NEWTON'S THIRD LAW: ACTION AND REACTION
3 marks · NormalWhen a swimmer pushes water backwards with her hands while swimming, she moves forwards.
Explain, using Newton's third law, how the swimmer is able to move forward through the water. Include a clear identification of the pairs of forces involved.
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 swimmer pushes backward on the water, the water pushes forward on her with an equal and opposite force. This reaction force from the water propels her forward. The action is the swimmer's hand applying a backward force on the water, and the reaction is the water applying an equal forward force on the swimmer's hand.
Also accept: The swimmer applies a backward force on the water and, at the same time, experiences an equal forward force from the water, causing her to move forward.
Common mistakes: Only stating that the swimmer moves forward without mentioning the reaction force from the water; Mixing up action-reaction pair locations
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly states that the swimmer pushes water backwards.
B2 (a) · 1 mark · States that the water pushes the swimmer forward equally.
B3 (a) · 1 mark · Links this to Newton's third law in terms of action and reaction force pairs acting on different objects.
Worked solution
The swimmer applies a force backwards on the water using her hands. According to Newton's third law, the water pushes back on the swimmer's hands with an equal force in the opposite direction (forwards). The action and reaction forces act on different objects. The reaction force from the water pushes the swimmer forward through the water.