General > General high-speed discussion

Speed Calculations

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wally_x:
Hello, I was wondering if someone could help me with the math on calculating the speed of a moving object that I record with my Chronos.

I've built what I call a "speed chart" (black and whiteboard behind the moving object like on Mythbusters) and I know my capture frame rate and my playback frame rate, but not sure what the equation is. Especially with the capture speed vs the save or "playback speed".

Any help would be much appreciated!

BiduleOhm:
The playback speed doesn't matter.

What you want to do is to count the number of frames the object takes to move from the start point to the end point you chose on your speed chart.

Then as you know the framerate of the capture you can know the time (frames number / fps = time in seconds) and you have the distance (let's say it's a foot) so you just have to do distance / time to have the speed in ft/s (or whatever distance you chose on your reference chart, per second) ;)

wally_x:
So basically if my object moves 2 ft on my chart in 10 frames and I was shooting at 2000fps then the math is something like:

10 divided by 2000 = .005 seconds. That divided by 2 gives me .0025 ft/sec, which I can convert to mph or anything else. This sound right?

BiduleOhm:
Here you've done time over distance, not distance over time, so that's not good. The speed is actually 400 ft/s (2 / 0.005). Everything else is right ;)

wally_x:
Awesome! Thanks again for you help!!

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