NEWTON'S THIRD LAW: ACTION AND REACTION
2 marks · NormalA person jumps off a small boat onto a pier. As she jumps forward, the boat moves backwards. Explain why the boat moves backwards using Newton's third law.
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Also accept: When the person pushes forward off the boat (action), the boat pushes back with an equal force in the opposite direction (reaction), causing it to move backwards.; The force that the person exerts on the boat causes the boat to exert an equal and opposite force back on the person, so the boat moves backwards.
Common mistakes: Describing only the movement not the forces; Omitting explanation of equal and opposite
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Marking points
B1 · 1 mark · Mentions that the forward push causes a backward force on the boat due to Newton's third law.
B2 · 1 mark · States forces are equal in size and opposite in direction.
Worked solution
When the person jumps forward off the boat, she exerts a force on the boat backwards. According to Newton’s third law, the boat exerts an equal and opposite force on the person. This equal and opposite pair of forces causes the boat to move backwards.