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# Crumple zones watch

1. Stuck on this question. Any help appreciated. Calculate the force required to stop a 750 kg car which is travelling at 22 m/s if the crumple zone crumples by 70 centimetres. How do I work this out?
2. I assume "crumple zone" equates to the distance over which the car is slowed down.

Start by using a suvat equation to find the acceleration. Use that in F=ma to find force.
3. NO NO NO! use Energy!
4. And does increasing the weight of car affect the thinking distance?
5. And how does increasing the weight of car affect the thinking distance?
6. Energy!!! Work out the kinetic energy of the car (1/2mv^2). This is equal to the work done in crumpling the crumple zone (force*distance)
Edit: Does being in a heavy car make you think more slowly?

Yes it does!!!!?!

Wut?
7. (Original post by running from demons)
Energy!!! Work out the kinetic energy of the car (1/2mv^2). This is equal to the work done in crumpling the crumple zone (force*distance)
Edit: Does being in a heavy car make you think more slowly?

Yes it does!!!!?!

Wut?
So would it be 1/2 * 750 * 22 = f * 0.8
but that means f = 10312.5 ??
8. Dont forget to square v.
9. (Original post by teachercol)
Dont forget to square v.
THanks so its 226875 Newtons.
10. I got 259286N

v^2=u^2+2as

0=484+1.4a

-484=1.4a

a=345.714286

f=ma

f=750*345.714286

f=259286N
11. Sorry. Last question. If I know the speed for the car, the driver's mass and therefore figure out the kinetic energy, how can I use energy transferred = average force * distance to find out the average force exerted by the seatbelt when energy is transferred to it due to stretching.
12. (Original post by shrep)
I got 259286N

v^2=u^2+2as

0=484+1.4a

-484=1.4a

a=345.714286

f=ma

f=750*345.714286

f=259286N
THanks. I was using 0.8 instead of 0.7. Could you help with the question I posted above?
13. I'm guessing that ud use the distance that the seatbelt stretched as the distance in that question. Btw i know its quicker 2 use the KE in the first part but I cant get SUVAT out of my head thanks to M1 lol.
14. (Original post by shrep)
I'm guessing that ud use the distance that the seatbelt stretched as the distance in that question. Btw i know its quicker 2 use the KE in the first part but I cant get SUVAT out of my head thanks to M1 lol.
This is a spearate question though and there is no distance given which is why Im stuck
15. Well then i guess u assume his acceleration is the same as the car/the distance is the same. So just change the mass of the car to the mass of the driver.
16. (Original post by D-Day)
I assume "crumple zone" equates to the distance over which the car is slowed down.

Start by using a suvat equation to find the acceleration. Use that in F=ma to find force.
"crumple zone" , "thinking distance" , sounds like a terribly vague GCSE "science" questions.That would be a pretty poor assumption but it's the only one you can really make given the info in the question.
17. Its AS Physics

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