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Velocity problem with the step size post and replies

The size of the step change the value for velocity?
Ismael Lozano
2 Posts

The tracker software is great, in our lab resulted very useful. I have a question regarding a problem with my data. I measure the speed of escape of fish larvae. I am following the fish with a mass point step by step. I noted that if i register the speed (v) with a step size of 1 the maximum speed of the fish is 25.5. But if a register the same sequence with a step size of 36 the maximum speed is 2.5. I use the same calibration stick. My question is: is the speed changing with the step size? am i doing something wrong? It supposed that the speed is the same. Thank you for your time. Congratulations for the software
P.d.: sorry for the english, i am a spanish speaker

Replies to The size of the step change the value for velocity?

Re: The size of the step change the value for velocity? - Douglas Brown
353 Posts

Hello Ismael. Tracker estimates the instantaneous velocity at a given step n by calculating the average velocity between the previous step n-1 and the following step n+1:

vx[n] = (x[n+1] - x[n-1]) / (t[n+1] - t[n-1])

This means that for objects moving with constant speed in a straight line the step size makes no difference, but for objects (like your fish) that dart around with rapidly changing velocities then the step size can make a huge difference. Basically, using a larger step size gives you average speeds over longer time periods which will always be slower than the fastest instantaneous speeds reached.

Bottom line: if you need to determine the maximum speeds reached by your fish, you should use a step size of 1.  Doug

Thank you very much! -
Ismael Lozano
2 Posts

Thanks for clear the situation, now make sense. The reason that i use a step size of 36 is for obtain swimming cruise. For the maximum speed i used a step size of 1. The problem arise when i was measuring the cruise speed and the fish made a burst (very quickly swim). I previously measured that burst and i noted the difference in v.  Thank you again!

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