Consider a pendulum (a mass at the end of a light string) swinging back and forth. Ignore air resistance. Is mechanical energy conserved?

1. Yes--there are no nonconservative forces present.
2. Yes--nonconservative forces are present but do no work.
3. Yes--nonconservative forces are present and do work, but the work done by some exactly cancels the work done by others.
4. No, mechanical energy is not conserved.