- All collisions are momentum conserving. But some collisions retain more KE than others.
- A (perfectly) elastic collision is one that retains 100% of its initial total KE.
- A perfectly inelastic collision is one that retains the minimum amount of KE.

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- Depicted above are just 5 possible outcomes of head-on collisions of two equal masses
*m*with equal initial speed*u*. - Notice that the total momentum (of zero) is conserved for all collisions.
- At the top is the (perfectly) elastic collision.
- At the bottom is the perfectly inelastic collision.

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- Depicted above are just 5 possible outcomes of head-on collisions of two equal masses
*m*, one of them with initial speed of*u*, and the other initially at rest. - Notice that the total momentum (of
*mu*rightward) is conserved for all collisions. - At the top is the (perfectly) elastic collision.
- At the bottom is the perfectly inelastic collision. (Losing all its initial KE is impossible because of momentum conservation)