A car hits a wall and stops. The driver's body is still moving. Something has to stop the driver too, and whatever does the stopping must exert a force. The question a car designer asks: how do you make that force small?
You have the pieces already. Earlier in this topic you saw that spreading a stop over a longer time means a smaller average force, and F = Δp/Δt made it exact. This lesson uses those pieces on the real devices: the airbag and the crumple zone.
In a crash, the car stops in a fraction of a second. The driver keeps moving at the car's old speed until some force stops them.
The driver's momentum change is fixed by the crash. Their momentum must go from its highway value to zero.
No safety device can shrink that momentum change. What a device can change is the time the stop takes.
An airbag inflates in front of the driver. The driver's head sinks into the bag, and the stop stretches over roughly ten times the time a hard dashboard would take.
A crumple zone does the same job for the whole car: the front of the car folds up, and the car, with everyone in it, stops over a longer time.
Stretching the stop to ten times the time cuts the average force to one tenth. The momentum change is the same; only the time grew.
A pickup truck's crumple zone folds up in a crash instead of staying stiff. Compared with a stiff front end, what does the folding change?
A seat belt with a built-in stretch section stops a passenger's chest in 0.3 s; a stiff belt would take 0.1 s. The chest's momentum change is the same either way. Why is the stretch-section belt's average force smaller?
In a crash, an airbag stretches the stop of a driver's head from 0.05 s to 0.5 s. The head's momentum change is the same either way. Explain why the average force on the head is smaller with the airbag.
The head's momentum change is the same with or without the airbag.
F = Δp/Δt divides that fixed momentum change by the stopping time.
The airbag makes the stopping time ten times longer: 0.5 s instead of 0.05 s.
Dividing the same momentum change by ten times the time gives one tenth the average force.
The airbag stretches the stop to ten times the time, so the average force on the head drops to one tenth.
A delivery truck's front end is built to fold up through 0.5 m in a crash. Explain how the folding protects the driver.
The driver's momentum change in the crash is fixed: their momentum must reach zero.
The folding front makes the truck take longer to stop.
For the same momentum change, spreading the change over a longer time means a smaller average force (F = Δp/Δt).
The longer stop means a smaller average force on the driver.
Isla says: a crumple zone does not change the car's momentum change in a crash; it stretches the time the change takes, so the average force is smaller. Is she correct?
A gym wall has a thick foam pad. A runner who fails to slow down sinks into the pad before stopping. Why is the pad's average force on the runner smaller than a bare wall's would be?
Which statement says how an airbag protects a passenger in a crash?
A crash dummy's head is stopped twice with the same size of momentum change: once by a hard dashboard in 0.06 s, once by an airbag in 0.6 s. The dashboard's average force is 900 N. Enter the airbag's average force in N.