Question

Every action force has an opposite and equal reaction force. Determine which of these are action/reaction pairs. Check all that apply.

A girl leans against a wall and the wall pushes on the girl.
A baseball hits a glove and the glove pushes on the ball.
A boy pushes on a door and the door swings open.
A soccer player kicks a ball and then another soccer player kicks the same ball.
A cat rubs up against a tree and the tree pushes against the cat.
A student pushes another student standing in a line, and that student falls into the next student.

Answer 1

Correct action/reaction pairs:
- A girl leans against a wall and the wall pushes on the girl.
- A baseball hits a glove and the glove pushes on the ball.
- A cat rubs up against a tree and the tree pushes against the cat.

Why:
- Newton’s 3rd law pairs are two forces of the same contact/interaction, equal in magnitude, opposite in direction, acting on two different objects simultaneously (e.g. girl→wall and wall→girl).
- The boy pushing a door / door swings open is not a listed action/reaction pair — the reaction to the boy’s push is the door pushing back on the boy, not the door’s subsequent motion.
- Two soccer players kicking the same ball are separate, sequential interactions; one kick and the other kick are not an action/reaction pair to each other.
- The student pushing the next student and that student falling into another describes a chain of events, not the simultaneous action/reaction pair (the reaction to the push is the pushed student pushing back on the pusher).