20 



THE HUMAN MOTOR 



Fio. 35. 



Restraints can be replaced 

 by forces, because the surface 

 which everywhere opposes its 

 reaction to the ball and obliges 

 it to move thereupon can be re- 

 placed by a force equal to that 

 reaction. The tension of a 

 string ABC can be replaced by a force BF, if the string were 



cut at B and its restraint thus 

 removed (fig. 34). The surface 

 of liquids take the shape of a 

 b membrane held by tangential 

 forces F lf F 2 . . . ' . equivalent to 



F, -^ Mercury pfs^ the restraining effects of the walls 



jF a of the vessel. This is what is 

 i called the surface tension of 

 liquids (fig. 35). Such restraints 

 can be without friction or with 

 friction according to whether or not the surface offers a resist- 

 ance to the displacement of the material 

 point ; that resistance will be examined 

 later ( 39). It is important, in all cases, 

 not to make abstractions and to be sure 

 that the frictions are forces. Let there 

 be a force F applied to the moving point 

 M, which moves along the curve S (fig. 

 36) ; F can be resolved into a perpendicular 

 component MN, which exerts a pressure on FIO. 36. 



the surface and is neutralised by the re- 

 action of this latter ; and a tangential component MT will 

 produce the movement of the point M. Experiment shows 

 that up to a certain value of MT, movement is not produced, this 

 value being that of the friction on the surface. Therefore it is a 

 force, a resistance to movement. In fact, the various points of 

 a solid are subject to restraint and to friction, and as a solid is 

 always deformable it is subject to connexions modified by internal 

 forces, by opposition to external forces. The condition of equili- 

 brium demands that the resultant of all these forces shall be zero. 

 This condition is necessary ; otherwise exterior forces with a 

 zero resultant could break a body of which the internal forces 

 had not also a zero resultant. 



15. Reduction of a System of Forces. The number of forces 

 applied to a solid can be reduced because the intensity of a force 

 is not changed if it is produced in the same straight line from 

 one point in a solid to another, from A to B, for example (fig. 37), 

 and it is possible to transfer several concurrent forces to a 

 point where they can be replaced by a single resultant OR 



