Question

When do you say motion is uniform

Answer 1

An object is traveling in uniform motion when its velocity is constant. This means that the object is neither accelerating nor decelerating. When an object's velocity is constant, it is moving only with respect to a unique frame of reference. In a car with truly constant velocity -- no speed bumps, air resistance, or pot holes -- you would not be able to tell that you were moving. Only by looking outside at passing objects would you learn that you were in motion. Because objects traveling in uniform motion do not behave any differently from objects at stand still, motion itself is always relative to a certain observational frame of reference.