A football is kicked horizontally along a flat grass pitch. The ball slows down and eventually stops. Explain why the ball decelerates after it has been kicked, using Newton's laws of motion.

Eduqas GCSE Physics — 4.2 Forces, accelerations and Newton's laws of motion · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A friction force acts on the ball from the grass, opposing its motion.
The forces on the ball are unbalanced, giving a resultant force.
By Newton’s second law, the resultant force causes an acceleration.
Because the resultant (friction) force is opposite to the direction of motion, the ball decelerates.

Examiner tips

  • Use the word "friction" and state it opposes motion. Mention that the forces are unbalanced to justify a resultant. Link the resultant to deceleration via Newton’s second law. Keep the answer concise and use correct terminology.

Common mistakes

  • Forgetting to mention that friction opposes motion. Saying the ball slows due to air resistance instead of grass friction. Using vague terms like "something slows it down" without specifying the force.

Mark scheme (4 marks)

  1. A friction force acts on the ball (from the grass) opposing its motion
  2. The forces on the ball are unbalanced / there is a resultant force acting on the ball
  3. Newton's second law: a resultant force causes acceleration (or deceleration)
  4. The ball decelerates because the resultant (friction) force acts in the opposite direction to the ball's motion

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