Page:The Solar System - Six Lectures - Lowell.djvu/140



or line around which the moment is taken. The moment of momentum is thus twice the area swept out by the moving body about the fixed one in unit time.

When two bodies collide, the amount of motion is not changed. This truth is the result of experiment, and was first determined by Newton. If the two are perfectly inelastic, they move on after the collision as one mass with a loss of kinetic energy. If perfectly elastic, they rebound in such a manner that not only the amount of motion, but the kinetic energy remains unchanged. Now probably no bodies are perfectly inelastic, just as no bodies are perfectly elastic. In the case, therefore, of the bodies in nature, while the amount of motion is never altered, a part of the kinetic energy is lost by the shock. It is transformed into heat energy.

Now the moment of a velocity, and therefore of a momentum, clearly remains constant when unacted upon by any force, for its direction continues the same, and a perpendicular upon it from any point measures out the same area in the same time, as the perpendicular, too, is constant.

The like is true, if it be acted upon by a force constantly directed to the same point; for in that case the force can generate no velocity except along the perpendicular upon the line which