---
title: "Impulse and Crash Safety"
description: "Force equals the change in momentum divided by the time it takes. Stretching a collision over a longer time lowers the force, which is what airbags, crumple zones and crash mats are for."
canonical: https://lightmysky.com/learn/science/impulse-and-crash-safety-mt_BjLAFw2zjU
source: https://lightmysky.com/learn/science/impulse-and-crash-safety-mt_BjLAFw2zjU.md
retrieved: 2026-09-12
---

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# Impulse and Crash Safety

Force equals the change in momentum divided by the time it takes. Stretching a collision over a longer time lowers the force, which is what airbags, crumple zones and crash mats are for.

Subject: Science · Area: Forces & Motion · Ages 15 to 16
Page: https://lightmysky.com/learn/science/impulse-and-crash-safety-mt_BjLAFw2zjU

## Ready when they can

- Calculates the average force from a change in momentum and a contact time
- Explains a safety feature by naming the quantity it changes and the one it leaves alone
- Says why a longer stopping time gives a smaller force when the momentum change is fixed

## Lesson: Stretch the stop, shrink the force

Impulse links force, time, and momentum in one equation for you: average force times contact time = change in momentum. Rearranged, average force = momentum change divided by contact time. Short times mean brutal forces, which is exactly why helmets and mats exist.

**Example.** A crash dummy's momentum falls by 1200 kg m/s in a collision. Against a rigid wheel the stop takes 0.1 s, so the force is 1200 divided by 0.1 = 12000 N. A crumple zone stretches the same stop to 0.4 s, giving 1200 divided by 0.4 = 3000 N: same change, quarter the force.

Every safety feature answers two questions: what does it change, and what stays fixed. Airbags, seatbelts, and crumple zones stretch the stopping time. The momentum change stays fixed by the masses and speeds, so spreading it over longer always means a smaller force.

**Tip.** For explain questions, write the chain in order: state the momentum change, state the longer time, then conclude the force falls. Longer stopping time with the same momentum change always means a smaller force.

**Recap.** Same momentum change over a longer time means a smaller force, and that trade is what safety kit buys.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Momentum and Its Conservation](https://lightmysky.com/learn/science/momentum-and-its-conservation-mt_Nn5_DMbK1_)

## Opens up

- [Calculating Work Done and Power](https://lightmysky.com/learn/science/calculating-work-done-and-power-mt_udulQxseg1)