On Sunday 25 June 2006 23:06, Jos van Kan wrote:

Kevin Donnelly wrote:

...

My son is doing physics coursework, and has a set of figures plotting voltage against resistance. He's used Qtiplot to draw a graph of the resulting curve, but does anybody know of any software which would allow this line to be extrapolated to show likely resistance for a given level of voltage - ie go beyond the actual data from the experiment to show what might happen if the experiment had been extended? It may be that Qtiplot can do this, but I can't find out how. If not, are there any other candidates out there?

It's way off topic, but least squares would be the standard method here and I think that's exactly what the physics coursework would be asking your son to do. That would be a lot more illuminating as how to go about such problems in general than putting it into a black box and watch the numbers come out. There is software to do least squares, but this is exactly the sort of problem you should do by hand by hand.

No, he's already done the coursework, which involved plotting actual results by hand - he's in year 4 secondary, so least squares would be a bit beyond them! The above is for the conclusions part, where he wants to say something like "if you change a, b might change as follows". He can just extrapolate the curve manually on the graph paper, but I thought there might be something that could do that more neatly. No matter. Thanks to all for the responses. -- Pob hwyl / Best wishes Kevin Donnelly www.kyfieithu.co.uk - KDE yn Gymraeg www.eurfa.org.uk - Geiriadur rhydd i'r Gymraeg www.rhedadur.org.uk - Rhedeg berfau Cymraeg www.cymrux.org.uk - Linux Cymraeg ar un CD -- Pob hwyl / Best wishes Kevin Donnelly www.kyfieithu.co.uk - KDE yn Gymraeg www.eurfa.org.uk - Geiriadur rhydd i'r Gymraeg www.rhedadur.org.uk - Rhedeg berfau Cymraeg www.cymrux.org.uk - Linux Cymraeg ar un CD -- Check the headers for your unsubscription address For additional commands send e-mail to suse-linux-e-help@suse.com Also check the archives at http://lists.suse.com Please read the FAQs: suse-linux-e-faq@suse.com

QtiPlot which is what he is using, will plot out functions Just do a curve fit and in the top window Qtiplot will post the equation of the line. use this equation to plot out a separate function giving it the limits that you want to predict to. I just did this on my copy and it works great for a linear and a Polynomial fits. You have to play with the limits to get the graph the way you want it. BoB C

Kevin Donnelly writes:

...

No, he's already done the coursework, which involved plotting actual results by hand - he's in year 4 secondary, so least squares would be a bit beyond them!

He can add one or more artificial points and handle the problem as an interpolation. He can place them outside the visible axis range to make them "invisible".

...

He can just extrapolate the curve manually on the graph paper, but I thought there might be something that could do that more neatly.

There are many software packages which can fit a curve (based on an appropriate mathematical model) to a set of data points (http://root.cern.ch/, http://www.r-project.org/, ...) but they are written for scientists or statisticians ...

-- A.M.

-- Robert Cunningham Sr. Physics Laboratory Coordinator /RSO Kettering University -- Check the headers for your unsubscription address For additional commands send e-mail to suse-linux-e-help@suse.com Also check the archives at http://lists.suse.com Please read the FAQs: suse-linux-e-faq@suse.com

Kevin Donnelly later wrote:

No, he's already done the coursework, which involved plotting actual results by hand - he's in year 4 secondary, so least squares would be a bit beyond them! The above is for the conclusions part, where he wants to say something like "if you change a, b might change as follows". He can just extrapolate the curve manually on the graph paper, but I thought there might be something that could do that more neatly.

Depends what you mean by "neatly". If he's doing it by hand anyway, then simply drawing a longer line with the ruler is just as neat as the line he drew in the first place! However, if you're looking for an approach which in some sense calculates the extrapolation, rather than simply draws it (so if "neater" in the sense of "cleaner"), then you are entering the territory of least squares and all that! That being said, when it comes to drawing graphs and diagrams in the midst of nicely formatted text, don't ignore what's been available in Linux (and indeed Unix) since the year dot (which in the case of Unix is 1970). Namely the 'groff' package, which is what's used for producing the formatted man pages, but is capable of much much fancier stuff than that. As an example (which makes use of the 'pic' component of groff), oriented towards your query: 1. Make a text file containing the data on resistance (col 1) and voltage (col 2) as follows: 50 4.9 55 5.5 60 5.6 65 7.0 70 6.9 75 7.6 80 7.9 85 8.5 90 8.7 95 9.1 100 10.0 and call this file "data.txt" (only the above numbers, and no blank lines). 2. Make a text file containing the following text: .LP Here is my graph of Voltage versus Resistance. .PS circrad=0.5 yscale=5 start=1 define blob { circle radius circrad at ($1,$2*yscale) } x0=0 ; y0=0; x1=0; y1=0 define lines { if start then { x0=$1 ; y0=$2*yscale ; x1=x0; y1=y0 start=0 } else { x0=x1 ; y0=y1 ; x1=$1 ; y1=$2*yscale line from (x0,y0) to (x1,y1) } } .PE .PS 4i copy "data.txt" thru blob copy "data.txt" thru lines line from (0,0) to (150,0) "" "" "" "" "" "\fBResistance\fP" for i=0 to 15 do { r=10*i line from (r,0) to (r,-0.2*yscale) "" "" sprintf("%.0f",r) at end of last line } line from (0,0) to (0,16*yscale) for i=0 to 8 do { v=2*i line from (0,v*yscale) to (-2,v*yscale) sprintf("%.0f ",v) rjust at end of last line } line invis from (-15,0) to (-15,16*yscale) "\fBVoltage\fP" aligned dashwid=2 line dashed from (0,0.2*yscale) to (150,14.7*yscale) .PE .sp 3 .LP The dashed line was drawn by eye through the experimental data. It can be used for extrapolation. However, it does not go through the origin, which it should! and call this file "physics_ex.tr" 3. Now, on any Linux system, execute the command groff -p physics_ex.tr > physics_ex.ps and you will have a PostScript file with a very nice diagram in it. If you want neat, this gives you neat! The key to the plotting of the data points is the "copy thru" command, which in its first instance plots the points from the data file, and in the second instance draws the lines joining the points. Then there's a dashed line through two points which the student could have read off from graph paper having drawn a line by eye with a ruler. All the rest is furniture. The above may look formidable at first sight, but it's not nearly as bad as it looks when you get down to it (provided you get the syntax right). In fact, it's not a hundred miles from the good old Turtle Graphics which some of us might remember! In the past I've used this sort of thing with schoolchildren. For example with a 14-year-old who was by no means a mathemtician, and had been set a project task of designing a cut cardboard shape which was to be folded into a box with a lid, with folding tabs to glue the whole thing together. Once I'd set up the basic framework on similar lines to the above, the rest was a matter of choosing the numbers so as to produce a good result, and the child progressively caught on to the idea of using numbers (x,y) for the position of a point, and then to composing the elements of an object using such points (i.e. the rectangles for the sides and lid etc., and trapezoidal shapes for the folding tags, and putting these in the right positions); so in fact picked up a bit of coordinate geometry on the side! Best wsihes, Ted. -------------------------------------------------------------------- E-Mail: (Ted Harding) Fax-to-email: +44 (0)870 094 0861 Date: 26-Jun-06 Time: 12:16:07 ------------------------------ XFMail ------------------------------ -- Check the headers for your unsubscription address For additional commands send e-mail to suse-linux-e-help@suse.com Also check the archives at http://lists.suse.com Please read the FAQs: suse-linux-e-faq@suse.com