Forced vibration movement

[Skies912]

When you force a material to vibrate at its resonate frequency, what's the path of movement it vibrates at?

 

example :: Tuning fork - strike the fork against any surface. Does it always vibrate on a single axis or radial arc?

 

reason :: I would like to create an experiment where I can force vibration to occur only on one axis; continually moving back and forth. This action, coupled with a magnetic coil, might generate enough electricity to power a small device.

 

Also, if an object keeps vibrating at its resonate frequency, will it eventually degrade?

[imatfaal]

Welcome to the forum

 

Note sure of the path - but I think even tuning forks follow paths with hints of harmonics and they are roughly elliptical rather than linear or circular.

 

And if you are thinking of a money-for-nothing over unity machine stop right there. They do not work.

 

For this particular instance - if your metallic tuning fork was moving in a magnetic field small currents would appear called eddy currents; these are induced in an conductor in a moving magnetic field or a conductor moving through a magnetic field. Eddy currents oppose the motion of the conductor. They will therefore damp your motion. And you won't even break even as the conductor will heat up due to its own motion and the flow of current

 

http://en.wikipedia.org/wiki/Eddy_current

[swansont]

The vibration details will depend on how you construct the device. A tuning fork vibrates primarily in one dimension because the material is much stiffer for vibrations in the other dimension, so those resonances would be at a very different frequency and probably much smaller amplitude.

 

Even on resonance, there will be losses. They're just smaller than at other frequencies. Unless you are mechanically driving the oscillation, removing the energy via induction will quickly dampen it. The load may also tend to change the resonance.

[Skies912]

Thanks for the replies.

 

No, I am not looking into free energy; I agree it's impossible with our current knowledge of the universe & mathmatic understanding.

 

However, what I am looking into is converting mechanical vibration energy into electrical energy, assuming I have a way to keep the object in the coil vibrating at its resonating frequency.

[Enthalpy]

Resonant movements follow often - but not always - a sine function because the movements are small, which linearizes the object's behaviour.

 

Mechanical resonance modes are often numerous. By design, you can put the interesting mode at a frequency very different from the others (say, a spring and a mass), rather close to an other (the sound board of a piano), or have two modes at the same frequency (a round bell vibrates NS and EW).

 

Depending on where you provoque the vibration and with what material, you excite more one mode or an other. That's the effect of varied mallet coatings at percussion instruments; the musician also targets different locations to get varied sounds.

 

Vibrations do degrade materials. I expect a resonance to act exactly as any cyclic deformation, for which Woehler curves are known:

http://en.wikipedia.org/wiki/Fatigue_(material)

- Take margins versus the material's proof stress

- The material shall have a proof stress as high as possible and can be brittle

- The surface shall be smooth, and the polishing grooves be parallel to the elongation direction

 

In a mechanical vibration of adequate material, the biggest loss uses to be the fastenings, but this improves quickly if holding at the nodes with the proper compliance and material. Then, the second loss is sound radiation, which improves with small, open areas, and with a material with slow sound propagation like bell bronze or invar.

[swansont]

Resonant movements follow often - but not always - a sine function because the movements are small, which linearizes the object's behaviour.

 

 

If it's not a sine then more than one frequency is involved. Which I expect will tend to dissipate the energy and limit the energy buildup of the resonance.