483. Kinetics is that branch of mechanics which treats of the action
of forces in the production of motion. We shall find it rather more
difficult than the subjects with which we have been hitherto occupied;
the difficulties in kinetics arise from the introduction of the element
of _time_, into our calculations. The principles of kinetics were
unknown to the ancients. Galileo discovered some of its truths in
the seventeenth century; and, since his time, the science has grown
rapidly. The motion of a falling body was first correctly apprehended
by Galileo; and with this subject we can appropriately commence.