# Damped Oscillation

1. Jun 13, 2004

### cutesoqq

In my notes, there are two sentences make me feel strange...

As we know, the pendulum whose length equals to that of the friver pendulum, its natural frequency of oscilation if the same of the frequency of the driving one. This is known as resonance oscillation.

However, somewhere I found another sentence...

"The resonant pendulum, is always a quarter of an oscillation behind the friver pendulu, i.e.there is a phase difference of T/4"

I don't know why there is a phase difference, if there is, then I think it contradicts the definition of resonance oscillation.

2. Jun 13, 2004

### Gokul43201

Staff Emeritus
In general, the phase difference is a function of the frequency and damping. At the resonant frequency, and at optimal damping ($$\gamma=w_0$$), the phase difference is $$\pi/4$$.

This does not contradict the idea of resonance as this IS the frequency where the amplitude is maximum.

3. Jun 14, 2004

### turin

Objects resonate, and systems resonate, driving forces don't resonate, per se. I will have to look up this friver pendulum, but the driving point does not have to be in phase with an object at resonance unless it is directly driving the property of consideration. For instance, if you directly and rigidly grab the pendulum's cable and forcefully swing it back and forth, then you would need to stay in phase to induce resonance. If, however, you have a really loose spring attached to the driving point, then you have to take into account the delay in the spring. Delay in response at a certain frequency is the same thing as phase lag.

I couldn't find anything about a friver pendulum. Can you explain what it is? BTW, if you meant "driver pendulum," then I appoligize. I'm not trying to make fun of you or anything. Even if you did mean driver pendulum, I still don't quite have a picture in my mind of the set-up.

Something that just came to mind:
There may be two resonance conditions. One is the resonance of an individual pendulum and the other is the resonance of a coupled two-pendulum system. Even if these two pendulums have the same resonance frequency, their coupling can give you a new resonance frequency. In fact, there will be two natural frequencies for the coupled two-pendulum system.

Last edited: Jun 14, 2004
4. Jun 14, 2004

### Gokul43201

Staff Emeritus
I thought 'friver' was a typo for 'driver' !

5. Jun 15, 2004

### cutesoqq

right... driven = driving in my notes...i feel troublesome with these words too...

6. Jun 15, 2004

### turin

Well, I don't know what a driver pendulum is. Please explain.