CHAPTER II. 



GENERAL PRINCIPLES OF MECHANICS (Continued). 



STUDY OF FORCES DYNAMIC?. 



23. Definition. Dynamics is the study of the movements pro- 

 duced by given forces, and conversely the forces that must be 

 brought into play to produce a certain movement. 



If a material point is considered the forces acting on it can be 

 reduced to a single resultant, and in the practical case of material 

 systems the whole mass can be assumed as concentrated at the 

 centre of gravity and the forces or their resultant as acting at 

 that centre. To simplify calculations, friction, and, in particular, 

 the resistance which air opposes to the movement of bodies (see 

 42) are neglected. 



24. Movement of a Point. Projectiles. When a force, con- 

 stant in strength and direction, acts on a point M, the movement 

 is (..2): 



s = /2, and the speed v = ft. 



In the case of a body which 

 was in repose and which would 

 fall by the action of gravity it 

 follows that 



s = ^ gt 2 and v = gt. 



From which is deduced 



V 2 = g 2 t 2 = 2 g X \ gt* = 2gS. 



Then: 



v = \/2gs. 



The height of the fall s is 

 usually designated by h, and 

 then 



X"" 



V 



FIG f>6. 



Also: 



