## Motion And Time

### UNIT

Measurement of any physical quantity is expressed in terms of an internationally accepted certain basic standard called unit.
Length, time & mass are measured in metre, sec, and kilogram

### PHYSICAL QUANTITIES

The quantities which can be measured by an instrument and by means of which we can describe the laws of physics are called physical quantities.
Eg.    length, velocity, acceleration, force, time, pressure, mass, density etc.

### REST & MOTION

An object is said to be at rest if it does not change its position with respect to its surrounding with the passage of time.
A body is said to be in motion if its position changes continuously with respect to the surroundings (or with respect to an observer) with the passage of time.

### UNIFORM & NON UNIFORM MOTION

(a)    Uniform Motion :
A body has a uniform motion if it travels equal distances in equal intervals of time, no matter how small these time intervals may be.
For example, a car running at a constant speed of say, 10 m/s, will cover equal distances of 10 metres, in every second, so its motion will be uniform.

(b)     Non-Uniform motion :
A body has a non-uniform motion if it travels unequal distance in equal intervals of time.
For example, if we drop a ball from the roof of a building, we will find that it covers unequal distances in equal intervals of time.
It covers :
4.9 meters in the 1st second
14.7 metres in the 2nd second
24.5 metres in the 3rd second  and so on.

### (a)    Distance & displacement :

(i)     Distance : Distance is the actual path travelled by a body in a given time.
Consider a body travelling from A to B along any path between A and B. The actual length of the path that a body travels between A and B is known as the distance. Here if the body goes from A to B via C, the distance travel will be ACB.