Question

when do we say that a body is at rest and when do we say that it is movie

Answer 1

Whether a object is in rest or motion depends upon the reference frame by which you are observing , Different reference frame may give different answers.Example : If you are travelling in a train and a woman is sitting near you. you will see the woman near you as stationary but if someone is observing the woman from outside the train he will see as if the woman is moving with the speed of train.Hence the two different reference point has given two results.

Answer 2

the body is said to be in rest when a body is stationery in respective to the other.
the body is said to be in motion when a body is moving in respective to other.


example--> a car is standing still on road
an observer on the same road observers it and for him the car is in rest as per the definition.

but at the same time it is in motion as if the earth is continuously rotating.

so basically the state of rest n motion is completely dependent upon the observer