# Frame of Reference, Inertial and Non-inertial frames, RELATIVITY

### Frame of Reference ( Inertial frame of Reference ) : –

“A system of coordinate axis which defines the position of of a particle in two or three dimensional space is called a frame of refwrence.”

When we say that something is moving, we mean that its position relative to something is changing.

Example : – A passenger moves relative to an aeroplane. The aeroplane moves relative to the earth, the earth moves relative to the sun. Sun moves to the galexy of which it is a member and so on. In each case a frame of reference provides the discription of the motion.

An inertial frame of reference is one in which Newton’s law of motion hold. In such a frame. An object at rest in motion continuous to move with constant velocity if no force acts on it. Any frame of reference that moves at constant velocity relative to on inertial frame is itself on inertial frame of reference.

Let S and S’ are two frames of reference as shown in figure —

fig….

Let P be any moving particle. Let o and o’ be two observers in frames s and s’ respectively. Now if the frames s ans s’ are at rest, o and o’ will observe the same motion of P. But if s and s’ are in relative motion, their observations for the motion of P will not be the same.