There are, as noted in Chapter III (Mechanical Principles Of Machines. 22. Why Machines Are Used), many forms of energy, such as chemical, electrical, muscular, mechanical, etc. Any one form may be transformed into any other form. For instance, electrical energy may be transformed into chemical energy by charging a storage battery; muscular energy into mechanical energy by sawing a board with a hand-saw; mechanical energy into electrical energy by means of a dynamo. It is impossible to create or destroy energy, but it is easy to transform it. A pile-driver head weighing 75 lbs. suspended 25 ft. above the ground possesses energy, to the extent of 1875 ft.-lbs. due to its position. This energy is known as static or potential because it is stationary. When the weight is released and falls, the energy is called kinetic energy, that is, energy released or due to motion. Potential energy is sometimes called energy of stress; for example, the spring in a spring balance is under tension when a weight is suspended from the hook. Of course in all cases the weight times height equals the energy of the body,

E (potential) = W X H, although sometimes the velocity is given instead of the height. Then the:

E (kinetic) = W X V2 / 2 Force of gravity or

K =WV2 /2g

This is obtained by substituting in the formula for energy, for the height its value

(Velocity)2 / 2 Force of gravity or

V 2 / 2g

Impulse equals force times time. Impulse may be defined as the force multiplied by the length of time it acts.

Momentum = Weight X Velocity / 32

The energy stored in a revolving fly-wheel is kinetic, and is, therefore, represented by the formula

K = WV2 - 2g.

W stands for, or is equal to, the weight of the wheel in pounds, g for 32, attraction force of gravity, V for velocity of a definite point in the iron in feet per second. At this definite point the whole weight is assumed to be collected.