82 ELEMENTARY GENERAL SCIENCE CHAP. 



forces is represented by the line GF, and the centre of the 

 forces is the point G. The point G, through which the resul- 

 tant (GF) of the parallel forces due to the weights of the indivi- 

 dual particles of the stone passes, is 

 known as the centre of gravity. For 

 the stone to be in equilibrium, the string 

 must be attached to a point in the lino 

 GF, or GF produced. 



Every material object has a centre of 

 gravity, and the position of this point 

 for a particular object is the same so 

 long as the object retains the same 

 form. 



Experimental Methods of Deter- 



FIG. ss.-paraiiei Forces mining Centre of Gravity. The centre 

 due to Gravity. of gravity of such geometrical figures as 



circles, squares, and parallelograms is 



really the centre of the figures, and can therefore be determined 

 geometrically. In the case of unsymmetrical figures, however, 

 the centre of gravity cannot be so easily found by geometry, 

 and is best determined by experiment. The following experi- 

 ments illustrate the method employed, and the principle 

 involved : 



EXPT. 65. Procure an uncut pencil. Tie a piece of thread 

 round the pencil and adjust the thread in such a position that 

 the pencil is suspended horizontally from it. Measure the 

 distance of the thread from each extremity of the pencil. 



The pencil may be regarded as a straight line, and the 

 observations will show that the centre of gravity is in the 

 middle of it. 



EXPT. 66. Obtain several sheets of wood, zinc or cardboard 

 cut into various shapes as shown in Fig. 34, and drill holes in 

 the positions indicated. Using one at a time, tie pieces of 

 string to the sheet, passing each through one of the holes. 

 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 

 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 



