Two bumpers are on offer for the same car: a rigid steel bar, and a bumper designed to fold up in a crash. One of them is gentler on the car and its passengers in the same crash. Which one, and how can you tell without a single number?
The last lesson gave the mechanism: for the same momentum change, a longer stop means a smaller average force. Judging a design is one read: which design stretches the stopping time more?
Here is the same car driven into the same test wall twice.
With the rigid steel bumper, nothing folds. The car stops almost at once.
With the folding bumper, the front folds up through 0.4 m. The car keeps moving while the bumper folds, so the stop takes longer.
The crash is the same, so the car's momentum change is the same both times.
The folding bumper spreads that same momentum change over a longer time. So the folding bumper exerts the smaller average force.
A van can be fitted with a rigid steel bumper or with a bumper that folds up in a crash. In the same crash, which bumper exerts less average force on the van?
A falling phone can land in a stiff shell case that stops it in 0.004 s, or in a soft case that stops it in 0.04 s. Which case exerts less average force on the phone?
A design is never gentler because it looks softer or weighs more. The only thing that matters is the time it adds to the stop.
Two bike helmets are tested in the same impact. Helmet A has a thin, stiff liner that barely compresses. Helmet B has a foam liner that compresses through 2 cm before the head stops. Which helmet exerts less average force on the head?
The impact is the same, so the head's momentum change is the same in both helmets.
Helmet B's foam compresses, so the head keeps moving while the foam gives way. The stop takes longer.
The same momentum change over a longer time means a smaller average force.
Helmet B exerts the smaller average force, because it stretches the stop over a longer time.
Jorge says: of two harnesses that stop the same rider, the one that takes longer to bring the rider to rest exerts the smaller average force, because the rider's momentum change is the same either way. Is he correct?
A climbing gym is choosing what to put under its wall. Which choice exerts the least average force on a falling climber?
A car maker replaces a rigid bumper with one that folds in a crash. The car's momentum change in a given crash is unchanged. What makes the folding bumper's average force smaller?
A playground replaces its rubber tiles with concrete. For the same fall, which statement is correct?