Safety Devices
Seat belts, airbags, crumple zones, and helmets
Do Now: Old cars were built rigid and strong, and barely dented in crashes. Modern cars crumple. Which is safer for the people inside? Why?
Modern cars. A car that crumples takes longer to stop, so the people inside feel a smaller force. A rigid car stops almost instantly.
A. Crumple Zones
B. Seat Belts and Airbags
Seat belts make you slow down with the car instead of flying forward and stopping suddenly against the dashboard or windshield. The belt stretches a little, adding stopping time, and spreads the force across your strong hips and chest.
Airbags inflate in a few hundredths of a second and then let gas out as you push into them. This adds stopping time and spreads the force over your head and chest.
Airbags work with seat belts, not instead of them. Without a belt, you can hit the airbag too fast or in the wrong position.
C. Helmets
Increase time: the foam crushes, so your head stops over a longer time.
Spread force: the hard shell spreads the force over a larger area of your head.
D. Same Physics Everywhere
| Device | How It Increases Stopping Time |
|---|---|
| Crumple zone | The car’s front crushes gradually |
| Seat belt | Stretches slightly; keeps you slowing with the car |
| Airbag | Cushions and slowly deflates |
| Helmet | Foam liner crushes |
| Football pads | Foam compresses |
| Highway crash barrels | Sand-filled barrels spread a crash over a longer distance |
| Bending your knees | Your legs slow your body over a longer time |
| Parachute | Lowers your landing speed, so there’s less momentum to lose |
Explore: Crash Test Lab
Choose a speed and protection, then press Crash! Compare the force graph with the previous trial.
Find the combination of protection that gives the smallest force at 20 m/s. How many times smaller is it than no seat belt in a rigid car?
Crumple zone + seat belt + airbag gives the longest stopping time (0.18 s) vs 0.010 s with no belt — 18 times smaller force.
Check Your Understanding
5 quick questions — pick an answer to see if you’re right.
1. A crumple zone makes a car safer because it
2. Why should a bike helmet be replaced after a big crash?
3. Airbags reduce injuries by
4. Which does NOT reduce the force by increasing stopping time?
5. A seat belt helps mainly because it