# Kinetic Energy Lost in Crumple Zones

Hi guys, i am working on a problem involving crumple zones, and i was wondering which formula would be best suited to discover the amount of energy lost due to heat and sound, during a collision between a car and a wall

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HallsofIvy
Homework Helper
What was the car's initial kinetic energy? What is the car's kinetic energy after the collision? What do you think happened to all the missing energy?

how do i work out the cars kenetic energy after the impact if its only hitting a wall, do i make an assumption about its mass after the impact or am i able to work it out and is the mass after the impact even necessary

Doc Al
Mentor
Hint: How fast is the car going after its collision with the wall?

id say 0, im assuming all of the kinetic energy is absorbed by the wall, is that possible or does the car have to continue moving ?

HallsofIvy
Homework Helper
"id say 0, im assuming all of the kinetic energy is absorbed by the wall, is that possible or does the car have to continue moving ?"

It's not only possible, it's certain. The whole point of a "crumple zone" is to stop the car!

You also asked "do i make an assumption about its mass after the impact or am i able to work it out and is the mass after the impact even necessary?"

Yes, the mass after the impact is necessary since kinetic energy = (1/2)m v2. Even if the car fell apart, you would still use the entire car in the calculation! Why would its mass change at all?

So: you know the car's speed and mass and therefore kinetic energy before the collision. You know the car's speed and mass and therefore kinetic energy after the collision. What happened to the energy the car lost?