Kinetic Energy.—The energy of the falling water or stone is called Kinetic energy. In both cases the power developed came from within themselves and not from any exterior source.
The difference between Potential and Kinetic Energy is therefore that Potential Energy represents the capacity to do work, while Kinetic Energy is the actual performance of work.
Friction.—In every form of energy there is always something to detract from it or take away a portion of its full force, called friction. When a shaft turns, it rubs against the bearings, and more or less power is absorbed.
When a wheel travels over the ground friction is ever present. The dislodging of the stone required ten pounds of energy, but a thousand pounds was developed by the fall. The water rushing along its rocky bed has friction all along its path.
Resistance.—This friction is a resistance to the movement of a body, and is ever present. It is necessary to go back and examine the reason for this. As long as the stone was poised at the top of the precipice it had latent or potential energy, which might be termed power at rest. When it fell it had power in motion. In both cases gravity acted upon the stone, and in like manner on the water pouring over the falls.
Inertia.—Inertia or momentum is inherent in all things and represents the resistance of any body or matter, to change its condition of rest or standing still into motion, and is then called Inertia of Rest, or the resistance it offers to increase or decrease its speed when moving, and is then called Inertia of Motion.
Inertia or momentum is composed by the weight of the body and its speed and is measured by multiplying its weight by its speed.
The law is, that when a body is at rest it will remain at rest eternally, and when in motion it will continue in motion forever, unless acted on by some external force or resistance. An object lying on the ground has the frictional resistance of the earth to prevent its moving. When the object is flying through space it meets the air and has also the downward pull of gravity, which seek to bring it to rest.
These resisting forces are less in water, and still less in gases, and there is, therefore, a state of mobility in them which is not found in solids.