Concepts
A heavy ball hits a lighter ball at rest.
What is total momentum before = total momentum after (Conservation of Momentum)
Two balls of the same mass moving at the same velocity towards each other collide.
What is they reverse movement, now moving in the opposite directions at the same velocity?
The tendency of an object to keep moving.
What is inertia?
You try to calculate the momentum of a 20-ton train at rest.
What is zero?