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[acscott]

My daughters science project is based on determing if the positioning of a figure skaters arms will affect the rate of speed or number of rotations in a spin. Her hypothesis is that a spin will speed up and the number of rotations would increase when the skaters arms are closer to the body.
She based the hypothesis on many factors. First, she has been a figure skater for eight years. Secondly, according to Conceptual Physics a spin is done on the longitudinal axis which is the center of gravity. That is where most of the mass is located. The rotational inertia of an out stretched arms is three times more than the inertia of tucked in arms. A perfect spin is obtain by speed, balance, and a feel of being in the center. Spinning in place (centering a spin) is one of the most difficult aspects of skating.

My question is what would be the best way for her to accurately measure the change. We will be using a video camera to record the change but there needs to be a way to actually measure it. Any ideas?

[deleted-71572]

A good question (I myself always have trouble counting how many times the skaters spin around when I watch the Olympics. Makes me dizzy just thinking about it, actually.) -- this gets to the importance of measurement and reporting possible errors associated with your measurement in science.

In order to get speed, you need to measure two things: The number of rotations, divided by the amount of time. Your daughter will have to decide if it is better to see how long it takes to do, say, ten complete revolutions with her arms out versus her arms in, OR how many complete revolutions she can do in a fixed amount of time (say a minute). Either way, you need a stopwatch/timer, and the ability to count revolutions (maybe one of those clicker things they use to count people going into concerts?). Will you have the ability to slow down the video? If so, that seems like the easiest way to double check the count. You can put some sort of fixed object (maybe a pole) in the frame of the picture, and say, every time the skaters left shoulder passes the pole, that counts as one.

[deleted-71576]

If you have a good video camera which can hook up to video editing software (many video cameras come with reasonable software these days) you can just go frame-by-frame and be able to get very accurate timings of the turns.

Video is shot at 30 frames per second (actually, 60 interlaced frames (60i), but for this discussion think of them as 30 full frames a second). So each frame is separated in time from the next frame by 1/30 or 0.033 seconds. It's extremely accurate, and given a set point of reference (the pole that Lucinda mentioned, for example) you can get very accurate data, which you can analyze in the warmth of home, and not miss readings due to measurement error. A digital video recorder can be an amazing stopwatch for science fair experiments.

If you used real film, it would likely be at 24 frames per second, and you would need to adjust that accordingly. But a simple digital video recorder would work just fine.

[MelissaB]

I agree with the other experts about measuring #of turns--but I think another problem is going to be measuring how far her arms are extended. I suggest she use three different positions for her arms--all the way out, all the way in, and midway, then repeat these several times (since there may be some variation associated with choosing 'midway'. I've never figure skated, but if you can't go into a spin with your arms in a certain position, it may be worth having an audio signal 'Now!' to tell the camera when she feels her arms are in the 'right' position.