Impulse and Crash Safety
Force equals the change in momentum divided by the time it takes. Stretching a collision over a longer time lowers the force, which is what airbags, crumple zones and crash mats are for.
What a learner can do afterwards
- Calculates the average force from a change in momentum and a contact time
- Explains a safety feature by naming the quantity it changes and the one it leaves alone
- Says why a longer stopping time gives a smaller force when the momentum change is fixed
1 · Read
Impulse links force, time, and momentum in one equation for you: average force times contact time = change in momentum. Rearranged, average force = momentum change divided by contact time. Short times mean brutal forces, which is exactly why helmets and mats exist.
A crash dummy's momentum falls by 1200 kg m/s in a collision. Against a rigid wheel the stop takes 0.1 s, so the force is 1200 divided by 0.1 = 12000 N. A crumple zone stretches the same stop to 0.4 s, giving 1200 divided by 0.4 = 3000 N: same change, quarter the force.
Every safety feature answers two questions: what does it change, and what stays fixed. Airbags, seatbelts, and crumple zones stretch the stopping time. The momentum change stays fixed by the masses and speeds, so spreading it over longer always means a smaller force.
For explain questions, write the chain in order: state the momentum change, state the longer time, then conclude the force falls. Longer stopping time with the same momentum change always means a smaller force.
Same momentum change over a longer time means a smaller force, and that trade is what safety kit buys.
More to read
8.2 Impulse
Impulse is the push behind a momentum change. Multiply the average force by the contact time to get it, and the result equals the change in momentum exactly. A small force acting for ages can shift as much momentum as a huge force in a blink.
Crash safety is impulse in action. Seatbelts, airbags, and crumple zones all stretch out the stopping time. The momentum change stays fixed, so a longer time forces the average force down to something the body can survive.
8.1 Linear Momentum and Force
Force and momentum are tied by time. A steady force acting longer builds a bigger momentum change, which is why follow-through matters in every sport. Sprinters push on the blocks as long as they can to bank the most speed.
To find the average force in a crash, divide the momentum change by the contact time. Short times mean brutal forces, which is exactly why helmets and mats exist. Name the quantity each feature changes to explain how it works.
8.3 Conservation of Momentum
Momentum conservation sets the size of the job, and impulse decides how violent it feels. The total momentum change in a collision is fixed by the speeds and masses, whatever the padding. Safety gear cannot shrink that total, it can only spread it out.
This is the golden rule for every explain question. State the momentum change, state the longer time, then conclude the force falls. Longer stopping time with the same momentum change always means a smaller force.
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2 · Watch
Take it off screen
Where it sits
Learn first
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.