A cinder block is sitting on a platform 20m high. It weighs 79N.

A cinder block is stationary it means it only has potential energy.

Explanation:

                         Potential energy (PE) = mgh

where,

m= mass of the block

g= acceleration due to gravity

h= height of the block

Now,

                         PE= 79 × 20 J

                   or, PE = 1580 J

The energy stored by the block as potential energy is 1580 J.

the minimum amperage that the current in this wire would have to have so that its weight is completely supported by the magnetic force due to earth's field, assuming that no other forces except gravity act on it