DYNAMICS.-FORCE AND MOTION. GRAVITATION. 



FALLING BODIES. THE PENDULUM. 



ENERGY. 



ECTJON I. 



\. 



FORCE AND MOTIO N. 



63. Dynamics. Dynamics is that branch of 

 physics which treats of forces and their effects. 



These effects may be of two kinds. 



(a.) The forces employed may be counterbalanced. If they thus 

 act upon a body at rest, that body will remain at rest ; if they act 

 upon a body in motion, the motion will not be changed thereby. 

 The branch of dynamics that treats of forces thus balanced is called 

 Static*. 



(&.) The forces employed may act against the inertia of matter 

 ( 38), and produce motion or change of motion. The branch of 

 dynamics that treats of forces thus used is called Kinetics. If we 

 have a problem relating to the forces that may produce equilibrium 

 in a lever, as in the act of weighing goods, it is a static problem ; 

 if a problem refer to the velocity of a falling body, or the amount 

 of work that may be done by the uncoiling of a watch-spring, it is 

 a kinetic problem. 



Note. No attempt will be made to maintain the distinction be- 

 tween the static and kinetic effects of forces. 



64. What is Force? The word force is difficult 

 of satisfactory definition. As generally used, it signifies 



2 



