Section 1 of 5The Physics Behind Safety
The Physics Behind Safety
Same momentum change
- A person moving at speed must lose all their momentum in a crash, however long the stop takes.
Force equation
- Since F = Δp ÷ t, a longer stopping time for the same Δp gives a smaller average force.
Smaller force
- A smaller force on the body means less chance of serious injury or damage to organs.
Energy
- Safety features also absorb kinetic energy by deforming, so less energy is transferred to the passenger.
Section 2 of 5Safety Features in Vehicles
Crumple zones
- The front and rear of a car collapse in a controlled way, which lengthens the collision time and absorbs energy.
Seat belts
- They stretch slightly as they hold a passenger, which slows the person over a longer time and prevents hitting the dashboard.
Air bags
- The bag inflates and cushions the head and chest, increasing the stopping time so the force on them is reduced.
Head restraints
- They stop the head snapping backwards in a rear-end crash, so the neck is not forced to change momentum suddenly.
Section 3 of 5Other Examples
1
Cycle and motorcycle helmets contain a foam layer that crushes on impact, so the head stops over a longer time.
2
Crash mats in gymnastics compress when a gymnast lands, so the landing takes longer and the force is smaller.
3
Bending your knees when you land lets your body slow down gradually instead of stopping suddenly.
4
Playground surfaces made of rubber give slightly on impact, reducing the force when a child falls.
| Without safety feature | With safety feature |
|---|---|
| Short stopping time | Longer stopping time |
| Large force on passenger | Smaller force on passenger |
| Greater risk of serious injury | Reduced risk of injury |
Section 4 of 5Worked Example
Question
- A passenger of mass 70 kg moving at 15 m/s is brought to rest in a crash.
- Calculate the average force when the stopping time is (a) 0.10 s with an air bag and (b) 0.020 s without one.
- Change in momentum = 70 × 15 = 1050 kg m/s.
- (a) F = 1050 ÷ 0.10 = 10 500 N.
- (b) F = 1050 ÷ 0.020 = 52 500 N.
- The time without the air bag is five times shorter, so the force is five times larger.
Answer
- (a) 10 500 N with the air bag;
- (b) 52 500 N without it.
Section 5 of 5Exam Checklist & Tips
1.3.4 – Key Points for the Exam
- Crumple zone A part of a vehicle designed to deform and absorb energy in a collision.
- Impact time The time over which a moving object comes to rest during a collision.
- Average force The change in momentum divided by the time over which it happens.
- Air bag A bag that inflates in a crash to cushion a passenger and lengthen the stopping time.
- Seat belt A restraint that stretches slightly to slow a passenger gradually.
Common Mistakes
- Saying safety features reduce the momentum change; they reduce the force by increasing the time.
- Writing that the force is smaller because the impact is "softer" without mentioning time.
- Forgetting to use the equation F = Δp ÷ t when asked to explain in terms of force.
- Saying a crumple zone makes the car stop faster; it makes the car stop more slowly.