State Newton's third law of motion. Describe how two spring balances can be used to verify it.

1. 1 · State the law. To every action there is an equal and opposite reaction, and the two act on different bodies.
2. 2 · Set up the balances. Hook balance B to a fixed block and balance A to B. Pull A with a string passing over a pulley and carrying weights.
3. 3 · Draw the set-up. [Diagram: block, balance B, balance A, string over a pulley to a hanger with weights, all in a line; label both balances]
4. 4 · Take readings. Add 1 N weights one by one. Note the readings of A and B each time.
5. 5 · Observe. Both balances always show the same reading: A pulls B as hard as B pulls A.
6. 6 · Conclude. Action and reaction are equal, opposite and act on different bodies. The law is verified.

Scoring tip: Draw the block, both balances, the pulley and the hanging weights in one straight line. Record your readings in a table.