NEWTON'S THIRD LAW: ACTION AND REACTION
3 marks · NormalA student places a block on a spring scale (newton meter) hanging from a clamp stand. The block hangs at rest.
State the action and the reaction force involved between the block and the spring scale.
Explain, using Newton's third law, why the reading on the scale does not change while the block is hanging and at rest.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) —
(b) —
Common mistakes: Giving balanced forces on a single object rather than interaction pairs
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Marking points
B1 (a) · 1 mark · Correctly states the action (block pulls down on scale).
B2 (a) · 1 mark · Correctly states the reaction (scale pulls up on block with equal force).
B3 (b) · 1 mark · Explains reading is constant because action and reaction are equal and opposite (Newton’s third law).
Worked solution
(a) The action is the block pulling down on the scale (due to its weight). The reaction is the scale pulling upwards on the block with an equal force.
(b) According to Newton's third law, these forces are always equal and opposite, so the reading does not change while the block is at rest.