Centre of Gravity 115 



Exercise for Student. 



Which of the positions named above would you term " stable " 

 and which " unstable " ? Make a further list of positions in which 

 the brick may be balanced, classifying them under these heads. 

 Which position do you consider that of greatest stability ? 



70. There is yet another form of equilibrium to be 

 considered. If a ball be placed on a horizontal surface, 

 it will remain in any spot on which it is placed. If 

 now it be displaced from this position by a small 

 disturbing force, it neither returns to its former position, 

 nor moves further from that position. Hence its 

 condition cannot be said to fall under either of 

 the headings of stable or unstable equilibrium, as 

 previously defined. Such a body as the ball is said 

 to be in a position of "neutral" equilibrium. Its 

 behaviour when displaced may be explained thus : 

 Its centre of gravity coincides with its geometrical 

 centre. The height of its centre of gravity above its 

 base of support is therefore equal to its radius. When 

 displaced, the height of its centre of gravity is neither 

 increased nor diminished, and the centre is still vertically 

 over the new point of support. Thus there is no moment 

 of its weight about the point of support tending either 

 to displace it further or to bring it back to its former 

 position. 



Exercise for Student. 



What is the state of equilibrium of a cone made of homogeneous 

 material when resting (a) on its point ; (b) on its base ; (c) on its 

 curved surface ? 



71. Other interesting points concerning the centre 

 of gravity may be investigated by performing 



EXPERIMENT 38. Using the millboard figures of 

 which the centres of gravity have already been found 



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