Question

Which is an action/reaction force pair? Check all that apply.

Earth pulls on a book, and the book pushes against a shelf.
A hockey stick hits a puck, and the puck pushes against the stick.
A pencil pushes against a piece of paper, and the paper pushes against the desk.
A finger pulls on a rubber band, and the rubber band pushes against the finger.
A dog pulls on a leash, and the owner pulls back on the leash.

Answer 1

Correct: 2 and 4.

Brief reasoning:
- Earth pulls on a book, and the book pushes against a shelf — Not a pair. The reaction to Earth pulling the book is the book pulling on the Earth (gravity). The book pushing the shelf is a different interaction.
- A hockey stick hits a puck, and the puck pushes against the stick — Yes. Stick on puck and puck on stick are equal and opposite forces on two different objects.
- A pencil pushes against a piece of paper, and the paper pushes against the desk — Not a pair. The pencil↔paper reaction is paper on pencil; the paper↔desk reaction is desk on paper.
- A finger pulls on a rubber band, and the rubber band pushes against the finger — Yes (interpret “pushes” as “exerts a force on”). The finger pulls the band and the band exerts an equal and opposite force on the finger (in practice the band pulls the finger).
- A dog pulls on a leash, and the owner pulls back on the leash — Not a pair. These are two different forces both acting on the leash. The reaction to the dog pulling the leash is the leash pulling the dog; similarly for the owner.