The surfaces absorb energy from the air. Friction changes energy of motion to heat energy. Moving objects produce heat energy. Energy is transferred between the two surfaces.
I gave my students this multiple choice question: Why do two surfaces get warm when they are rubbed together?

Only one answer is correct among these 4 choices:
- The surfaces absorb energy from the air.
- Friction changes energy of motion to heat energy.
- Moving objects produce heat energy.
- Energy is transferred between the two surfaces.

Could you tell me which one is correct?

Friction changes energy of motion to heat energy.