Video

The camera on most phones is a valuable tool for vibration measurement because the physical object is represented in the field of view. Including a high-precision clock in the field of view can allow you to count cycles of vibration. This tool is limited primarily by the frame rate of the camera. Common frame rates are 30 fps and 60 fps. The iPhone Pro models have SLO-MO with frame rates of 120 fps and 240 fps, which significantly improves the precision of measured frequencies. This method is also limited by your own ability to identify similar points in subsequent cycles and accurately select the frame.

Example

Try this yourself with this video, which was shot at 60 fps. You can pause the video and use the "." and "," keys to advance or reverse the video by one frame. You should find a natural period of around 0.37 seconds, which is a natural frequency of 2.70 Hz.

But, how well do we know this? The resolution of the clock is 30 Hz, or 0.0333 seconds, which you can confirm by looking at the time in each frame. So, that natural period has an uncertainty of 0.0333/0.37, or almost 10%! Consider your own ability to to discern which frames have the cantilever tip in the same location and you increase the uncertainty. That makes this method great for checking other more precise results but not as accurate to use on its own.

You can get yourself a "millisecond clock" here, but be forewarned, it's resolution is actually only 1/60th of a second, not 1/1000th as its name suggests.

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