Momentum is transfered and conserved during collisions between objects.
Prior knowledge & experience:
Why do billiard balls trade velocities when they strike each other?
Why does a tennis ball bounce so high when dropped with a larger ball (volleyball or basketball) beneath it?
Momentum from the massive ball transfers to the less massive tennis ball giving it a large velocity.
Momentum is not always conserved. It is difficult for students to understand that the momentum of objects is always conserved if the entire system is viewed. Typically momentum from an object is transfered to the very massive Earth. The student sees one object stop, but nothing else move and concludes that the momentum went away. It is important to point out that the momentum is in the Earth's movement. This results in a very small velocity, one that is almost undetectable unless the collision is extremely large (asteroid).
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