如果有個motor d coil得上面個part cut 到field line下面個part cut唔到..

Once started, the coil of the motor could keep on rotating. It is only that the torque given by the motor is reduced by half.

For only half of the magnetic field remains, there is always half of the coil inside the magnetic field when it is rotating. Thus half (or one-sdie) of the coil is under the action of magnetic force (compare with two sides of the coil is acted by magnetic forces if the magnetic field covers the whole coil). This force would still keep the coil rotating.