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It may be shown without difficulty, that the example which we have just given obeys the law of “action and reaction.”

Before Impact. After Impact. Mass of A 8 Mass of A 8 Velocity of A 9 Common velocity 3 Quantity of motion in direction AC 8×9 or 72 Quantity of motion in direction AC 8×3 or 24 Mass of B 6 Mass of B 6 Velocity of B 5 Common velocity 3 Quantity of motion in direction BC 6×5 or 30 Quantity of motion in direction AC 6×3 = 18

Hence it appears that the quantity of motion in the direction AC of which A has been deprived by the impact is 48, the difference between 72 and 24. On the other hand, B loses by the impact the quantity 30 in the direction BC, which is equivalent to receiving 30 in the direction AC. But it also acquires a quantity 18 in the direction AC, which, added to the former 30, gives a total of 48 received by B in the direction AC. Thus the same quantity of motion which A loses in the direction AC, is received by B in the same direction. The law of “action and reaction” is, therefore, fulfilled.