300 



THE HUMAN MOTOR 



Consider the different cases already mentioned ( 209 et seq.) 

 and distinguish carefully the useful work of the instrument on 

 the one hand, and the muscular work on the other. For example, 

 the useful work of a truck, a wheelbarrow, or any vehicle, is the 

 product of the friction by the distance covered. The manner 

 of measuring the effort due to the friction F is known. Measure 

 the distance covered L, then 



T - F X L. 



But it is the horizontal component of the muscular effort which 

 gives the value F ; if it is a fraction ^ of that total effort, it can 

 be said that the muscles make an expenditure of force, m X F x t 

 in the time /, during which the work is performed. 



The diagram, fig. 213, gives an example of the method of 

 analysing the effort exerted on the tool. 



Fw. 213. 



Analysis of the effort in pushing a truck. 



The maintenance of the handles at a certain height constitutes 

 a static effort, F' x t, so that for work done, T = F x /, in the 

 time t, the expenditure of muscular force is : 



+ 



x /. 



226. The measurement of the work done in using pruning shears 

 was made by Prof. Imbert by finding the distance fallen by a 

 weight capable of cutting a given vine-shoot. For this purpose 

 the pruning shears were arranged as if for the measurement of 

 the effort of cutting (fig. 189). In the centre of SN was suspended 

 a flexible cord, terminated by a hook to receive the weight, the 

 latter being caused to fall along a vertical scale, giving the height 

 of the fall. 



Let P be the weight and A the height ; the work done in cutting 

 will be : 



T = Ph. 



