Changes in momentum (physics only) — AQA GCSE Physics
Test yourself on Changes in momentum (physics only) with AQA GCSE practice questions.
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Changes in momentum (physics only) explained
The equations in this topic interlock. Acceleration is the rate of change of velocity, a = Δv ÷ t; resultant force is mass × acceleration, F = m a; and putting them together gives F = m Δv ÷ t, which is the rate of change of momentum.
Read the full explanation
Reading them together shows why a longer collision time means a smaller force: the change of velocity is fixed, so a larger t makes a smaller a and therefore a smaller F. The same chain explains why a heavier vehicle needs a larger force to change its velocity in the same time. Choose the equation that contains the quantity you want and the quantities you are given. Units are the quickest check on a rearrangement, since a force must come out in kg m/s², which is the newton.
Your focus
- Write down the equation linking acceleration, change in velocity and time taken, using change in velocity rather than velocity, and rearrange it to make the change in velocity the subject.
- Calculate the average force on a passenger from their mass, change of velocity and the time of a collision.
- Explain, using two linked equations, why doubling the collision time halves the force on the object.
Changes in momentum (physics only) exam tips
Marking Points
- one mark for selecting the correct equation, a = Δv ÷ t or resultant force = mass × acceleration
- one mark for a correct substitution with consistent units
- one mark for showing the rearrangement of the equation
- one mark for the answer with the correct unit
- one mark for linking a longer contact time to a smaller acceleration and so a smaller force
Examiner Tips
- 💡List what you are given with units before choosing an equation; the choice then makes itself.
- 💡Show the rearranged equation as its own line so a substitution slip does not cost every mark.
- 💡Where two equations are needed, calculate the acceleration first and carry the full value forward.
Common Mistakes
- writing a = t ÷ Δv, which mark schemes specifically refuse
- using a final velocity where a change in velocity is required
- mixing a mass in grams with a force in newtons in the same substitution
- giving acceleration the unit m/s instead of m/s²