Home / IGCSE 0625 / Motion, forces, energy / Impulse
IGCSE 0625 / Section 1.6 / Extended
Supplement (Extended) content

Spread the change over more time.

A china plate shatters on concrete but survives on a carpet, from the very same fall. The change in momentum is identical; what differs is the time taken to stop. Stretch that time out and the force drops, which is the whole idea behind airbags, crumple zones, and bending your knees as you land.

Concept mapMomentum → Skill mapOpen the star map →
The key idea

The resultant force equals the rate of change of momentum: F = Δp ÷ Δt = (mv − mu) ÷ t. For a given change in momentum, a longer time produces a smaller force, and a shorter time a larger one.

Definition · learn the exact words

The resultant force on an object is equal to the rate of change of its momentum.

F = (mv − mu) / t

For a fixed change in momentum, a longer time of change gives a smaller force.

Section 01

Stretch the time, soften the blow.

Set the same change in momentum, then vary the time it takes. Watch the force readout: the slower the change, the smaller the force, even though the momentum change is identical.

Stage 1 · Learn

Check force and momentum change

Four quick checks on the rate of change of momentum and the time effect. Each correct answer earns XP and lights this skill on your star map.

Quick check+10 XP

The resultant force on an object equals the...

Quick check+10 XP

For a given change in momentum, increasing the time over which it happens makes the force...

Quick check+10 XP

Car crumple zones reduce the force in a crash by...

Quick check+10 XP

Which equation gives the resultant force from a change in momentum?

Section 02

Why time is the lever.

Worked example

A 0.40 kg ball travelling at 5.0 m/s is caught and brought to rest in 0.10 s. Find the average force on the ball.

Step 1 · Change in momentumΔp = m(v − u) = 0.40 × (0 − 5.0) = −2.0 kg m/s
Step 2 · Force from F = Δp / tF = 2.0 ÷ 0.10 = 20 N
Step 3 · AnswerThe average force is 20 N, acting to slow the ball down.
Examiner trap

When a question asks why a safety feature reduces the force, the answer is the time. The change in momentum is set by the masses and speeds and cannot be altered. It is the longer time of the collision that lowers the force, since F = change in momentum divided by time.

Stage 2 · Exam

Exam-style questions

Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 2.

Finish the four checks above to unlock the exam questions
Exam style+20 XP

A 0.50 kg ball hits a wall at 8.0 m/s and bounces straight back at 8.0 m/s. If the contact lasts 0.20 s, the average force is...

Exam style+20 XP

A 2.0 kg trolley is brought to rest from 6.0 m/s in 3.0 s. The average resultant force is...

Exam style+20 XP

A cricketer moves their hands backwards while catching a fast ball. This reduces the force on their hands because it...

Skill unlocked

Impulse and force, mastered.

This skill is now lit gold on your star map. That completes the momentum topic.

-Rank -Level -Score -Topics
Go deeper · practice
Six original Cambridge-style questions
Force as the rate of change of momentum, the F = (mv minus mu)/t calculation, and why lengthening a collision time reduces the force. Attempt each, then reveal the worked solution.
Stage 3 · master the unit
Motion, forces and energy challenge
Mixed questions across the whole unit, each one worth XP. Start this only when you feel confident across every topic in the unit, not just impulse.
Start the unit challenge →