QUESTION: if a body is kept at height 'h' from the ground, its potential energy is 'mgh',but its ability is to fall till the centre of the earth.then why not in place of 'h',[(radius of earth)+(h)] is taken?
ANSWER: There's no such thing as an absolute value of potential energy. It can be defined any way you want. Just pick a reference point. A convenient one is the surface of the Earth. If you want to use the center of the Earth, that's fine.
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QUESTION: why so? if there is an absolute value of all the other energy forms,why not potential energy?energy means ability,and its ability is there.so there should be a absolute value as 'mgh'.taking 'h' as 'radius of earth+h'.
That's just the way it is. Potential energy means energy of position. A rock on the floor has no potential energy relative to the floor, but if you do work on it and raise it a distance h above the floor it now has PE = mgh, relative to the floor. You can describe kinetic energy exactly since it equals (1/2)mv^2 but there's no exactness about potential energy.