the pressure P is:

P = F / A

Teacher's Note

When a student walks on soft ground, their footprints are deeper when they stand still versus when they walk, demonstrating how the same force (weight) creates different pressures based on contact area.

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