A football player kicks a stationary ball. The ball accelerates rapidly and travels across the pitch. Explain, using Newton's laws of motion, why the ball accelerates when kicked and why it eventually slows down and stops.

Edexcel A-Level Physics (9PH0) — 2.2 Forces, Newton's laws and momentum · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

The foot applies a force to the ball, giving a resultant force that changes its velocity – Newton’s second law (F=ma) shows the ball accelerates.
The ball’s motion creates an equal and opposite force on the foot – Newton’s third law.
Once the ball is moving, friction with the ground and air resistance act opposite to its direction of travel.
These resistive forces form a resultant force that opposes the ball’s motion, so by Newton’s first law the ball decelerates and eventually stops.

Examiner tips

  • Use the correct law names (first, second, third) and link each to the described force. Show the direction of forces relative to motion. Mention both friction and air resistance as resistive forces.
  • common_mistakes
  • :
  • Confusing the direction of the force with the direction of acceleration. Forgetting to mention the ball’s equal and opposite force on the foot. Saying the ball slows because of “gravity” instead of friction/air resistance.

Mark scheme (5 marks)

  1. The player's foot exerts a force on the ball (Newton's second or first law application — a resultant force causes acceleration)
  2. A resultant force causes the ball to accelerate (Newton's second law — resultant force causes a change in velocity / acceleration)
  3. Newton's third law: the ball exerts an equal and opposite force back on the foot
  4. Once the ball is in motion, friction (from the ground) and / or air resistance act on the ball in the opposite direction to motion
  5. These resistive forces produce a resultant force opposing motion, causing the ball to decelerate and eventually stop (Newton's first law — a resultant force changes the state of motion)

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