CBSE 9th Physics | Force and Laws of Motion: Part 1

Force and Laws of Motion: Part 1

Force

Definition :

A pull or push acting on a body which changes or tends to change the speed of the body or direction of motion of the body or the shape and size of the body is known as force.

A force influences the shaped and motion of an object. A single force will change its velocity (i.e. accelerate it) and possibly its shape.

Two equal and opposite force may change its shape or size. It is a vector quantity, having both magnitude and direction, and is measured in Newtons.

The main types of force are gravitational, magnetic, electric and nuclear.

The S.I. unit of force is Newton and denoted by N.

The CGS unit of force is Dyne and denoted by D.

Effect of Force:

A force when applied on an object may show the following effects:

(i) A force may cause motion in an object at rest

(ii) A force can bring the moving object to rest

(iii) A force can change the speed of a moving body

(iv) A force may change the size and shape of the object.

(v) A force may change the direction of the motion of a moving object.

Note:

A force acting on an object does not necessarily show the above mentioned effects. For example, when we push a wall (ie force is applied on the wall), the wall does not move.

Similarly, if we press a steel ball, it is not deformed. This is because, in these cases, the force applied by us is not sufficient to cause the changes

Balanced and Unbalanced Force

When the forces acting on a body cancel out the effects of each other in such a way that the resultant force is zero.

Then these forces are known as balanced force. Balanced force may change the shape or the size of an object.

Example: –

Let us consider a rigid almirah kept at rest on a horizontal surface. Let two forces F1 and F2 act simultaneously on the almirah from opposite directions.

If the two forces are equal the effects produced by one force get cancelled by the effect produced by other. The net force or the resultant force is then zero. The almirah continues to remain at rest.

Unbalanced force: –

When two forces of unequal magnitudes, act in opposite directions on an object simultaneously then the object moves in the directions of the larger force.

These forces acting on the object are known as unbalanced forces.

Example: –

When a boy drags a box on the floor, then an unbalanced force is acting on the box. The force of pull is applied by the boy acts on the box in the direction of pull and the force of friction between the lower surface of the box and the surface of the floor acts in the opposite direction.

The box moves in the direction of unbalanced force (= force of pull- force of friction) i.e. in the direction of pulling force.

Note: –

When balanced forces act on a body the acceleration of the body is zero. In otherwards, if a number of forces acting on a body produce zero acceleration, then the forces are balanced.

And if a number of forces acting on a body produce an acceleration, then the forces are unbalanced.

Newton’s Law of Motion

Aristotle, a great Greek philosopher believed that, to maintain the motion of a body, a continuous force was needed.

However this concept of motion was totally rejected by a great Italian physicist Galileo Galileo in 17th century. He gave the famous principle of inertia, which will be discussed a bit latter.

The idea of Galileo helped Newton to state the first law of motion which is also known as law of inertia.

1. Newton’s first law of motion: –

According to this law, a body continues to be in its state of rest or uniform motion in a straight line unless compelled by some external force (i.e. unbalanced force) to change that state.

Newton’s first law of motion gives the definition of force-

Inertia:-

The tendency of a body to oppose any change in its state of rest or uniform motion or direction is called inertia of the body.

The principle of inertia was given by Galileo Galilei in 17th century. He verified this fact by performing the following experiment.

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Consider a double inclined plane ABCD as shown in fig. 2.1 (a). The surface of these planes are very smooth so that there is no force of friction.

A glass ball released from height h was observed. It was found that it rises up to the some height h on the plane CD.

Now, the inclination of eth plane CD was decreased as shown in fig. 2.1 (b). Again the ball released from height h on the plan AB was observed.

It was found that again the ball reached to the some height h on the plane CD after traveling more distance. Now the inclination of the plane CD was reduced to zero so that the plane CD was made horizontal as shown in fig. 2.1 (c).

The ball released from height h on the plane AB was found to travel forever.

Inertia is the inherent property of all the objects.

Types of inertia: –

(i) Inertia of rest

(ii) Inertia of motion

(iii) Inertia of direction

(i) Inertia of rest : –

The tendency of a body to oppose any change in its state of rest is known as inertia of rest.

For example: – When a bus suddenly starts moving forward, the passengers in the bus fall backward. This is because, the lower part of the bodies of the passengers being in contact with the floor of the bus, come in motion along with the bus.

On the other hand, the upper part of their bodies remains at rest due to inertia of rest. Hence the passengers fall backward.

(ii) Inertia of motion: –

The tendency of a body to oppose any change in its state of uniform motion is known as inertia of motion.

Example: –

The passengers fall forward when a fast moving bus stops suddenly.

This happens because, the lower part of the bodies of the passengers come to rest as soon as the bus stops. But the upper part of their bodies continue to move forward due to the inertia of motion.

(iii) Inertia of direction: –

The tendency of a body to oppose any change in its direction of motion is known as inertia of direction.

Fore Example: – When a bus suddenly takes a turn, the passengers sitting casually experience a jerk in the outward direction.

This happens because the passengers tend to remain in their original direction of motion due to inertia of direction.

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