The Laws of Motion. 



to fall from the steeple of Westminster Abbey. 

 But to make the matter still plainer: if the 

 roller a (fig. 85) leans against the obstacle b, it 

 will be found incapable of overturning 1 L v but if 

 a is taken up to c, and suffered to roll down the 

 inclined plane against #, it will overturn it in- 

 stantly. It is plain, therefore, that by its conti- 

 nued motion the roller a has acquired a force 

 which it had not in itself. The stroke which a 

 strikes at b is called its momentum. Hence re- 

 sults the well-known maxim in philosophy, which 

 I have before had occasion to repeat to you 

 " That the whole momentum, or quantity of 

 force, of any moving body, is estimated by the 

 quantity of matter multiplied by the velocity or 

 swiftness with which it moves." When the pro- 

 ducts, therefore, arising from multiplying the 

 quantity of matter in any two bodies by their 

 respective velocities, are equal, we say their mo- 

 menta, or moving forces, are the same. Thus, 

 if a body, which I call A, Aveighs forty pounds, 

 and moves at the rate of two miles in a minute ; 

 and another body, which I call B, weighs only 

 four pounds, and moves at the rate of twenty 

 miles in a minute, the entire force with which 

 these two bodies will strike each other would be 

 equal, and each of them would require an equal 

 force to stop it. For forty multiplied by two 

 gives eighty, the force of A ; and twenty multi- 

 plied by four is eighty, the force of B. 



Upon this easy principle depends much of 



