## Collision dynamics revisited Energy transfer

When a particle collides elastically with another particle the laws of conservation of momentum and energy are strictly observed. (Since the collision is elastic, 'energy' means kinetic energy.) However, both energy and momentun are in general transferred from one particle to the other. How much of each is transferred depends on the relative masses of the colliding particles and on the angle of scatter.

In snooker, when the cue ball strikes a stationary ball of equal mass head-on, all the energy and all the momentum are transferred from the moving ball to the target ball. The cue ball stops dead, and the target ball continues with the cue ball's original velocity. If the moving ball only makes a glancing collision, it keeps most of its momentum and energy. The greater the change of direction of the moving ball, the more energy is transferred to the target ball.

### Bouncing off a big target

When the target is much bigger than the projectile, it may receive a lot of momentum but very little kinetic energy. For example, if we bounce a tennis ball off a bus, the ball will bounce back with the same speed and practically no loss of KE. The momentum of the ball is reversed. To compensate, the bus recoils with twice the original momentum of the tennis ball. The momentum transfer may be larger than the original momentum of the moving particle, yet practically no KE is transferred.

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