In an electric motor, periodically changing the electromagnet can cause the axle to spin because

a. the electromagnet loses its magnetism
b. mechanical energy is converted to electric energy
c. the force is constant
d. the magnetic field reverses direction

We want to know why does the axle spin in the electric motor. When we periodically change the electromagnet via alternate current, the magnetic field reverses directions which creates a force on the axle and so the axle starts spinning. So the correct answer is d. the magnetic field reverses direction. 

The answer is  the potential energy of the bob at position b is 1.47 joules.if the maximum height of the bob is 0.45 meters when it swings back,1.323   joules of energy was transformed to thermal energy.hope this