An object is in motion if it is moving relative to a

Question

An object is in motion if it is moving relative to a

Answer 1

a point of reference

Answer 2

...stationery object.

Answer 3

...reference frame.

In the normal course of day to day living, our reference frame is typically the ground we stand on or the building we are in.  We call that reference frame stationary; that is, it has no velocity that we need to account for in determining the velocity of other things.

But we might also observe motion relative to a nonstationary reference frame...say, a train car that we are in.  And that train car is, in turn, moving relative to the ground.  So, to find our motion relative to the ground, we need to factor in the motion of the train.  For example, V = w + v; where V is our motion wrt ground, w is our motion wrt the train car, and v is the velocity of the train car wrt the ground.

If the train is just starting out and is moving forward at v = 5 kph wrt the ground, and you are walking to the rear of the car with w = 3 kph wrt the car, your velocity V = -3 + 5 = 2 kph wrt the ground.  The minus sign means to the rear and plus signs mean to the front.  So wrt the ground, you are moving 2 kph forward in the direction of the train.