Question

Using newton's third law, explain what is meant by action and reaction pairs of forces.

Answer 1

Newton’s third law: For every force (the “action”) exerted by body A on body B there is a force (the “reaction”) exerted by body B on body A that is equal in magnitude, opposite in direction, and of the same type.

Key points
- Pair statement (vector form): FAB = −FBA.
- The two forces act on different bodies, not on the same object. That is why they do not cancel each other on a single body.
- They are simultaneous and collinear (they lie along the same line of action).
- The labels “action” and “reaction” are arbitrary — either force can be called the action.

Common examples
- Push on a wall: when you push the wall with force F, the wall pushes you back with −F.
- Walking: your foot pushes backward on the ground; the ground pushes your foot forward (propulsion).
- Rocket: hot gases are expelled backward; the gases push the rocket forward.
- Gravity: Earth pulls the apple downward and the apple pulls the Earth upward with the same magnitude (the Earth’s acceleration is just negligible).

Why this matters
- For a single object, only forces acting on that object affect its motion. Action–reaction pairs act on different objects, so they don’t directly cancel in an object’s free-body diagram.
- For the entire isolated system, internal action–reaction pairs cancel; only external forces change the system’s momentum.

Note: In some advanced contexts (electromagnetism, relativity) momentum carried by fields must be included to account fully for the third-law balance, but the basic pairwise statement above holds in ordinary mechanics.