APS Excellence in Physics Education Award
November 2019

Science SPORE Prize
November 2011

The Open Source Physics Project is supported by NSF DUE-0442581.

how is vx calculated? post and replies

how is vx calculated?
Lev Yampolsky
5 Posts

I would imagine that for each time point t, vx should be calculated as

vx(t) = (x(t) - x(t-d))/d,

where d is the time step.

I see that it is not calculated this way.
In a set of measurements done at 0.5 sec steps I found out that vx is calculated as

vx(t) = x(t+d) - x(t-d).

Strange, but ok. One can argue that this sliding averaging over two consecutive measurements is a good idea.

But when I look at more detailed data, with every frame being a step (d=0.04 sec)

the same formula does not apply, i.e.,

vx(t) != (x(t+d) - x(t-d))/0.08.

(I found in help that this is the default algorithm, but it is clearly not in my data. Help mentioned that there is another algorithm, Bounce - maybe I accidentally turned it on? I could not figure out how to switch between algorithms).

I also checked if maybe velocities are averaged not over two two consecutive measurements, but over the whole 1 second. Would be very detrimental to capturing quick changes in velocity, but ok. But this is not the case either. Sometimes x clearly increases over several observations, but vx changes sign! Sometimes x is changes direction, but vx does not change sign. Something is not right.

I mean, I really don't have to care, let's assume that x and y are reported correctly and I'll calculate velocities myself, but just curious.

Replies to how is vx calculated?

Re: how is vx calculated? -
Lev Yampolsky
5 Posts

I have a feeling that the observed problem is somehow related to the mysterious statement in the Autotracker help:

The specific track and point marked at the target position must be selected prior to autotracking.

What is a track and point and how do I mark it on the target? It is not mentioned anywhere downstream in the help. I am worrying that I did not select it prior to autotracking and thus all velocities are calculated relative to some wrong reference point.

ok, I see what is happening. -
Lev Yampolsky
5 Posts

after a lot of experiments I am convinced that my problem is with the mysterious  track and point thing. A lot of times, even when I clicked at the correct object, Autotracker keeps calculating velocities of the reflection (hence opposite signs relative to the actual object's trajectory). This happens because when I shift-control-click on the image I set the target, but I do not set the new "track and point" (should show un on the video as a thick cross). A couple of times I saw this cross while clicking, but could not figure out how to make it show up. No combination of shift-control-alt-command brings it in.

Sounds like a mighty important feature of the Autotracker and totally counterintuitive (why is it separate from the target and why can it be offset from it?)- wouldn't it be a good idea to describe it in help?