Why does collision analysis ignore potential energy?

[astro2cosmos]

in collision we all talk about only momentum and energy. but which form of energy only kinetic no sign of potential energy.why??
suppose a ball is rolled from top of a inclined surface at height L from ground and hit another ball placed at bottom of inclined surface, there occurs a collision b/w these balls. so how the potential energy is implemented in this event?

 

[Doc Al]

In your example the potential energy has been transformed to kinetic energy before the collision takes place.

 

[Ali Inam]

remember whenever a body starts motion, its potential energy is converted into kinetic energy.
At the point before the collision, the kinetic energy of the body is maximum while the potential energy is said to be zero. So, no potential energy can be implemented.
You can start considering potential energy when the ball starts to come into rest, and its motion is somehow decreased.

 

[freireib]

Ali Inam's response is exactly correct, however, it would probably be helpful for you (astro2cosmos) to work through the derivation including the terms you think are missing and see what happens. This is the only way you'll ever be convinced that they don't matter.