Momentum Matters
Mass on the move. Master momentum and its conservation, then use the force-momentum equation to explain how airbags and crumple zones save lives by stretching a crash out in time.
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Momentum Matters ⚡
A rolling marble is easy to stop; a rolling lorry is not. The difference is **momentum** — a measure of how much motion an object carries. Momentum turns out to be one of the most powerful ideas in physics: it is always **conserved**, and it explains how car safety works. (Higher tier.)
Momentum 🚂
**Momentum** is a property of every moving object: **p = m v** (momentum = mass × velocity) It is measured in **kg·m/s**. Like velocity, momentum is a **vector** — it has a direction, so momentum in opposite directions can cancel out.
Find the momentum
An interactive activity.
Conservation of momentum 🔄
In a **closed system** — one with no external forces — **momentum is conserved**. This means the **total momentum before** an event equals the **total momentum after** it. It holds for **collisions** (objects hitting each other) and **explosions** (objects pushing apart) alike.
A sticky collision
An interactive activity.
Two ice skaters
An interactive activity.
True about momentum
Select ALL THREE statements about momentum that are TRUE.
- Momentum = mass × velocity
- In a closed system, total momentum is conserved
- Momentum is a vector, so direction matters
- Only objects that are speeding up have momentum
- Total momentum increases in every collision
- A stationary object has a large momentum
Force changes momentum 📉
A resultant force changes an object's momentum. In fact the force equals the **rate of change of momentum**: **F = m Δv / Δt** The key insight: the **same change in momentum** spread over a **longer time (Δt)** needs a **smaller force**.
The force of a crash
An interactive activity.
Saving lives 🛡️
This is the science behind car safety. **Seatbelts, airbags and crumple zones** all work the same way: they **increase the time** over which a passenger's momentum falls to zero in a crash. Because F = m Δv / Δt, a **longer time** means a **smaller force** on the body — and a smaller force means fewer and less serious injuries.
Match each safety feature to how it works
- Crumple zone
- Airbag
- Seatbelt
- Cycle helmet
- The front of the car folds up, making the collision last longer
- A cushion inflates so the head decelerates more gradually
- It stretches a little to spread the stop over more time
- Its foam crushes to extend the time the head takes to stop
Why an airbag helps
How does an airbag reduce the force on a passenger in a crash?
- It increases the time over which the passenger's momentum changes, so the force is smaller
- It increases the passenger's momentum
- It reduces the passenger's mass
- It makes the collision happen in a shorter time
How a crumple zone protects you
An interactive activity.
Momentum summary
Momentum is p = m _____, measured in kg·m/s. In a closed system, total momentum is _____: the total before an event equals the total after. Force is the rate of change of momentum, so spreading a momentum change over a _____ time reduces the force — which is how airbags and crumple zones _____ injury.