## Saturday, 20 July 2013

### Frames of Reference

If your friend is on a moving train then he is in motion with respect to you. But if you are sitting on a moving train with your friend then your friend is at rest with respect to you. In this example, you are the frame of reference as the motion or rest state of your friend changes with respect to you.

In simple words Motion is a combined property of object under study and the observer. There is no meaning of rest and motion without the viewer. Nothing is in absolute rest ot absolute motion (except for speed of light which would be discussed later). A book is at rest with respect to an observer on Earth and in motion with respect to a observer on Moon.

In a 3 Dimensional motion, The position of a body at a given time is determined by three coordinates x,y and z. If the coordinates of the body doesn't change with time then it is at rest with respect to the given frame of reference. These reference frames was first given by Galileo Galilei.

There are two types of frames of reference :

Inertial Frame of reference : Newton's first law of motion says that a body continues to be at rest ot in motion unless it is acted upon by an external force that changes its velocity or direction. The frames of reference in which Newton's first law of motion is applicable are called Inertial Frames of reference. Inertial frame of reference is applicable on all motion described by Classical Physics.

Non-inertial Frame of reference : Unlike Inertial frame of reference, Newton's first law of motion is not applicable in Non-inertial frame of reference. Speed of light is an example of Non-inertial frame of reference.

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