4. A student is sitting still on a swing at the playground. He asks his friend to give him a push. She walks behind him and pushes him forward, and he begins swinging back and forth. When did the student experience unbalanced forces?
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1 point
when he sat on the swing without moving
when he was pushed forward by his friend
when the forces acting on him were equal from each side
when the forces acting on him were equal from above and below