What is action/reaction?

• A. A force that opposes motion
• B. For every action there is a reaction
• C. Action and reaction occur at different times
• D. The result of balanced forces

Answer: (B)

Action/reaction is a pair of forces described by Newton’s third law: every force exerted on a body has a counterforce, equal in magnitude and opposite in direction, exerted back on the original object. They form a pair and act on two different objects, happening simultaneously. This is why when you push against something, you feel it push back against you with the same strength in the opposite direction.

Examples help it click: pushing off a boat makes the boat move backward while you move forward; a gun fires and the gun barrel experiences a backward force while the bullet speeds forward; stepping onto a dock from a boat pushes the boat away as you are propelled toward shore.

The other choices don’t capture the idea of a force pair in interaction. A force that opposes motion describes friction, not the general action/reaction concept. Saying they occur at different times is incorrect because the forces act simultaneously. Saying they’re the result of balanced forces describes a different situation where there’s no net force on a single object.