Question

when does motion (or a change in motion) of an object happen? when forces are balanced when forces are unbalanced

Explanation:

According to Newton's first law of motion, an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. So, motion (or a change in motion) of an object happens when forces are unbalanced.

Answer:

when forces are unbalanced