# Homework Help: Basic concept of waves

1. Feb 18, 2005

### primarygun

Hello everyone,
Today I saw some waves in my physics lessons. A stone drops into water, and waves are created and spread from source to surroundings. Finally, the water became calm again.
What's the most proper explanation for that?

I think: Water molecules gain energy after being hitting, they collide the others.
By law of conservation of momentum, it is rather obvious to see the total amount of kinetic energy decreases after a collision. Finally, it becomes 0.
Is my explanation correct?

2. Feb 18, 2005

### HallsofIvy

"By law of conservation of momentum, it is rather obvious to see the total amount of kinetic energy decreases after a collision."

That is certainly not true. "conservation of momentum" has nothing to do with kinetic energy decreasing. If fact, in the standard "perfectly elastic" collision problems both momentum and kinetic energy are conserved.

What causes water waves to "die down" is the fact that the collisions are NOT perfectly elastic. Friction degrades some energy to heat.