A bowling ball rolling along a smooth lane strikes a stationary skittle. The skittle flies forwards rapidly whilst the bowling ball slows down but continues to move in the same direction. Explain, using Newton's laws of motion and the concept of momentum, why the bowling ball slows down when it strikes the skittle, and why the skittle moves off faster than the bowling ball after the collision.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A bowling ball of large mass rolls along a smooth lane at a steady speed and collides with a single, much lighter stationary skittle. After the collision, the skittle shoots forwards at high speed whilst the bowling ball continues forwards but at a noticeably slower speed.
Model answer (5 marks)
Before the collision only the bowling ball has momentum, the skittle is stationary.
During the collision the skittle exerts an equal and opposite force on the ball (Newton’s third law). This backward force on the ball causes it to decelerate, so its momentum decreases (Newton’s second law).
The total momentum of the system is conserved. Therefore the momentum lost by the ball is gained by the skittle.
Because the skittle’s mass is much smaller than the ball’s, the same amount of momentum gives it a much larger velocity, so it flies off faster than the ball.
During the collision the skittle exerts an equal and opposite force on the ball (Newton’s third law). This backward force on the ball causes it to decelerate, so its momentum decreases (Newton’s second law).
The total momentum of the system is conserved. Therefore the momentum lost by the ball is gained by the skittle.
Because the skittle’s mass is much smaller than the ball’s, the same amount of momentum gives it a much larger velocity, so it flies off faster than the ball.
Examiner tips
- Use the exact wording ‘momentum’ and ‘conserved’ – examiners look for these terms.
- Show the chain: third law → force on ball → second law → deceleration → conservation → velocity difference.
- Keep the answer short and to the point – 5 marks can be earned with 5 clear points.
Common mistakes
- Confusing force with momentum – students write ‘force causes the ball to slow’ without linking to momentum.
- Using ‘energy’ instead of ‘momentum’ – the question is about momentum conservation.
- Failing to mention the skittle’s small mass as the reason for its higher speed.
Mark scheme (5 marks)
- Before the collision only the bowling ball has momentum (the skittle is stationary / has zero momentum)
- Newton's third law: the skittle exerts an equal and opposite force on the bowling ball (as the bowling ball exerts on the skittle)
- The resultant (backwards/decelerating) force on the bowling ball causes it to decelerate / lose momentum (Newton's second law)
- Total momentum is conserved (the total momentum after the collision equals the total momentum before)
- Because the skittle has a much smaller mass than the bowling ball, it must have a much greater velocity to account for the momentum it gains, so it moves off faster than the bowling ball
Key terms in this question
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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