THE DIFFERENT KINDS OF LEVERS. 81 



and, secondarily, the soft parts attached to these. When 

 muscles move bones the latter are almost invariably to be 

 regarded as levers whose fulcra lie at the joint where the 

 movement takes place. Examples of the three forms of 

 levers recognized in mechanics are found in the human 

 body. 



Levers of the first order. In this form (Fig. 36), the 

 fulcrum or fixed supporting point, F, lies between the 

 weight to be moved and the moving power. The distance 



i 



FIG. 36. A lever of the first order. F, fulcrum ; P, power ; W, resistance or 

 weight. 



PF from the power to the fulcrum is called the power- 

 arm of the lever, and the distance WF is the weight- 

 arm. When power-arm and weight-arm are equal (as in 

 an ordinary pair of scales), no mechanical advantage is 

 gained ; to lift a pound at W, P must be pressed down 

 with a force greater than a pound ; and the end W will 

 go up just as far as the end P goes down. If PF be 

 longer than WF then a small weight at P will balance a 

 larger one at W, the gain being greater the greater the 

 difference in the length of the arms, but the distance 

 through which W is moved will be less than that 

 through which P moves ; for example, if PF be twice as 



How are the bones moved by muscles to be regarded? Where 

 do the fulcra of these levers lie? How many kinds of levers are 

 found in the body? 



Describe a lever of the first order. Define power-arm and weight- 

 arm. When is a mechanical advantage gained by such a lever? 



