# I Collision of a billiard ball against a wall

1. Dec 21, 2017

### themagiciant95

The collision of a billiard ball against a wall is considered elastic. Intuitively i can accept it. But analytically i dont understand it.

Why does the wall not take any relevant amount of energy from the ball? Is it connected to the change in temperature of the wall ?
What scenarios can we aproximate as elastic ? Under what characteristics ?

2. Dec 21, 2017

### lekh2003

The wall definitely feels the momentum of the ball. The wall is just very firm. It isn't going to move. Most of the energy just gets transferred into sound, not as much in heat.

3. Dec 21, 2017

### themagiciant95

So the only fact that allows us to consider the collision as elastic, is that the energy dissipated by the wall is very low ?

4. Dec 21, 2017

### A.T.

That is the definition of elastic collision.

5. Dec 21, 2017

### lekh2003

A.T. is correct, an elastic collision is defined as a collision where the energy is transferred to other methods of energy transfer besides collisions.

6. Dec 21, 2017

### A.T.

Nice that we agree, but I don't understand your definition at all. Why not keep it simple:

https://en.wikipedia.org/wiki/Elastic_collision

"An elastic collision is an encounter between two bodies in which the total kinetic energy of the two bodies after the encounter is equal to their total kinetic energy before the encounter."

7. Dec 21, 2017

### lekh2003

Sure, I might be going on a tangent. But yes, the total energy is conserved, even if that requires energy going into sound.

8. Dec 21, 2017

### A.T.

In an ideal elastic collision the total macroscopic kinetic energy is conserved. No energy goes into sound.

9. Dec 21, 2017

### lekh2003

But in the OP's question, it is not perfectly elastic, some of the energy goes into sound. I find that it would be better not to discuss perfectly elastic collisions since the OP is familiar with this.

The OP is questioning how the collision can be perfectly elastic which it is not.