Preventive safety devices (active safety)
These work before a crash, to help the driver avoid it.
- ABS (anti-lock braking system): sensors on each wheel notice when a wheel is about to lock. The system releases and presses the brake many times a second, so the wheel keeps rolling. You keep steering control and often stop in a shorter distance.
- Traction control: cuts engine power if a driven wheel spins on a slippery road.
- ESC (electronic stability control): senses a skid and brakes single wheels to bring the car back on its path.
- Automatic emergency braking (AEB): radar or camera watches the road, warns, and then brakes by itself if the driver does not.
- Others: lane keeping, blind-spot warning, adaptive cruise control, rear camera, tyre pressure warning, good lights and wipers, and a driver-attention alert.
All of them depend on sensors, a computer and brake or engine control, but they cannot beat the laws of physics: speed still matters most.
Collision safety devices (passive safety)
These work during a crash, to cut the harm to people.
- Crumple zones and rigid cell: the front and back crush and take energy; the cabin stays whole.
- Seat belt: holds the body to the seat so it slows with the car. A pretensioner tightens the belt in the first instant, and a load limiter lets it give a little so the chest is not hurt.
- Airbags: sensors detect a hard hit and a gas inflates the bag in about 0.03 seconds. It spreads the force over a larger area of head and chest. It works with the belt, not instead of it.
- Head restraints, side airbags and curtains, child seats and pedestrian-friendly bonnets also help.
The main idea: make the person stop over a longer time and a larger area. Force = change in momentum ÷ time, so more time means less force.
Stopping distance: why speed matters
Stopping distance = reaction distance + braking distance. Reaction distance = speed × reaction time (about 1 s for a person, about 0.25 s for radar braking). Braking distance = v² ÷ (2a), where a is the braking deceleration (about 7 m/s² on dry road). Double the speed: reaction distance doubles, braking distance becomes four times. Wet roads cut a to about 4 m/s².
Key formulas and definitions
- Stopping distance = speed × reaction time + v² ÷ (2a)
- v in m/s = km/h ÷ 3.6
- Force = change in momentum ÷ time (longer time = smaller force)
- Active (preventive) = avoids the crash; passive (collision) = protects in the crash
- Key terms: ABS, ESC, AEB, crumple zone, pretensioner, airbag
Worked examples
1. Convert 72 km/h to m/s.
72 ÷ 3.6 = 20 m/s.
2. A driver at 20 m/s reacts in 1 s. How far does the car go before braking starts?
Reaction distance = 20 × 1 = 20 m.
3. At 20 m/s with deceleration 8 m/s², find the braking distance.
v² ÷ (2a) = 400 ÷ 16 = 25 m.
4. Total stopping distance for the driver in examples 2 and 3 (use a = 8)?
20 m + 25 m = 45 m.
5. With auto brake (reaction 0.25 s) the same car at 20 m/s, a = 8. Find the stopping distance.
20 × 0.25 = 5 m, plus 25 m = 30 m. It saves 15 m.
6. A 70 kg person moving at 10 m/s is stopped in 0.05 s without a belt, and in 0.2 s with a belt and airbag. Compare the average forces.
Change in momentum = 70 × 10 = 700 kg·m/s. Without: 700 ÷ 0.05 = 14 000 N. With: 700 ÷ 0.2 = 3 500 N. The force is four times smaller.
Common mistakes
- Thinking airbags replace the seat belt. They need the belt; without it an airbag can hurt.
- Thinking ABS always shortens the stopping distance. On loose gravel or snow it can be longer, but you keep steering.
- Thinking safety devices let you drive faster. Physics is the same; speed still decides the crash force.
- Mixing active and passive safety. Active tries to avoid a crash; passive protects inside the crash.