Question

the weight of a body decreases in a coal mine. why ?

Answer 1

That means that as you descend into the earth, all of the mass in all the shells above you gets canceled. If the center of the earth were not molten iron, but instead hollowed out, you would be weightless inside.

The physics problem is actually a little more general. It applies to inverse square force laws such as the electrical force. So if you put an electron inside a positively or negatively charged sphere, it will also have a net force on it of zero.