A stationary football is kicked by a player. Explain, using Newton's laws, why the ball accelerates when kicked and what happens to the player's foot as a result of kicking the ball.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A footballer places a ball on the ground before taking a free kick. Before the kick, both the ball and the player's foot are stationary.
Model answer (4 marks)
Before the kick the ball is at rest because the forces on it cancel, so its resultant force is zero (Newton’s First Law). When the player’s foot strikes the ball a net force is applied to the ball, so it accelerates in the direction of that force (Newton’s Second Law). By Newton’s Third Law the ball exerts an equal and opposite force on the player’s foot. The foot and the ball are different objects, so the forces act on different bodies and do not cancel each other out.
Examiner tips
- State the three relevant laws explicitly
- Explain the direction of the forces and the resulting acceleration
- Show that the forces act on different objects so they don’t cancel
Common mistakes
- Confusing the direction of the force on the ball with the force on the foot
- Saying the forces cancel out instead of acting on different bodies
- Forgetting to mention the ball’s initial rest state
Mark scheme (4 marks)
- Before the kick, the ball is stationary because the resultant force on it is zero (Newton's First Law)
- The kick applies a resultant force to the ball, causing it to accelerate (Newton's Second Law)
- The ball exerts an equal and opposite force back on the player's foot (Newton's Third Law)
- The foot and ball are different objects / the Newton's Third Law forces act on different objects so they do not cancel out
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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