How can I measure the speed of a figure skater's spin? My daughter is a figure skater and needs to measure how fast she is spinning before she loses her position. This is a science project for 7th grade. I have looked for devices to possibly measure her speed such as a "tachometer" and "stroboscope" but I don't think these will work. Any suggestions?
 A: In this link you can find a video analyzer tool. This tool works well with videos taken even on a smartphone, and is free and made for educational purposes.
https://www.cabrillo.edu/~dbrown/tracker/
Have her hold a distinct red kerchief or ball in one hand. This way the tracker can easily track her spinning. 
You can get frequency as well as number of spins.
A: If you look at this video which claims to be a "world record spin", they claim they measured the speed (308 rpm!) with a "Sport Tach" - see for example review at http://figureskating.about.com/od/accessories/gr/sporttach.htm . Tried to follow links to their website - it seems their domain is for sale so I suspect they have gone out of business.
When you google "long range non contact tachometer", you will find a device listed on eBay for very little money - http://www.ebay.com/itm/like/171278492137?lpid=82 . For less than $15 you might just see if it works...
The Sport Tach was described as an expensive item, but the principle was described as a "infrared tachometer". They claim they put a "transmitter" on the clothing of the skater.
I think that if you took an IR LED (from an old remote control) with a small battery, a switch and a series resistor (to limit the current), and mounted it (on your head or elsewhere) as a "transmitter" (think very fast lighthouse), then you could record the flashes of IR light with a receiver that has a matching IR window (like you have on the IR remote control of a TV - it limits the amount of ambient light for better SNR).
I don't know what kinds of equipment you have available, but for relatively little money you can get a cheap "oscilloscope" for your iphone that would be able to record the signal. The electronics skill needed to do this herself might be outside of her range, although at this speed you could use a phototransistor, a battery and a large series resistor, and watch the signal turn on and off. Personally I would probably do this with an Arduino plus a breadboard - super easy to program, and fun too. But maybe more dad's (ors mom's) project than the daughter's...
But bringing this back to the level of a 7th grader - I would recommend to use the slo-mo video of the current iPhones, and play it frame by frame to determine the spin speed. It records at 120 fps, meaning that at 300 rpm (the "world record" above was 308) you would have one spin for every 24 frames. Frankly even if you recorded at normal framing rate (29.97 fps - close enough to 30) you should be able to count the number of frames for each complete revolution, and make a simple plot:

Here I plot the frame number on the bottom, and the spin number along the Y axis. Excel allows a simple parabolic fit which can be used to estimate the approximate RPM with a simple formula (leaving that as an exercise - it's her science fair project, not mine). Note that if you are off by one frame, it will smooth out over the curve. Don't try to use the conversion "7 frames = (30 / 7) * 60 rpm, because that would become a very bumpy curve very quickly.
I think this last method is the most in line with science fair technology.
A: If you don't want to do video, you could try with a smartphone. There are apps which can record the sensor measurements, including the accelerometer and the gyroscope. I have no idea how noisy the data will be for a skater, but if the device is firmly attached to her body close to her center of mass, it may deliver useful results. From there the sky is the limit. There are many companies which provide technological solutions for different applications from medicine, sports to CGI effects for movies. Do a google search for motion tracking. 
A: you can use GOM playef pres F to count revolutions  if you count  6 frames per revolution 6x5= 30 frames  then you spin is 5 rev per second thats is tha same 5x60 second=  300 rpms,
or use  iphone  Spinorama free app
