Notes on Relativity and Cosmology - Physics Department, UCSB

282 CHAPTER 9. BLACK HOLES
(c) What would we see and feel if, instead of staying out side, we fell into
the black hole? Would we ever see the star’s surface reach r = 0?
9. Suppose that you are a static observer (perhaps you have built a platform
to stand on) far from a black hole and that you tie a long string (of
negligible mass) to a 1kg rock. You then drop the rock and let it fall
toward the black hole, pulling the string with it. You also tie the string
to an electric generator so that, as the rock falls, it powers the generator.
The generator will take energy from the motion of the rock and turn it
into electrical energy.
Suppose that the rock falls all the way to the photon sphere before stopping.
If all of the energy lost by the rock is made into electricity (i.e., a
100% efficiency generator), how much electricity will be made??? Is there
a difference between using a ‘large’ black hole and a ‘small’ black hole? If
so, explain what it is.