Coefficient of restitution, Karate and deformation energy

[fisico30]

Hello Forum,

I was reading a book about the physics of sports. It talks about karate and deformation energy that gets transferred 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 transferred 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

 

[eljose79]

Hello fisico30,

Thank you for sharing your thoughts on the physics of sports and the concept of deformation energy. I can understand your confusion about the coefficient of restitution and its relation to hard and soft body parts.

Firstly, let's clarify the definition of coefficient of restitution. It is a measure of the elasticity of a collision between two objects and is defined as the ratio of the relative velocity of separation to the relative velocity of approach. A higher coefficient of restitution indicates a more elastic collision, while a lower coefficient indicates a more inelastic collision.

Now, in terms of hard and soft body parts, it is important to note that the coefficient of restitution is not solely determined by the rigidity of the surface. Other factors such as the material's density, shape, and surface texture also play a role. For example, a rubber surface may have a lower coefficient of restitution than a hard surface due to its ability to absorb and dissipate energy.

In the context of sports, we want to maximize the transfer of kinetic energy to the target, as you mentioned. This can be achieved through an elastic or inelastic collision, depending on the specific sport and desired outcome. For example, in karate, a more inelastic collision may be preferred to break through a target, while in basketball, a more elastic collision is desired to make a successful shot.

I hope this helps clarify the concept of coefficient of restitution and its relation to hard and soft body parts. Keep exploring the fascinating world of sports physics!