Some of the energy is destroyed as the machine operates. The machine must overcome the constant force of magnetism. Useful energy decreases as energy is transferred to the surroundings. Some of the energy is released as heat when converted.
I gave my students this multiple choice question: Why is the output energy of a machine always less than the input energy?

Only one answer is correct among these 4 choices:
- Some of the energy is destroyed as the machine operates.
- The machine must overcome the constant force of magnetism.
- Useful energy decreases as energy is transferred to the surroundings.
- Some of the energy is released as heat when converted.

Could you tell me which one is correct?

Useful energy decreases as energy is transferred to the surroundings.