## SHM Damping coefficients help!

### dfx

Posted 26 December 2005 - 11:36 PM

Hiyya,

I'm investigating eddy current damping of Simple Harmonic motion and I'm going to be investigating how the damping coefficient varies with the magnetic field. A "magnetic tunnel" creates eddy currents which damp the metallic glider on an air track. It's an almost classical experiment.

However, since I am using a glider on an air track and a motion sensor, I've to attach a target on the glider which the motion sensor tracks (to record the simple harmonic motion). The problem is that this target is of such a shape that it'll offer plenty of air resistance (it's a flat rectangular sheet). So the problem is that air resistance will no longer be negligible, but has to be taken into account. So say I found the damping coefficient with just air resistance (no magnetic tunnel), and then with the magnetic field AND air resistance, how do you get the damping coefficient due to the magnetic field ONLY?

i.e. if I wanted the damping coefficient due to eddy currents (magnetic field) ONLY, would I SUBTRACT the damping coefficient of AIR from damping coefficient of (air + eddy currents)?? (i.e. to get an accurate value, so as to remove the effect of air resistance damping from the value). Basically, are there any known theories/mathematics for "combined damping"?

I hope someone can help. There is practically nothing on this on the internet, and if I'm not mistaken it's slightly into uni level physics!!!

Any feedback/help much appreciated. Thanks!

### Vixus

Posted 27 December 2005 - 09:27 PM

You really went for the gusto with this project, here I am with coefficients of static friction. Pretty lame compared to yours. I don't know whether you can do a simple subtraction. Which variables are you controlling and which are you measuring?

You may have to repeat the experiment with different sizes of sheet to get your results.

### dfx

Posted 28 December 2005 - 10:08 AM

I know! I've been coerced several times into just forgetting it and doing something far simpler - for the marks. However, I spent ages researching this and it is fairly rare lol and I've setup the apparatus and it works an absolute treat. The best bit, ironically, is that the motion sensor takes all the measurements.

As far as research shows, the distance of the magnets from the metal is a key variable (apparently an inverse square relationship). Also temperature has interesting effects. If you cool it to near absolute zero you get a superconductor with the meisner effect. But I'm not doing anything that fancy. Just want to vary maybe over a range 30 centigrade or so.

### Vixus

Posted 30 December 2005 - 07:18 PM

Then I'd just ignore all effects of air resistance if they're small enough, or reduce the size of card you're using so they are. Make sure to mention that in your evaluation.