Question

Trent builds a model electric motor. When he connects a battery to the motor, the motor spins through a half rotation and stops.

Which is the most likely solution to the problem?

replacing the battery, because it is not providing current
installing new brushes, because they are not making a good connection
checking the commutator, because the current is not reversing
adding more wire coils in the armature, because the electromagnet is too weak

Answer 1

checking the commutator, because the current is not reversing

Explanation: a DC motor needs the commutator to reverse the current every half turn to keep torque in the same direction. If the commutator/segment or its connection is faulty the coil will turn about half a turn and then reach the neutral position with no torque and stop.