Forced Frequency Homework: Which of These Acts as a Force?

[Redjakk1]

Homework Statement

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.

Homework Equations

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.

 

[rude man]

Homework Statement

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.

Homework Equations

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.

Right, all three.
The pendulum swings due to initial condition(s), e.g. a finite displacement & then let go. It would continue swinging (assuming no friction) the same way forever. However, since the pendulum touches water at its low point that forces a change to the oscillatory motion.
The sound applies a forcing function to the glass, otherwise it would not vibrate unless again a finite initial condition were imposed, e.g. the glass is stresses (bent) and then let go.

 

[Redjakk1]

Right, all three.
The pendulum swings due to initial condition(s), e.g. a finite displacement & then let go. It would continue swinging (assuming no friction) the same way forever. However, since the pendulum touches water at its low point that forces a change to the oscillatory motion.
The sound applies a forcing function to the glass, otherwise it would not vibrate unless again a finite initial condition were imposed, e.g. the glass is stresses (bent) and then let go.

Thank you very much for the explanation. I understand it a lot better now.