These are “famous” collisions whose outcomes are either obvious or can be calculated mentally.

**(Perfectly) Elastic “Sitting Duck” Collisions**

- Notice that,
- the total momentum is conserved.
- when one mass is much larger than the other, the velocity of the heavier mass is hardly changed by the collision.
- The two masses approach and separate from each other at relative speed of
*u.*

–

**(Perfectly) Elastic “Jousting” Collisions**

- Notice that
- the total momentum is conserved.
- when one mass is much larger than the other, the velocity of the heavier mass is hardly changed by the collision.
- The two masses approach and separate from each other at relative speed of 2
*u.*

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