Everybody talks about modern cars having crumple zones that “absorb” the impact in a collision so that the people inside don’t get injured, and that’s awesome! But I’m confused about the conservation of energy.. if two 2,000 lb cars crash into each other at 50mph, does ALL that energy get converted into heat and noise in the collision? Does it somehow get stored as potential energy in the crumple zone? Does it do something else?
In: Physics
It pretty much all ends up as heat and/or noise. It takes energy to deform metal by breaking bonds between atoms, and then the energy of the bond is released as heat/sound.
With enough impact applied to a small amount of material, you can really see this effect more clearly. You can take a cold piece of metal and make it hot in seconds just by hammering on it. https://www.youtube.com/watch?v=tXF60MOWUeY&ab_channel=SalemSaugusNPS
>Does it somehow get stored as potential energy in the crumple zone
Not “stored”, used to bend the metal. It’s not turned into *potential* energy, it’s turned into work aka “producing a piece of metal that’s a different shape than before”. It takes force to bend metal right? Force x distance (the amount that it gets bent by) = work. The kinetic energy of the cars is converted into work done on the metal to change its shape.
But yes a bunch is converted into heat as well, and a small amount into sound.
Heat, noise, the mechanical work required to displace the metal from its prior state, the kinetic energy imparted to the air molecules around the impact, lots of things.
Most of it goes into permanently deforming the metal, which results in the metal getting hot. You can try it yourself. Take a paperclip and bend it back and forth quickly 10 times or so. The metal should be warm to the touch in the area where the bending occurred.
Some is also dissipated as the cars or their parts skid across the road after the collision. A small amount of the energy does get stored as potential energy as “elastic” deformations of the metal, and as you said a tiny bit goes out as noise.
in the military, modern troop carriers are designed to shear away from the passenger compartment in the event of an explosion. Ie. take the energy exerted by a mine, bomb, whatever and “throw” parts of the vehicle. If they were built to absorb the blast, that energy would be transferred through the vehicle and into the passengers.
cars are made to crumple (use energy to crumple) instead of withstand the impact which would transfer the energy into you (the passenger.)
As others have mentioned, deformation of metal makes heat. Another factor into what these crumple zones do is make it take longer for the car to go from moving to stopped. The time is still very short. But if it takes 100 milliseconds instead of 50 that takes half the force because it took twice the time.
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