32 JUNIOR GRADE SCIENCE 



line from the point of suspension. Tie a piece of thread across the basket 

 in the direction of the plumb-line ; then suspend the basket from another, 

 point, and notice where the plumb-line crosses the thread. The point of 

 intersection, which need not be actually on the framework itself, is the 

 centre of gravity. 



Centre of gravity. Consider a large number of weights, some 

 heavier than others, suspended from a horizontal rod. A certain 

 position can be found at which a spring balance would have to be 

 attached in order to keep the rod in equilibrium. When the rod is 

 hung from this point the tendency to turn in one direction is counter- 

 acted by the tendency to turn in the other, so the rod remains horizontal. 

 The weights may be regarded as parallel forces, 

 and the pull of the spring balance as equal to 

 their resultant. Now consider a stone, or any 

 other object, suspended by a string. Every 

 particle of the stone is being pulled downwards 

 by the force of gravity, as indicated in Fig. 24. 

 The resultant of these parallel forces is repre- 

 sented by the line OF, and the centre of the 

 FIG. 24. Parallel forces forces is the point 0. The point G, through 

 which the resultant (GF) of the parallel forces 



due to the weights of the individual particles of the stone always 

 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 line GF, 

 produced upwards. 



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 determining centres of gravity. 

 The centre of gravity of such geometrical figures as circles, squares, 

 and parallelograns 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 experimental method adopted for determining the centre of 

 gravity of any material body depends upon the considerations set 

 forth in the preceding paragraphs. The body, the centre of gravity 

 of which is required, is allowed to hang quite freely, either by means 

 of a cord or on a smooth peg, and when it has come to rest a vertical 

 line through the point of support is marked upon it. If a string is 

 employed, this vertical line will be a continuation of the string, and 

 at once drawn by the help of a ruler. If the body which is being 



