# Coefficient of restitution, Karate and deformation energy

1. Jun 20, 2012

### fisico30

Hello Forum,

I was reading a book about the physics of sports. It talks about karate and deformation energy that gets transfered to a target when a punch hits it....

The formula for the deformation energy contains the coefficient of restitution and say that a small coefficient belongs to hard body parts while a smaller coefficient to softer parts...
That seems wrong: a hard surface is more rigid and offer an more elastic collision with another object that a rubber surface.....

But by the way the article describe things, deformation damage computes the amount of energy required to push an object beyond its breaking point.
More elastic things (skin) require more energy to break because they can contort to a greater degree before breaking. Less elastic things (a board) only need to bend a little before they break but require more energy in order to make that initial bend.

So, when we hit something, we want as much of our kinetic energy to be transfered to the other object. We want the collision to be elastic or inelastic? I think inelastic... but then they say that hard body parts (like the leg) has a low coefficient of restitution while the groins a larger one...

I am confused..

thanks
fisico30

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