CENTRE OF GRAVITY. 



relative motion of the mass round the centre of gravity may be found by con- 

 sidering the centre of gravity as a fixed point, round which the mass is free to 

 move, and then determining the motion which the applied forces would pro- 

 duce. This motion being supposed to continue uninterrupted, let all the forces 

 be imagined to be applied in their proper directions and quantities to the centre 

 of gravity. By the principles for the composition of force they will be me- 

 chanically equivalent to a single force through that point. In the direction of 

 this single force the centre of gravity will move, and have the same velocity 

 as if the whole mass were there concentrated and received the impelling 

 forces. 



These general properties, which are entirely independent of gravity, render 

 the " centre of gravity" an inadequate title for this important point. Some 

 physical writers have consequently called it the ' centre of inertia." The 

 " centre of gravity," however, is the name by which it is still generally desig- 

 nated. 



