A 4000 kg freight car moving at 3 km/hr collides and couples with an 8000 kg freight car which is initially at rest. The final speed of the combined cars is

b.

explanation:

10.0

iron would move it is magnetic there is a mag field around the current in the wire

force required to detach it is given by

charges are taken to be fundamental charge

now we have

so above is the minimum distance at which it will detach