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(62.) On the other hand, the quantity of motion does not depend on the mass only, but also on the speed. If a certain determinate mass move with a certain determinate speed, another equal mass which moves with twice the speed, that is, which moves over twice the space in the same time, will have twice the quantity of motion. In this manner, the mass being the same, the quantity of motion will increase or diminish in the same proportion as the velocity.
(63.) The true estimate, then, of the quantity of motion is found by multiplying together the numbers which express the mass and the velocity. Thus, in the example which has been last given of the impact of masses, the quantities of motion before and after impact appear to be as follow:
Before Impact. After Impact. Mass of A 8 Mass of A 8 Velocity of A 17 Common velocity 14 Quantity of motion of A
* The sign × placed between two numbers meant that they are to be multiplied together.