Recognising and applying the law in real-life situations

Identifying an action-reaction pair

To correctly identify an action-reaction pair, three criteria must be met:

1. Both forces have the same magnitude (intensity).
2. They have the same direction but opposite sense.
3. They act on two different objects (never on the same one).

If any one of these criteria is missing, it is not an action-reaction pair within the meaning of the third law.

Common pitfall: confusing with equilibrium

Many pupils confuse the principle of reciprocal actions with the equilibrium of an object (weight balanced by the reaction of the support). These are two different things:

• Equilibrium: two forces acting on the SAME object (e.g. weight and normal reaction on a book resting on a table) which cancel each other out because the sum of the forces is zero.
• Action-reaction: two forces acting on two DIFFERENT objects (the book on the table, the table on the book), which never add up to each other.

Example: walking

When you walk, your foot pushes the ground backwards (action). In response, the ground pushes your foot forwards, propelling you along. Without this reaction from the ground (for example, on a very slippery surface such as ice), you would not be able to move forwards effectively.

Example: rocket propulsion

A rocket ejects hot gases downwards at very high speed (action). In response, the gases exert an upward thrust on the rocket (reaction), even in the vacuum of space where there is nothing to ‘push against’ – contrary to popular belief.

Key points

• Mass (A) × acceleration (A) results from the force exerted by B, not from the force that A exerts on B.
• The two forces in the pair never act on the same system: they must therefore never be added together in a force analysis involving a single object.