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# seat belts and crumple zones watch

1. i have read about these two things and i don't get how they work?
2. seat belts prevent the body continuing forwards under its own momentum and colliding with the stationary dashboard/steering wheel/window

crumple zones transform kinetic energy into work/sound/heat
3. but they talk about the seatbelt stretching and deceleration being less in the mark scheme
4. The crumple zone works to bring you to a more gradual stop, rather than simply clanging into something. Watch a crash test dummy in slow-motion, you'll see the car decelerating as the front compresses, which takes a lot of the force out of the collision.
5. Both reduce the rate of change of momentum in a car crash. Hence both reduce the resultant force acting on a person involved in a car crash. This means that they are less likely to be seriously injured. understand?

Imagine if you jump in a puddle of water very, very quickly. The rate of change of momentum of your foot is quite high. Hnece the resultant force exerted on the puddle is quite big. this explains why a lot of the water is splashed out and displaced. However if you jump in the puddle very slowly and your foot's initial speed when hitting the puddle is much less, the rate of change of momentum of your foot is far less. Therefore the resultant force your foot exerts on the puddle is far less. Less water then gets splashed out and so displaced. Understand??
6. (Original post by trimalchio)
i have read about these two things and i don't get how they work?
When moving you have kinetic energy. To stop you have to get rid of this kinetic energy. It gets turned into other forms, including deformation and breaking of your body and bones.
To stop you the force on you does work.
Work is force x distance.
The greater the force, the more damage it will do on you.
If you have a certain amount of kinetic energy then
Force x distance = kinetic energy needed to be "lost"
The smaller the distance the larger the force needs to be.
The greater the distance the smaller the force needs to be.
Crumple zones and seatbelts make the distance greater.
Therefore they make the stopping force smaller.
Therefore they make it less likely you will be damaged by this force.

This is what the exam board is talking about.

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