49. 



THE HUMAN MOTOR 



3. If a composite body is under 

 consideration, the centre of gravity 

 of each part can be found, and 

 the forces of gravity composed 

 from these. Thus, for a dumb- 

 bell with equal spheres, p, p and 

 p' would be resolved into a force 

 P acting at the centre of the bar 

 if it is homogeneous, or at another 

 point if it is heterogeneous (fig. 



49) ; the point G is obtained by the ordinary method for the 



composition of parallel forces. 

 For a system of weights P p 



P 2 , P 3 , P 4 , joined together 

 by rigid bars (the weights 

 of which can be neglected) 

 the result will be : The 

 weights of 1 and 3 kilo- 

 grammes (lig. 50) have a re- 

 sultant at a quarter of the 

 distance OO ' at g ; this result- 

 ant (of 4 kilogrammes) and 

 the weight of 2 kilogrammes 

 have resultant of 6 kilo- 

 grammes placed at a third of the distance gO" at g' ; finally this 

 resultant of 6 kilogrammes is composed with the 6 kilogrammes 

 of the fourth weight and the final resultant will be at G in the 

 middle of O'O", and will be equivalent to 12 kilogrammes. 



The system of the weights must not, if articulated at the points 

 O, O', 0*, O", change its formation during the above determina- 

 tion, the general centre of gravity being relative to an invariable 

 position of the weights, of which the individual centres of gravity 

 have been found. In fact, if the body O" (dotted line) is dis- 

 placed, the centre G in its turn will be displaced in space to G'. 



21. Stability of Equilibrium. The conditions of equilibrium 

 of a body depends on the effects of gravity and the reaction on 

 the body of its support. 



F/. so. 



. 51 



