Re: [Phys-L] Question on data analysis

[Peter Schoch <pschoch@fandm.edu>]

Hello,

If you do an Atwood’s machine experiment and graph the applied force on the y-axis and the acceleration (derived from the time) on the x-axis, the trend line should not be forced through the intercept — in theory it could/should give you an estimate of the friction in the pulley.  (Ignoring the moment of Inertia.)

Rather than “telling them” what Ohm’s law is, if you have students measure the voltage and amperage across/through a resistor and then graph the data I would force the intercept to zero, because if there is no amperage in the circuit there is no voltage.

Motion on an air track, graphing distance vs. t^2 — if there is no elapsed time there should be no motion, then force it to zero.

I guess the answer you’re seeking is:  Based on the physical analysis of the system, and perhaps the theory you are trying to illustrate, you must make a determination of whether it is logical to have a (0,0) coordinate.  _Sometimes_ the theory in freshman physics would say that the intercept should be zero (Atwood’s machine above), but we know that “in the real world” this is not true, it just arises from the simplifications we do in freshman physics.  When you know that the intercept may give you interesting results, and give the students something to ponder (like my Atwood’s example, with which I ‘torture’ my students) then have them do the regression both ways (easy enough with a spreadsheet) graph it all (data and both fit lines — R^2 values help in their analysis) and then have them think about it.  It can oftentimes be a great teachable moment.

BTW, if you are using MS-Excel you might want to use the built-in functionality and then use the formulas to also calculate the same results — you’ll notice a difference in the 2 answers!  OTOH, if you use Libre Office, you will not notice the same difference in the answers.

Peter Schoch


> On May 21, 2015, at 4:50 AM, Savinainen Antti <antti.savinainen@kuopio.fi> wrote:
>
> Hi,
>
> spreadsheet programs usually have a feature which allows to force a trendline through the origin. Of course,
> the same thing can be done manually as well. My question is: when it is reasonable to force a trendline through the origin? I mean, a non-zero y-xis intercept may have a physical interpretation or it may indicate systematic error in measurements or (more realistically) may be a combination of both. Given these, what could be a benefit for the forcing of the trendline through the origin?
>
> The question was motivated by an observation that a chemistry colleague insists that her studenst must force a trendine through the origin (in case there is no chemical interpretation, I suppose) for the y-axis intercept.
>
> Regards,
>
> Antti
> Finland
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