What mistakes do students make with Changes in momentum (physics only)?

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.

What are the exam tips for Changes in momentum (physics only) in Physics AQA GCSE?

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.

AQA · GCSE · Physics

Course version: 8463 · Version from Sept 2016

Students should be able to apply equations relating force, mass, velocity…

Full specification wording

Changes in momentum (physics only): Students should be able to apply equations relating force, mass, velocity and acceleration to explain how the changes involved are inter-related.

The equations in this topic interlock.

Checked against the AQA 8463 specification (opens in a new tab) on 22 September 2026.

Point 3 of 3
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Selecting the correct equation, a = Δv ÷ t or resultant force = mass × acceleration.
A correct substitution with consistent units.
Showing the rearrangement of the equation.
The answer with the correct unit.
Linking a longer contact time to a smaller acceleration and so a smaller force.

Common mistakes and how to put them right

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.

Exam 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.

Try the lesson

The start of the Changes in momentum (physics only): Students should be able to apply equations relating force, mass, velocity and acceleration to explain how the changes involved are inter-related. lesson, made from checked teaching notes for this specification point.

Have a go first

A 1500 kg car accelerates from rest to 20 m/s in 8 s. Calculate the resultant force acting on the car.

Tip: press 1 to 4 to choose

Listen

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. 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. A strong answer earns marks for these points. selecting the correct equation, a = Δv ÷ t or resultant force = mass × acceleration. a correct substitution with consistent units. showing the rearrangement of the equation. the answer with the correct unit. Exam tip: List what you are given with units before choosing an equation; the choice then makes itself.

See it

A flowchart running from top to bottom. A box labelled 'Change in velocity Δv and time t' leads to a box 'a = Δv ÷ t', which leads to 'Acceleration a'. A separate box 'Mass m' leads to 'F = m a', and 'Acceleration a' also leads into 'F = m a', which leads to 'Resultant force F'. 'a = Δv ÷ t' and 'Mass m' both lead into 'F = m Δv ÷ t', which also leads to 'Resultant force F'. Finally, 'Longer collision time t' leads to 'Smaller acceleration a', which leads to 'Smaller force F'. — It shows how the equations a = Δv ÷ t and F = m a combine into F = m Δv ÷ t, and how a longer collision time leads to a smaller acceleration and so a smaller force.

Key words

Say what each word means before you turn the card.

a = Δv ÷ t

Try answering aloud or in your head before turning the card.

F = m a

Try answering aloud or in your head before turning the card.

See every definition

a = Δv ÷ t: Acceleration is the rate of change of velocity, found by dividing the change in velocity by the time taken.
F = m a: Resultant force is mass multiplied by acceleration.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A flowchart running from top to bottom. A box labelled 'Change in velocity Δv and time t' leads to a box 'a = Δv ÷ t', which leads to 'Acceleration a'. A separate box 'Mass m' leads to 'F = m a', and 'Acceleration a' also leads into 'F = m a', which leads to 'Resultant force F'. 'a = Δv ÷ t' and 'Mass m' both lead into 'F = m Δv ÷ t', which also leads to 'Resultant force F'. Finally, 'Longer collision time t' leads to 'Smaller acceleration a', which leads to 'Smaller force F'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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