Question

select all that apply
which force pairs must be equal because they are action/reaction pairs?
the upward normal force on a book at rest on the table and its downward weight.
the rightward force of you pushing on a wall and the leftward force of the wall pushing on you.
the rightward force of a pull on a box traveling at constant velocity and the leftward friction force.
the upward normal force on a car and the downward push of the car on the ground.

Explanation:

Action/reaction pairs (Newton's 3rd Law) are forces that two objects exert on each other, acting on separate objects.

1. The normal force on the book and the book's weight both act on the book (same object) – they are balanced forces, not action/reaction.
2. You push the wall, and the wall pushes you: these are two forces between two objects (you and the wall), acting on separate objects – this is an action/reaction pair.
3. The pull on the box and friction on the box both act on the box (same object) – they are balanced forces, not action/reaction.
4. The ground's upward normal force on the car and the car's downward push on the ground are forces between two objects (car and ground), acting on separate objects – this is an action/reaction pair.

Answer:

B. The rightward force of you pushing on a wall and the leftward force of the wall pushing on you.
D. The upward normal force on a car and the downward push of the car on the ground.