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Many concepts are easy to understand but difficult to define. One such concept is centre of gravity. It is defined as a point where the entire mass of the body is supposed to be concentrated.
Irregular shaped cardboard piece, nail, string and nut
Make a plumb line by tying a nut at one end of the string and nail at the other. Punch three holes (a bit larger than the nail) at different points near the edges of the cardboard. Insert the nail into one of the holes and hold the setup by the nail. Once the setup becomes steady, mark a line along the thread on the cardboard. Hold the setup by the nail in other holes and draw lines along the plumb line. You will notice that all the lines intersect at the same point. This point is the centre of gravity of the cardboard.
An object remains stable only when its centre of gravity lies directly below the point of suspension. The CG of the cardboard should lie somewhere along the plumb lines. Point of intersection is the only point, which is common in all lines.
Try to balance the cardboard on your finger tip. It balances only at its CG i.e., the cardboard behaves as if all its mass is concentrated at CG.
Try this method to find CG of a body having shape of an arc e.g., metallic handle of the bucket etc. You cannot draw a line but can tie a thread on the handle along the plumb line. You will find that CG lies outside the body.