58 



THE HUMAN MOTOR 



73. 



must be perpendicular to the 

 plane ABG. It is in this way 

 that, in a hammer, vide fig. 

 73, the percussion takes place 

 in the direction PP' and 



CC' = /' -f -77-; the head of 



the hammer throws the centre 

 of gravity back between C and 

 C'. In these conditions the 

 percussion is nought on the axis QA, that is to say, at the hand. 



The quantity C'G = ^-p is always small. 



A knowledge of the laws of impact is important in the study of 

 machines, and finds an interesting application in the game of 

 billiards, and a useful application in the driving in of stakes or 

 piles for foundations. 



The effect of impact, as expressed by the resultant movement, 

 depends as much on the speed as on the mass of the body. Thus 

 a stone thrown by hand against a door would make it turn on its 

 hinges, whereas a shot from a cannon passes through the door 

 without displacing it, the movement not having had time to 

 spread around the part struck. A jet of sand projected at a high 

 speed is employed to grind glass, metals are cut by means of 

 circular saws without teeth moving at a great speed, and high 

 speeds are usefully applied in other interesting ways. The effect 

 of mass is no less obvious : a nail does not penetrate a plank unless 

 the latter is supported on something ; it is better to use a fairly 

 heavy hammer to knock in nails than a light one, because M and 

 m being the mass of the hammer and of the nail respectively, the 

 loss of energy due to the impact is : 



(V ). 

 The relation of that loss to the energy MV 2 of the hammer is 



m /i _ 2. \ 2 ' 

 M -f m \ V/ 



that is to say that the relative loss will be less in 

 proportion as the mass M of the hammer is greater 

 and its speed V slower. Thus shoemakers use a 

 stone to receive the blow of the hammer, and that 

 large anvils do not spoil bars. Lastly, take a 

 mass M suspended by a string a, and carrying an 

 identical string a'. If a' is pulled sharply it is 

 broken, but if pulled steadily the upper string a 

 is broken, because, in the first case, the rapid 



