184 Dynamics. 



mer would first draw the latter, and afterward be drawn by it ; 

 there would indeed be an instant of rest, but it would be that in 

 which the former had lost all the velocity impressed upon it, and 

 this state would continue only for an instant. 



294. Thus the forco of bodies in motion cannot be estimated 

 by weights, that is, by the simple tendency of gravity in bodies 

 destitute of local motion ; but only by other forces of the same 

 kind, as those of heavy bodies having fallen from a certain height. 

 Hence, in order to have an idea of the force of a body of 3 

 pounds, carried with a velocity of 50 feet in a second, I should 

 seek by the method of article 277, from what height a heavy 

 body must fall to acquire a velocity of 50 feet in a second, and 

 I should find it to be 38,8 feet nearly. I should conclude, there- 

 fore, that a body of 3 pounds, urged with a velocity of 50 feet in 

 a second, must strike as if it had fallen from a height of 38,8 

 feet. 



295. The force which bodies in motion are capable of exert- 

 ing, is called percussion. 



The force of percussion cannot, therefore, in any way be 

 compared with simple pressure, or the effort which a mass is 

 capable of making by its weight without local motion. A blow 

 of a hammer, though feeble, will drive a nail into a block of 

 wood ; also a body of small mass, which by its fall had acquired 

 but little velocity, would be attended with the same result, while 

 fc very considerable weight would produce no effect. 



The reason of this difference is, that in the former case, all 

 the degrees of velocity possessed by the body in motion, are 

 exerted in an instant ; whereas in the latter, the weight which 

 exerts only a pressure, receives its degrees of force successively, 

 and imparts them in the same manner to the nail and the sur- 

 rounding mass ; and as each of these degrees is infinitely small, 

 it is absorbed as soon as it is received. 



