Newton's third law of motion states that for every action there is an equal and opposite reaction. Do the action force and the reaction force act on the same body or on two different bodies?
Show Hint
Identify which object each force acts on. The action and the reaction of Newton's third law always act on two separate bodies, never on the same one.
Think of Newton's third law using the recoil of a gun as a fresh example instead of a swimmer.
When the trigger is pulled, the exploding gunpowder pushes the bullet forward. This forward push on the bullet is the action force, and it acts only on the bullet.
At the same instant, the bullet pushes back on the gun with an equal and opposite force. This backward push is the reaction force, and it acts only on the gun, which is why the gun jerks backward.
Notice that the two forces of the pair belong to two separate objects, the bullet and the gun. Neither object experiences both forces at once.
This is exactly why Newton's third law is described as applying between two different bodies interacting with each other, never as two forces landing on one single body.