Unit 5, lesson 26. About 15 minutes.

Why seatbelts and helmets save lives

Why does a soft landing hurt less than a hard one, at the same speed?

Play

Getting the experiment ready…

Explain

Soft things help you stop slowly.

Stopping slowly keeps you safe.

When a moving body stops, it needs a force. Stop it in a tiny moment, and the force is huge.

A helmet, a seatbelt or an airbag makes the stop take a little longer. That makes the force much smaller.

That's why they save lives. Always wear your seatbelt, and your helmet on a bike.

Impulse: F × t = Δp. For a given change in momentum, more stopping time means less force.

Crumple zones, airbags, seatbelts and helmets all stretch out the stopping time.

Worked example: 40 kg stops from 10 m/s. In 0.01 s, F = 400 / 0.01 = 40 000 N. In 0.2 s, F = 2000 N.

Sudden stop: big force
Slower stop: small force
Bodies stay safe

Want the full story?

These are average forces, and the peak force is higher. The stopping times are rough examples. Real crash protection is about spreading the force over your body as well as reducing it.

The Formula

force = change in momentum ÷ stopping time

F \times t = \Delta p \Rightarrow F = \frac{m \Delta v}{t}

$F$: the average force while stopping, measured in newtons (N)
$t$: the stopping time, measured in seconds (s)
$\Delta p$: the change in momentum, measured in kilogram metres per second (kg m/s)
$m$: the mass, measured in kilograms (kg)
$\Delta v$: the change in speed, measured in metres per second (m/s)

Try the numbers

Mass: 40 kgSpeed: 10 m/sStopping time: 0.1 s

Force while stopping: 4,000 N

Try it at home

You need a soft ball, a pillow, a table.

You need: a soft ball, a pillow, a table.

Drop a ball on a pillow. So soft!

Drop a soft ball on a hard floor and listen.
Drop it from the same height onto a pillow.
Compare how each ball stops.

What's happening: the pillow gives way, so the ball takes longer to stop, and the force is smaller.

Physics is life

Your body

You bend your knees when you land.

Bending your knees when you land makes your stop take longer, which protects your legs.

Daily life

Seatbelts keep you safe.

Seatbelts and airbags greatly reduce injuries in crashes.

Nature

Cats land softly on bendy legs.

Cats bend their legs as they land, which makes their stop take longer and keeps them safe.

Space

Airbags helped robots land on Mars.

Some Mars rovers bounced to a safe landing, using giant airbags.

Quick check

Question 1 of 3

How does a seatbelt help?

Words to know

Impulse: Force multiplied by time. It equals the change in momentum.

Impulse: Force multiplied by time. It equals the change in momentum.
Momentum: Mass multiplied by velocity. It tells you how hard something is to stop.
Force: A push or a pull on something.

Next up

Levers