# Physics of Power

Energy, like momentum, is always conserved. However, sometimes it changes from kinetic energy, which is easily observed by measuring velocities and masses, to other forms that are harder to measure, most notably heat. The process of changing kinetic energy to heat is usually damaging to the material being heated. If the material is human tissue, it can be crushed, torn, or broken by the conversion of kinetic energy to heat. If the material is wood, it will break. A process that slowly or gradually converts kinetic energy to heat is usually called friction. A process that suddenly converts kinetic energy to heat is called an

## Examples of inelastic collisions

• Example 1: When billiard balls collide at normal speeds, they suffer no measurable damage because their collisions are almost perfectly elastic. All collisions conserve momentum, but only elastic ones conserve kinetic energy. So, if one ball with a certain velocity strikes a stationary ball on-center, it will transfer all of its momentum and kinetic energy to the stationary ball, stop, and cause the other ball to move away at the same velocity as the striking ball. If a perfectly elastic Taekwondo student struck a perfectly elastic target, the target would fly off undamaged, but with lots of kinetic energy, perhaps sustaining damage when it hits the floor.
• Example 2: If, instead of hard balls, we use balls made of soft clay, then, when one ball strikes a stationary ball, both balls will mush together and move away with half the velocity of the striking ball. The kinetic energy before the collision is MV2/2. The kinetic energy after the collision is MV2/4. Half of the kinetic energy has gone into damaging the balls. Since both balls are equally damaged, each ball got damaged in the amount MV2/8.
• Example 3: If a hard ball strikes a stationary clay ball, only the clay ball will be damaged. Therefore, all of the lost kinetic energy MV2/4 went into damaging the clay ball.
• Example 4: If a clay ball strikes an anchored hard ball, all the momentum of the clay ball will be transferred to the earth, and all of its kinetic energy MV2/2 will be expended in damaging the clay ball. This is twice the damage of example 3, and four times the per-ball damage of example 2.