NEWTON'S THIRD LAW: ACTION AND REACTION
2 marks · EasyWhen a fish swims by pushing water backwards with its fins, it moves forward. Use Newton's third law to explain why the fish moves forward.
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The fish pushes water backwards (action). According to Newton’s third law, the water pushes the fish forward with an equal and opposite force (reaction), moving the fish forward.
Must show: Fish pushes water backwards, Water pushes fish forward with equal and opposite force
Also accept: The reaction force from the water pushes the fish forwards
Common mistakes: Saying fish moves forward only because of its muscles; Not mentioning Newton’s third law or opposite direction
Show mark scheme and worked solution
Marking points
B1 · 1 mark · States that the fish pushes water backwards (action)
B2 · 1 mark · States that the water pushes the fish forwards with equal and opposite force (reaction)
Worked solution
When the fish pushes water backwards with its fins, the water pushes the fish forward with an equal and opposite force, according to Newton’s third law. This makes the fish move forward.