DETERMINATION OF CENTRES OF GRAVITY 



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14. DETEEMINATION OF CENTRES OF GRAVITY. 



i. Experimental methods of determining centres of gravity. (a) Procure 

 a disc of sheet cardboard and find by trial the point on which it may be 

 balanced, that is, the centre of gravity of the disc. Make a hole in the card 

 near the edge, and take a plumb-line consisting of a thread with a piece of 

 lead tied at one end and a hook of thin wire at the other. Hang the disc 

 from the hook, and then suspend both as shown in Fig. 23 (a), so that the 

 disc and lead are both suspended and the thread passes over the point of 

 suspension. The thread also passes through the centre of gravity. Do this 

 for various holes in the edge of the disc, and see that in all cases the vertical 

 line through the point of suspension passes through the centre of gravity. 



FIG. 23 (a). Determination of 

 centre of gravity of a disc. 



FIG. 23(&). A method of determining centre 

 of gravity. 



(6) Another way to find the centre of gravity of a plane figure, such as 

 a triangular plate of metal or card, is to tie strings at each corner of the 

 plate. Then hang the plate by one of the strings to a support such as one 

 of the rings of a retort stand. Allow it to come to rest, and, using a straight 

 edge, draw a chalk mark across the plate in the same straight line with the 

 string as shown by the dotted line in the figure. Now attach the same 

 plate by one of the other threads exactly as before, and again make a mark 

 in continuation of the string. The two chalk marks intersect at a point 

 marked G, Fig. 23 (6). Untie and do the same with another string, the third 

 line passes through the intersection of the first two. Obtain a similar point 

 for each of the other plates. Also determine in the same way a similar 

 point for irregular plates of wood, zinc, or cardboard. 



ii. Centres of gravity of skeleton solids. (a) Procure a skeleton cube or 

 tetrahedron, and suspend it as in the preceding experiments. Mark the 

 verticals through the point of suspension by light wires attached by wax, 

 and thus find the position of the centre of gravity. Th3 centre of gravity 

 will not be on any of the bars of the skeleton solid used. 



(6) Find the centre of gravity of an open wickerwork basket, such as a 

 waste-paper basket. To do this, suspend the basket, and hang a plumb- 



