1. A table sitting on the floor a. What is the action force? b. What is the reaction force? 2. A bowling ball striking a pin a. What is the action force? b. What is the reaction force?

Answer 1

The best way I teach my students about "action" vs "reaction" forces is that the object applying the new action force is the one causing some sort of change (even if you can't see it). The object doing the reaction force was just "minding its own business" until the two objects interacted. The object doing the reaction force doesn't want to change, so it's going to apply some amount of force against the other object.

Example 1:

The floor was just "minding its own business" until a table was put on top of it. The table is applying a new force onto the floor, so the floor is going to try to resist it. In this case, the force applied by the table is the action force, while the floor responded with a reaction floor onto the table.

Example 2:

The pin was just sitting on the lane "minding it's own business" until the bowling ball came flying towards it! Therefore, the bowling ball applied the action force, while the pin applied the reaction force back onto the ball.

If the confusion is about why one situation caused a visible change (ie the ball caused the pin to move) and the other didn't (ie the table rested on the floor), I could try to explain. But if you just need to identify which is the "action" force and which is the "reaction" force, just remember that the reaction force was just "minding its own business".