1. The problem statement, all variables and given/known data Which of the following is a forced frequency acting on the oscillations? I. A pendulum skims the surface of a pool of water at the lowest point of motion. II. To keep a bell ringing, a bell ringer pulls on a bell rope. III. A singer shatters a glass. 2. Relevant equations 3. The attempt at a solution My guess would be all 3 as each of these situations has an external force. But I'm not quite sure as I read an example saying that the singer situation would be mechanical resonance. Does that mean it's still forced frequency? Also, I'm unsure as to whether the first situation is referring to the waves which result from the pendulum (I don't think so) or the oscillations of just the pendulum. In that case, I think the answers would be I and II. I just don't really understand.