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# Rest and motion

An idea about frame of reference is necessary before discussion on rest and motion.

## Frame of reference

While we observe a body that does not change its position with flow of time, we almost unknowingly refer to a second body which is nearby the first one. The second body should be a fixed one - it may be a domestic building or a tree, a lamppost or a prominent spot oh a road or a fixed star in the sky. This second body is called a reference body or a frame of reference.

## Definition

Frame of reference is any nearby or a distant fixed object from which the distance and position of a body is observed as time elapses.
Definitions of rest and motion

## Rest

A body is said to be at rest if it does not change its position with time from a fixed neighboring object.

## Motion

A body is in motion, if its position from a fixed neighboring object changes with time.

## Rest and motion are relative

A body seems to be at rest with respect to one neighboring fixed object but the same body may appear to be in motion with respect to some other fixed neighboring object So, rest and motion are relative terms.

## Displacement

When an object changes its position as time passes on, the distance measured in the direction from its initial to final position along a straight line is known as its displacement. Displacement is a vector quantity; it is directed from the initial position of a body to its final position. Unit in C.G.S system is cm and in M.K.S. system is m.

## Speed

Speed of a body is the distance covered by it in unit time in any direction.
Unit of speed in CGS is cm/s. Unit of speed in MKS is m/s. Speed has no definite direction, so it is a scalar quantity.

## Velocity

Velocity of a body is the distance covered by it in unit time in a specified direction. Thus velocity of a body is its speed in a specified direction. Hence, velocity is a vector quantity. The unit of velocity in C.G.S. unit is cm/s and in M.K.S m/s.

## Acceleration

Increase or decrease of velocity per unit time is the acceleration of a body.
It is also defined as the rate of change of velocity with time. It’s a vector quantity. Unit of acceleration is C.G.S system is cm/s2 and in M.K.S system m/s2.