ion. ?<<: 



nc valut ubatancc* in question is known 



it, equilibrium is iinpoa.tr 



(2) oquAtion can be satisfied by ascribing 



us than me value, equilibrium may be 



possible. We say maybe possible, because tl>* i nua- 



uiay not be / equation of equilibrium, mid of 



r'jiulil- i* necessary that all i. 



equations be satisfied* 



We may illustrate the subject of friction by the solution of 



-lil.-in : 



A weigi 'laced on a rough horizontal plane ; a string 



is attached to W and passes over a fixed smooth nullv, and 



attached 



tin- In; the vertical of the string 



passes froi; :ixed pully, when there is equi- 



libri 



6 be the inclinat .is string to the vertical, 



tension of the string is equal t /'. The body on the rough 

 ntal plane is acted on by its own weight, by the resist- 

 of the plane, which \A at ri^ht angles to the plan. 



u along the plane: denote the resistance by II, 



Then resolving the forces horizontally and vertically we 



P sin 0, 



Hence, t ^ 7?, we have 



therefore 



where /j is j 



may, without any real loss of generality, suppose that 

 $ is a positive angle not exceeding a right angle. 



