Collision of two balls of different mass
1. What happens if you throw a light ball towards a heavy ball?
Just like a basketball ball bounces on the surface of large earth, the light ball bounces.
This is easy to understand.
2. When a heavy ball rushes towards a light ball
In this case, you have to think a bit.
It would help if you did a simple coordinate transformation. Remove the speed of v for each ball. Then, you can think that the light ball is approaching while the heavy ball is stopped.
Then, we can think of a light ball hitting a heavy ball surface and then bouncing back.
Then add speed v to each ball. As a result, the light ball bounces with a speed of 2v.
3. When two balls collide with each other at speed v
You can sum both cases. As a result of the crash, the light ball will pop out at speed 3v.