When I started teaching, the game of "shove ha'penny" was massively popular. How times have changed! Today I fired coin A into a stationary coin, coin B. Coin A stops and coin B moves on. Try A at different initial speeds.
Then I stuck two coins together.
When you repeat the experiment with coin B being twice the mass, it won't go as far. It's velocity after the collision is reduced. Or swap it round and fire the heavy one at the lighter one. If done correctly, the lighter one should go twice as far. All this shows that velocity after a collision depends on two things - velocity before the collision and mass. Hence we use momentum in calculations. TOTAL momentum of both coins together before the collision remains unchanged after the collision.
Then for fun, try this! Predict what you think will happen first!
