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Frame of reference:
When the position of a particle in space changes with time, it is said to be in motion. To describe motion, a frame of reference is needed first and foremost. The set of spatial coordinates (x, y, z) and time t describe the position of a particle at a particular time t with respect to the frame of reference, which is the aforesaid coordinate system. A clock that is not in motion with respect to that frame of reference can measure proper time in that frame. So the particle moving with velocity v with respect to one frame of reference(R1) might be at rest with respect to another frame of reference(R2) while R2 is moving with respect to R1 with velocity v.
Thus we conclude that motion is a relative phenomenon, and to describe motion, a frame of reference is always needed.
There are two types of frame of reference. A frame of reference that does not have acceleration and rotation is called an inertial frame of reference. All inertial frames move with respect to one another with constant velocity. Most importantly, the laws of Physics( including Newton's laws of motion) hold and are the same in all inertial frame of reference. This statement can not be made in the case of an accelerating or rotating frame. There, to express Newton's equation of motion, one must introduce pseudo forces that solely arise due to acceleration and/ or rotation of the frame.
