What object does not speed up or slow down?

Answer 1

"Speeding up" or "slowing down" are terms which describe an acceleration of an

object. Newton's second law of motion (F = ma) defines quantitatively the

relationship between force and accelerations. Thus an object that experiences no

force also experiences no acceleration and does not slow down or speed up.

However, consider a particle moving in a circle with constant speed v. The particle is

neither speeding up nor slowing down, but it DOES experience a force and thus an

acceleration (there must be a force which keeps the particle moving in a circle

instead of flying off in one direction). This force, however, is always pointing

perpendicular to the direction that the particle is moving.

So in general, an object that neither speeds up nor slows down is one which

experiences no force in the direction that it is moving.