Collider

You can set various initial states of the system, like the elasticity of balls and walls, gravity, etc.
If you want to understand the laws of conservation of energy and momentum, then this is exactly what you need!
This is not only useful and informative application, but also an excellent anti-stress!
This is a free version, you can download Pro here: https://play.google.com/store/apps/details?id=com.simonbashkirov.collider.pro