1. Frame of reference
A frame of reference is a conveniently chosen co-ordinate system. The position of particle is described in the frame of reference chosen.
2. Rest and motion
A body is at rest in a chosen frame of reference if its position does not change with time. On the other hand, if position of particle or body changes with time the particle or body is in motion. Rest and motion are relative. A body at rest in one frame, can be in motion in another frame of reference. There is nothing like abolute rest.
3. Position Vector
It describes the instantaneous position of a partile in a frame of reference. It is a vector joining the origin of co-ordinates to the instantaneous position of particle. For a particle at rest; position vector is same at all times. For a particle in motion, the position vector changes with time.
4. Distance and Displacement
The actual path along which a particle moves is known as the trajectory. The distance travelled by a particle equals the actual length of the path discribed. It is a scalar quantity. It is a positive number which does not decreases with time and can never be zero for a particle in motion.
