Question

which of the following describes an action - reaction pair?
a. a man pushes a car forward, and the car pulls the man forward.
b. the sun pulls on earth, and earth pulls on the moon.
c. you push down on a desk, and the desk pushes down on the ground.
d. you push up on a barbell, and the barbell pushes down on you.

Explanation:

According to Newton's third law of motion, for every action, there is an equal and opposite reaction. The two forces in an action - reaction pair act on different objects.

• In option A, if a man pushes a car forward, the reaction force should be the car pushing the man backward, not pulling the man forward.
• In option B, the Sun pulling on Earth and Earth pulling on the Moon are forces acting on different pairs of objects and not an action - reaction pair (the action - reaction pair for the Sun - Earth gravitational force is Earth pulling on the Sun).
• In option C, when you push down on a desk, the reaction force is the desk pushing up on you. The desk pushing down on the ground is a different force (related to the desk's weight and normal force from the ground, but not the reaction to your push on the desk).
• In option D, when you push up on a barbell (action force), the barbell pushes down on you (reaction force) which is in line with Newton's third law.

Answer:

D. You push up on a barbell, and the barbell pushes down on you.