PLEASE HELP! I know the answer to this, but I need an explanation.

Let r be the distance between the man and point Q. He applies a torque of

(r i ) × ((-800 N) j ) = ((-800 N) r ) k

pointing "into the page" with magnitude (800 N) r.

Meanwhile, the rope applies a torque of

((3.0 m) i ) × ((500 N) j ) = (1500 N•m) k

pointing out of the "page" with mag. 1500 N•m

The beam is held in place in a horizontal position, so the torques cancel and

(800 N) r = 1500 N•m

r = (1500 N•m)/(800 N) = 1.875 m ≈ 1.9 m

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