204 ACTION AND REACTION. 



resist that which is impressed upon them, that they will move with the same 

 force. But the lighter ball will move with twice the speed of the heavier. The 

 impressed force which is manifested by giving velocity to a double mass in the 

 one, is engaged in giving a double velocity to the other. 



If a cannon-ball were forty times the weight of a musket-ball, but the musket- 

 ball moved with forty times the velocity of the cannon-ball, both would strike 

 any obstacle with the same force and would overcome the same resistance ; for 

 the one would acquire from its velocity as much force as the other derives from 

 its weight. 



A very small velocity may be accompanied by enormous force, if the mass 

 which is moved with that velocity be proportionally great. A large ship float- 

 ing near the pier-wall may approach it with so small a velocity as to be scarcely 

 perceptible, and yet the force will be so great as to crush a small boat. 



A grain of shot, flung from the hand and striking the person, will occasion 

 no pain, and, indeed, will scarcely be felt, while a block of stone having the 

 same velocity would occasion death. 



If a body in motion strike a body at rest, the striking body must sustain as 

 great a shock from the collision as if it had been at rest and struck by the oth- 

 er body with the same force ; for the loss of force which it sustains in the one 

 direction is an effect of the same kind as if, being at rest, it had received as 

 much force in the opposite direction. If a man, walking rapidly, or running, 

 encounters another standing still, he suffers as much from the collision as the 

 man against whom he strikes. 



If a leaden bullet be discharged against a plank of hard wood, it will be found 

 that the round shape of the ball is destroyed, and that it has itself suffered a 

 force by the impact, which is equivalent to the effect which it produces upon 

 the plank. 



When two bodies moving in opposite directions meet, each body sustains as 

 great a shock as if, being at rest, it had been struck by the other body with the 

 united forces of the two. Thus, if two equal balls, moving at the rate of ten 

 feet in a second, meet, each will be struck with the same force as if, being at 

 rest, the other had moved against it at the rate of twenty feet in a second. In 

 this case, one part of the shock sustained arises from the loss of force in one 

 direction, and another from the reception of force in the opposite direction. 



For this reason, two persons walking in opposite directions receive from 

 their encounter a more violent shock than might be expected. If they be of 

 nearly equal weight, and one be walking at the rate of three, and the other four 

 miles an hour, each sustains the same shock as if he had been at rest, and struck 

 by the other running at the rate of seven miles an hour. 



This principle accounts for the destructive effects arising from ships running 

 foul of each other at sea. If two ships of 500 tons burden encounter each other, 

 sailing at ten knots an hour, each sustains the shock which, being at rest, it 

 would receive from a vessel of 1,000 tons burden sailing ten knots an hour. 



It is a mistake to suppose that, when a large and small body encounter, the 

 small body suffers a greater shock than the large one. The shock which they 

 sustain must be the same ; but the large body may be better able to bear it. 



When the fist of a pugilist strikes the body of his antagonist, it sustains as 

 great a shock as it gives ; but the part being more fitted to endure the blow, the 

 injury and pain are inflicted on his opponent. This is not the case, however, 

 when fist meets fist. Then the parts in collision are equally sensitive and vul- 

 nerable, and the effect is aggravated by both having approached each other with 

 great force. The effect of the blow is the same as if one fist, being held at rest, 

 were struck by the other with the combined force of both. 



