What is meant by the centre of gravity?
The easiest way to think of it is to get yourself a broomstick. Most have brooms on them so they are not perfectly symmetrical; see if you can get one that lets you remove the broom and just have the stick, or get a pole or rod of some type.Try to balance it by putting your index finger under it in the middle; if it balances with your finger in the middle the center of gravity is in the middle.Now hang something on one of the ends, say a hanger with a shirt on it. You will find you have to move your finger quite a ways over to keep the pole or stick balanced: the center of gravity has changes but the new combination of the total weight of the (stick + hanger and shirt) instead of just the weight of the stick. The center of gravity will be further off from center the heavier the item you hang on the hanger.Humans have a center of gravity, too, somewhere behind your breast bone. If you push on a person near it the person will feel compelled to take a step; the further from the center of gravity you push the less they will feel such a need, so in dancing with a partner the lead part tries to get the follower to feel a need to move their center of gravity by moving the leader’s center of gravity.Things are not equally dense, so a sponge and a piece of lead have different masses, but if they are perfectly symmetrical the center of gravity for both will be at their center.You can also see it at work on a playground see-saw, where 2 people of equal weights can play on it and balance parallel to the ground by sitting on the ends easily, but a heavy person will have to move closer to the center to balance the see-saw parallel to the ground.Perfect spheres like balls are easy: their center is their center of gravity. Weirdly shaped objects are harder, like human bodies, where our weight is not perfectly distributed: the human head is quite heavy, e.g., and your feet not so heavy. The center can be felt when dancing but not “seen” so easily.