The Physics of Vibrations

[musichic109]

[color=darkblue]I am 15 years old, in 9th grade, and I am trying to compete in a science fair. The project that I need to do on the Physics of Vibrations. I need to find the size of the sweet spot on 25 different racquets, and compare them. I got the idea from he topic listed under Tennis in "sports science". The paragraph says that i need a photodiode and analog-to-digital converter to do this project accuratly, but where can I get such items? I have called Radioshack and Circuit City, and they both have a photodiodes, but not an analog-to-digital converter. Even if I can somehow get these items, how will I know how to use them? How can I connect them to a computer? Is there another way to do this project? [/color][/i]

[Louise]

I am 15 years old, in 9th grade, and I am trying to compete in a science fair. The project that I need to do on the Physics of Vibrations. I need to find the size of the sweet spot on 25 different racquets, and compare them. I got the idea from he topic listed under Tennis in "sports science". The paragraph says that i need a photodiode and analog-to-digital converter to do this project accuratly, but where can I get such items? I have called Radioshack and Circuit City, and they both have a photodiodes, but not an analog-to-digital converter. Even if I can somehow get these items, how will I know how to use them? How can I connect them to a computer? Is there another way to do this project? [/i]

You are talking about the project on this page (this is more for other readers of this thread):
https://www.sciencebuddies.org/mentorin ... ?from=Home

This isn't a developed project, like the others on the science buddies page, so you will need to do more research yourself. This is why there are not sources for materials, costs, and detailed experimental procedures.

However, there are several references in this section- you should read them since they will provide more detail. It sounds like there is a simple way to do the measurement "loosely tape a card to the handle so that it will vibrate when the racquet, bat or club is tapped (Brody, 1987, 33)." compared to the complicated way, "If you want to go all out, you can measure the vibration of the card by monitoring light reflecting off the card with a photodiode and analog-to-digital converter."

As to where to get an A/D converter, the A/D converter will depend very much on the parameters that you need to measure. Basically, the photodiode will create an analog signal that you want to digitize and put in to a computer. You can then measure the frequency of the light on/light off. What type of A/D converter you use depends on a couple of things- what frequency is the system vibrating at? Some A/D converters are good for slow frequencies, and others are good for high frequencies. This is all listed in the specification for the converter. In order to chose the correct converter, you need to have an idea of what frequencies you will measure. Note: what you need might be very expensive.

If you are near a University or have a very nice high school physics lab, you might be able to just use their oscilloscope instead. Some of the newer oscilloscopes have all sorts of programs that will automatically calculate the signal frequency. On models that don't have these features, you can manually calculate this. [Note: a lot of this discussions apply to photodiodes too... some only "see" light changes very slowly, so some models may not be "fast" enough to observe the frequency you are interested in.]

I think you should start with reading the books cited and then asking more questions here when you have more information. Once you know information about the frequency ranges you are interested in, and have a better idea of the experiment, we can be more helpful!

Also, 25 tennis rackets sounds very labor intensive. It will take some time to map out the "sweet spot" of each racket, and you probably want to do this multiple times per racket to get some average values.

Good luck!
Louise

[deleted-71588]

You might want consider this as an engineering measurement problem. What aspects of vibration are you trying to measure? Amplitude? Frequency? Rate of decay?

Somebody proposed the idea of using a photo cell? What does it measure (input) and what does it provide (output)?

Analog to Digital converters typically will measure some electrical property (voltage or current) and provide a digitized result. DVM (digital volt meters or digital multi-meters) are pre-packaged units which can accurately measure slowly changing voltage, current, or resistance. The subtle properties of the A/D converters involve timing and sampling theory.

Vibrations are not "steady state" so using a photo cell and an A/D converter by themselves isn't a complete solution to any vibration measurement problem.

Try searching for "measuring vibrations" to get some more information and ideas.