Exam Focus: Force and Collisions

Frequently Examined

Across all four boards (AQA, Edexcel, OCR A and OCR B) this topic is about momentum in collisions. How far it goes depends on your board, so read the line for yours. Wherever a board asks for force and change in momentum, it is Higher tier content.

AQA: force equals the rate of change of momentum (F = Δp ÷ t), and explaining safety features such as air bags, seat belts, crash mats and cycle helmets, are Physics only (separate science) content. Both are Higher tier.

Edexcel: force = change in momentum ÷ time is Higher tier content. It is in both Physics and Combined Science.

OCR B: change in momentum = resultant force × time is Higher tier content. OCR B also says that the longer an impact lasts, the smaller the average force is for a given change in momentum.

OCR A: you need momentum (p = mv) and examples of momentum in collisions, at Higher tier. You do not need the equation linking force, change in momentum and time, so you can skip those parts of this page.

Questions use real world safety scenarios ("A car travelling at 12 m/s is brought to rest in 0.8 s…") and ask you to apply F = Δp ÷ t.

Key question types:

• Calculate the change in momentum: use Δp = mv − mu; show your working and give the unit kg m/s
• Calculate the average force: use F = Δp ÷ t, then link the answer to safety features
• Explain why safety features reduce injury: increasing the time of the collision reduces the force for the same change in momentum
• Elastic vs inelastic collision distinction: state which quantity is conserved in each
• Direction questions: an object that bounces back has a bigger change in momentum than one that stops

Exam style note: Questions often use the "Suggest" command word for unfamiliar safety scenarios. Apply the same rate of change of momentum idea even if the context is new (helmet, airbag, crumple zone, crash mat, catching a ball).