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# Newton's Laws of Motion

(a) First Law: Everybody continues in its state of rest or of uniform motion in a straight line except in so far as it be compelled by any external impressed force to change that state.
(b) Second law: The rate of change of momentum is directly proportional to the impressed force and takes place in the direction in which the force acts.
(c) Third law: To every action there is an equal and opposite reaction.

## Inertia of Rest and Inertia of Motion

Inertia of a body is its reluctance to change the state of rest or of uniform motion. It is also the degree of resistance a material body can offer to change of motion or rest condition. Inertia is of two types :
(i) inertia of rest, (ii) inertia of motion.

## Inertia of rest

Inertia of rest is the inherent property of a material body by virtue of which it continues in its state of rest until and unless no external unbalanced force compels it to move.

## Inertia of motion

The tendency of a moving body to maintain its uniform motion in a straight line in absence of an effective external force is called inertia of motion.

## Concept of Force

Newton's First Law clearly states that nothing moves unless it is moved. That is, a body at rest does not take any initiative to move, an external force of required magnitude is necessary for this purpose.
Similarly, a moving body may also be compelled to change its motion under this type of action. The change of motion may be either in the form of change in direction of velocity or change of magnitude of velocity or for both the reasons.
Force is an external agent that actually changes or tends to change the state of rest or uniform motion of a body.

## Conceptually correct definition of force

Force is an agent which acting on a body actually changes the state of rest or uniform motion of the body.
Force is a vector quantity; it has magnitude as well as direction.

## Units of force

The C.G.S. unit is Dyne = g-cm/s2; the M.K.S. unit is Newton (symbol N) = kg-m/s2

## Momentum

This physical quantity is dependent both on the mass and the velocity of a moving body. So, momentum is defined as the product of mass and velocity of the body concerned, i.e., Momentum = mass x velocity. It is a vector quantity; its direction is the direction of motion of the related body.

## Units of momentum

Momentum = mass x velocity,
The C.G.S. unit is g-cm/s and the M.K.S. unit is kg-m/s

## Action and Reaction forces

When one body exerts a force on the other, the force is called the action, the second body also offers at the same time an equal force on the first body in. the opposite direction and this force is called reaction. Since, every force of action is associated with an opposite force of reaction, no force can exist singly in nature, and that is, in nature forces always appear in pairs.
Action and reaction do not act at the same point. For example, when a cricket ball is hit with a bat, the cricket ball is the point of application of the action force the bat applied. The cricket ball also exerts equal reaction force on the bat. So, the bat is the point of application of the reaction force.