Momentum
Momentum is the quantity of motion of a moving body, and is the velocity multiplied by the weight.
Thus, a body weighing two pounds, moving at four feet per second, may be represented as having a momentum of eight.
A body weighing two pounds moving at the rate of six feet per second may be said to have a momentum of twelve.
A body weighing ten pounds moving at the rate of ten feet per second will have a momentum of one hundred—and so on.
Now, a step further. A body in motion striking another body free to move will lose part of its motion, and will impart some of its motion to the body moved against. The aggregate momentum after the striking is the same as before—that is to say—if a body weighing ten pounds have a velocity of twenty feet per second, its momentum we will call two hundred. Now, if in moving it strike another body either larger or smaller its motion will be somewhat retarded, and the body struck will possess some motion.
Multiply the weight of each by its motion after the striking, and it will be found that the sum of the products is two hundred. This may be illustrated by swinging balls like pendulums to cords of equal length from a beam, having the arrangement such that balls of different materials and sizes can be substituted at liberty. If a body be drawn back parallel to the beam, and released so as to swing against another swinging body, both will have motion. This motion will, in some cases be a rebounding motion, as in the case of a small elastic body swinging against and striking a larger elastic body, but in all cases the sum total of the momentum after the impingement is the same as before.
The following statement of the law then, is deducible:
The Momentum of one body in motion may be made to impart momentum to another body, the amount of momentum lost by the former being exactly equal to that thus acquired by the latter.
Before leaving these remarks on momentum the reader should observe carefully what momentum is and bear in mind it is the quantity of motion possessed by a moving body, and has to do only with mass and velocity—and takes no account of distance passed through.