Question

Difference between rest and motion.

Answer 1

Hint An object that you see is at rest then it will not be moving with respect to you. And when you see a body moving then there are two cases you might be moving or the object itself is moving with respect to you. The motion of a body depends on how you see that body; this depends on the frame of reference with which you are observing.

Complete step-by-step answer:
If two persons are sitting in train in front of each other than if you ask one person about the motion of the other person then he’ll say that he is at rest while some other person notice the same person that was at rest in first case then he’ll say that it is in motion, i.e. he is moving here the role of frame of reference.

Frame of reference is of two types

Inertial Frame of reference

Non-inertial frame of reference

Inertial frame of reference:
The frame of reference in which Newton’s law holds true. Which means if no external force is acting on a body it will stay at rest or remain in uniform motion (Law of motion). Now if a body is kept on the surface of the earth, for a person on earth seeing, it is at rest while for a person on the moon then it is in motion.
The term inertial frame is relative i.e. first we decide a reference frame to be the inertial frame of reference. So a more generic definition of an inertial frame will be: like Inertial frame is at rest or moves with constant velocity with respect to my decided inertial reference frame.

Non-inertial Frame of Reference:
We define a non-inertial frame as a frame that is accelerated w.r.t(with respect to) the decided inertial frame of reference. Newton’s law will not hold true in such frames. So in the above example if we assume earth to be an inertial reference frame the moon becomes a non-inertial reference frame as it is in accelerated motion w.r.t. earth. But if we want to make Newton’s law hold here we need to take some unknown forces also known as pseudo forces.

Note: By shifting your frame of reference in physics, sometimes you can easily solve the problems. So choose your frame of reference wisely because it may even complicate your question. Example throwing a ball in an upward direction from a moving train, if you see it, the motion of the ball will be in a straight line if you are sitting in the same train but it will be parabolic if you are standing on the ground.