Changes in Momentum and Impact Forces
Revise Changes in Momentum and Impact Forces for GCSE Physics with this free worksheet and full mark scheme — Higher-tier exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) physics topic: force is equal to the rate of change of momentum, so spreading an impact over a longer time reduces the force, as safety features do.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
This is a Higher tier only topic, so there's no Foundation paper — only the Higher worksheet and mark scheme below.
Topic overview
When a force acts on an object that is moving, or able to move, the object experiences a change in momentum. This is a Triple-only and Higher-only topic.
The equation is force = change in momentum ÷ time taken, F = Δp ÷ Δt. This is another form of Newton's second law, and it shows that force equals the rate of change of momentum.
The crucial consequence is that a longer time means a smaller force for the same change in momentum. This is why safety features work — crumple zones, air bags, seat belts, cycle helmets and crash mats all increase the time over which momentum changes, reducing the force on the person and therefore the risk of injury.
Revision notes
The equation
Force = change in momentum ÷ time taken, F = Δp ÷ Δt.
Force in newtons, change in momentum in kg m/s, time in seconds. Force equals the rate of change of momentum.
Why time matters
For a given change in momentum, increasing the time over which it happens decreases the force.
The change in momentum is fixed by the mass and velocity change, so the only way to reduce the force is to extend the time. This is the principle behind every safety feature.
Safety features
Crumple zones in cars deform on impact, extending the time of the collision.
Air bags, seat belts, cycle helmets, gymnasium crash mats and safety padding all work the same way. Each increases the impact time, reducing the force experienced and therefore the injury.
Key points
- Force = change in momentum ÷ time taken.
- Force equals the rate of change of momentum.
- This is another form of Newton's second law.
- A longer time gives a smaller force.
- Crumple zones extend the collision time.
- Air bags and seat belts work the same way.
Worked examples
Example 1
A 0.2 kg ball's momentum changes by 4 kg m/s in 0.5 s. Calculate the force. [2 marks]
Model answer
Force = change in momentum ÷ time = 4 ÷ 0.5 (1 mark) = 8 N (1 mark).
Example 2
Explain how a car's crumple zone reduces injury in a crash. [3 marks]
Model answer
The crumple zone deforms during the collision, increasing the time over which the car comes to a stop (1 mark). The change in momentum is the same, but force equals change in momentum divided by time (1 mark), so a longer time means a smaller force acting on the occupants and less injury (1 mark).
Example 3
Explain why a gymnast lands on a crash mat rather than a hard floor. [2 marks]
Model answer
The crash mat increases the time over which the gymnast comes to a stop (1 mark), so the force on them is smaller for the same change in momentum, reducing the risk of injury (1 mark).
Common mistakes
Saying the safety feature reduces the change in momentum.
The change in momentum is the same; it is the TIME that is extended.
Confusing force with momentum.
Force is the rate of change of momentum, measured in newtons.
Not stating the equation in the explanation.
Citing F = Δp ÷ Δt is what earns the explanation mark.
Giving only one safety feature.
Several are examinable and all work by the same mechanism.
Exam tips
- Always say the change in momentum is unchanged and the time is extended.
- Cite the equation when explaining safety features.
- Learn several examples that work by the same principle.
- Remember this is Triple-only AND Higher-only.
Key terms
- Rate of change of momentum
- How quickly momentum changes, equal to the force.
- Crumple zone
- A car region that deforms to extend collision time.
- Impact force
- The force experienced during a collision.
- Change in momentum
- Final momentum minus initial momentum.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.