A football is kicked and travels across a grass pitch before coming to a stop. Explain, using Newton's laws of motion, why the football eventually stops, and describe what would happen to the football's momentum as it slows down.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A footballer kicks a stationary ball. The ball rolls along the grass and gradually slows down before stopping completely.
Model answer (5 marks)
A resultant (unbalanced) force acts on the football as it rolls.
The frictional force from the grass acts opposite to the direction of motion.
According to Newton’s first law, a body in motion will remain in motion unless a resultant force acts on it; the friction provides this force, so the ball decelerates.
Newton’s second law states that a resultant force causes a change in momentum; the frictional force therefore reduces the ball’s momentum.
As the ball slows, its velocity – and hence its momentum – decreases until both reach zero when the ball stops.
The frictional force from the grass acts opposite to the direction of motion.
According to Newton’s first law, a body in motion will remain in motion unless a resultant force acts on it; the friction provides this force, so the ball decelerates.
Newton’s second law states that a resultant force causes a change in momentum; the frictional force therefore reduces the ball’s momentum.
As the ball slows, its velocity – and hence its momentum – decreases until both reach zero when the ball stops.
Examiner tips
- Use the exact terms ‘resultant force’, ‘friction’, ‘decelerate’, ‘momentum’, ‘velocity’
- Show the causal chain: friction → resultant force → deceleration → change in momentum
- Keep the answer concise – 5 points, not more than 5 sentences
Common mistakes
- Confusing friction with air resistance or ignoring that friction is the unbalanced force
- Failing to mention that momentum decreases because velocity decreases
- Using vague language such as ‘the ball slows down’ without linking to Newton’s laws
Mark scheme (5 marks)
- A resultant (unbalanced) force acts on the football as it rolls
- Friction (from the grass) acts in the opposite direction to the ball's motion
- Newton's first law: an object will remain in motion unless acted upon by a resultant force — the friction provides this resultant force and causes the ball to decelerate
- Newton's second law: the resultant force causes a change in momentum / the ball decelerates (its velocity decreases)
- The momentum of the ball decreases (as velocity decreases) until it reaches zero when the ball stops
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Newton's laws and momentum questions
- A delivery driver notices that when two identical electric vehicles (EVs) are be…
- A car travelling at high speed collides with a stationary barrier and comes to r…
- A skateboarder standing still on a flat, frictionless surface pushes against a w…
- A rocket is launched from rest into space. Once in deep space, far from any grav…