I raised a 20 kg rock 1.5 m off the ground. I am just holding it now. Nothing is moving (except for the muscles in my back and arms that are twitching like crazy--doing work--but that's internal energy/work). So how much mechanical work am I doing now

Electromagnetic induction. a coil of wire contains n turns and has an electrical resistance r. the radius of each turn is a. initially, inside the coil there exists a uniform magnetic field of magnitude b0 parallel to the axis of the coil. the magnetic field is then reduced slowly. the current induced in the coil is i. how long does it take for the magnitude of the uniform field to drop to zero?