CBSE Class 8 Science Force and Pressure Chapter Notes

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Force & Pressure

The word Force is associated with either pull or push, which causes the change in the state of the body. A force cannot be seen. A force can be judged only by the effects it can produce on various objects around us.

Effects of Force
i) Force can move a stationary object
ii) A force can stop a moving object
iii) Force can change speed of moving object
iv) Force can change the direction of moving object
v) A force can change the shape (or size) of an object.

Thus we can say that

1. Force is that agent which can change the state of rest or of uniform motion or shape or direction of motion of anybody. The S.I. unit of force is newton. It is a vector quantity (i.e. force has magnitude as well as direction).

Balanced and Unbalanced Forces

a) Balanced forces cannot change the speed of a body. They can change the shape of the object. If the resultant of the forces acting on a body is zero, the forces are said to be balanced.

b) Unbalanced forces can cause the change in speed or direction of motion of the body. If the resultant of the forces acting on the body is not zero, the forces are said to be unbalanced.

2. Resultant force: When two or more forces act on a body simultaneously, then the single force which produces the same effect as produced by all the forces acting together, is known as the resultant force.

3. Classification of Forces: Forces may be classified into the following two classes:

a) Contact Forces: The contact forces arise as a result of physical contact between two bodies, one by which force is exerted and the other on which force is exerted.
Example of contact forces:

i) Muscular Force: The force exerted by the muscles of our body is called muscular force. The force used to lift a bag, water bottle etc. are all example of muscular force.

ii) Frictional Force: A force arising when one body moves over the surface of another body and is directed opposite to the motion is called frictional force. Friction is a force that slows things down or prevents things from moving.

b) Non-contact Forces: The non-contact force or the action-at-a-distance corresponds to the force which do not involve any physical contact between the two bodies but act through the space between them.

i) Gravitational Force: The force of attraction between two objects because of their mass is called gravitational force.

ii) Magnetic Force: A magnet attracts from filings, nails and other objects made of iron, steel, nickel and cobalt . Magnets exerts forces of attraction and repulsion on other magnets. An important feature of magnetic force is that it can act from a distance, and is therefore a non-contact force.

iii) Electrostatic Force: When a plastic comb is rubbed with silk, it can pick up small bits of paper (figure). This is because the comb acquires an electric charge because of which it can exert a force called electrostatic force. Electrostatic force can also act from a distance and is therefore a non-contact force. A body with electrostatic charge can either attract or repel another charged body. Electrostatic force is used to separate solid pollutant particles from smoke given out from factories.

Equations of Uniformly Accelerated Motion
There are three equations for the motion of the bodies which travel with a uniform acceleration. These equations give relationship between initial velocity, final velocity, time taken, acceleration and distnace travelled by the bodies. We shall study these equations one by one.

1. First Equation of Motion: The first equation of motion: v = u + at. It gives the velocity acquired by a body in time t. We will now derive the first equation of motion.
Consider a body having initial velocity ‘u’. Suppose it is subjected to a uniform acceleration ‘a’ so that after time ‘t’ its final velocity beocmes ‘v’. Now, from the definition of acceleration we know that: 

The equation v = u + at is known as the first equation motion and it is used to find out the velocity ‘v’ acquired by body in time ‘t’, the body having an initial velocity ‘u’ and uniform acceleration ‘a’. In fact, this equation has four value in it, if any three values are known, the fourth value can be calculated. By paying due attention to the sign of acceleration this equation can also be applied to the problems of retardation.

2. Second Equation of Motion: The second equations of motion is: s =ut + 1/2 at² . It gives the distance travelled by a body in time t. Let us derive the second equation of motion.

Suppose a body has an initial velocity ‘u’ and a uniform acceleration ‘a’ for time ‘t’ so that its final velocity become ‘v’. Let the distance travelled by the body in this time be ‘t’. The distance travelled by a moving body in time ‘t’ can be found out by considering its average velocity. Since the initial velocity of the body is ‘u’ and its final velocity is ‘v’, the average velocity is given by:

This is the second equation of motion and it is used to calculate the distance travelled by a body in time t.

3. Third Equation of Motion: The third equation of motion: v2 = u² + 2as. It gives the velocity acquired by a body in travelling a distance s. We will show how to derive this third equation of motion.

The third equation of motion can be obtained by eliminating t between the first two equations of motions. This is done as follows:

From the second equation of motion we have:

Law of Motion

Newton’s First Law of Motion: According to Newton’s first law of motion, a body continues to be in a state of rest or in a state of uniform motion along a straight line unless an external unbalanced force is applied on the body.

Inertia: This inherent property of the body by virtue of which it is not able to change by itself its state of rest or state of uniform motion or direction of motion is called inertia of the body.

Inertia of rest: It is the property by virtue of which a body at rest cannot start moving on its own. Rather the body at rest opposes any force that tries to move it.

Example: When a carpet is suddenly jerked the dust fly off because due to sudden jerk the carpet comes in motion but dust particles tend to remain at rest due to inertia of rest and hence fall off.

Inertia of Motion: It is the property by virtue of which a body in motion cannot stop on its own. Rather the body in motion opposses any force that tries to stop it.

Inertia of Direction: It is the property by virtue of which a body moving along a particular straight line cannot change its direction of motion by itself. Rather the body opposses any force that tries to change its direction of motion.

Eample: The action of mud guard over the wheels of vehicles is based on inertia of direction.

Note: Mass of a body is a measure of inertia of the body. Heavier the body, greater is its inertia.

Eample: When a running car stops suddenly, the passenger is jerked forward. The reason is that in a running car, the whole body of passenger is in the state of motion. As the car stops suddenly, the lower part of his body being in contact with the car, comes to rest but his upper part remains in the state of motion due to the inertia of motion. Thus he gets jerked forward.

Linear Momentum and Newton’s Second Law of Motion

The concept of linear momentum is extremely important in physics. Whenever we examine moving object we must consider both its mass and velocity because the quantity of motion possessed by the body depends upon these two factors. Let us go through the following examples:

1. We know that a cricket ball is heavier than a tennis ball. Suppose we throw both, a cricket ball and a tennis ball with the same speed. It is found that more force is required to stop the cricket ball (which has more mass) and less force is required to stop the tennis ball (which has less mass). It can be concluded that if two bodies of different masses are moving with the same speed or velocity, the force needed to stop the heavier body is more than that required or the lighter body.

2. We now throw two cricket balls of the same mass with different speeds or velocitie It is found that more force is required to stop that cricket ball which is moving with a higher velocity and less force is required to stop the cricket ball moving with lower velocity. It can be concluded that if two bodies of the same mass are moving with different velocities, the force needed to stop the faster moving body is more than that required for the slower moving body. So, the force needed to stop a moving body is directly proportional to its mass and its velocity. This gives us the term known as momentum.

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