NEWTON'S THIRD LAW: ACTION AND REACTION
2 marks · NormalA student stands on a skateboard in a stationary position. She pushes against a wall with her hands and starts moving backwards.
Explain, using Newton's third law, why the student moves backwards when she pushes against the wall.
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.
Show answer
When the student pushes on the wall, the wall pushes back with an equal and opposite force, causing the student to move backwards as described by Newton's third law.
Also accept: The force student exerts on the wall is equal in size and opposite in direction to the force from the wall pushing her back, so she moves backwards.
Common mistakes: Describing only the action force and not the reaction; Confusing action-reaction with balanced forces
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States that when the student pushes on the wall, the wall pushes back on her in the opposite direction.
B2 (a) · 1 mark · Links this to Newton's third law (every action has an equal and opposite reaction).
Worked solution
When the student pushes the wall, according to Newton's third law, the wall pushes back with an equal and opposite force on the student. This reaction force causes the student to move backwards.