Question

A magnetised needle suspended freely in a uniform magnetic field experience a torque but no net force.an iron nail near a bar magnet however experience a force of attraction in addition to torque. Why?

Answer 1

Force of attraction always exists. A magnetized needle can rotate freely and obtains the direction, where all the torques are compensated an the net torque is 0. But there is a reaction force on the prop of the needle. & If you remove the prop, then needle will move under the action of magnetic force. Because the needle is magnetized, the optimal position in the space is when the north pole of the needle is directed to the south of the magnetic force lines and vice versa for the south pole of the needle.