CurveFitLab3 - For example if you see your data and have x...

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PHYSICS 103M – LAB 3: FORCE AND POTENTIAL ENERGY Help getting the curve fit You know how to perform a curve fit in KaleidaGraph (we learned it in the Computer tutorial, pages 34-35). Specially, you know very well how to perform a weighted least square fit. We will now define another curve U(x) and we’ll fit our data to this new curve. To define it, follow the steps bellow. 1. Once that you have plotted the x and y (which is U(x) in our case) points and their respective error bars, click Curve Fit on the main menu. 2. Click on General fit1 . 3. In the Curve Fit Selection click on Define… 4. A window called General Curve Fit Definition will appear. 5. In that window, in the middle part, you’ll see: m1 + m2 * M0; m1 = 1; m2 = 1 (or something similar) 6. Erase that line and type: m1 / (M0 + m2)^3; m1 = ###; m2 = ### 7. Instead of writing ### you need to write a number there, for m1 and m2. Look at your data and pick any value for x and its corresponding value of U.
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Unformatted text preview: For example, if you see your data and have x = 2 and U= 5, then your values of m1 and m2 will be given by: m1 = x 3 ⋅ U = x 3 ⋅ U = (2) 3 ⋅ 5 = 40 m2 = x = 2 You need to use your own values, of course. This is just an example. 8. So your line will now look like: m1 / (M0 + m2)^3; m1 = 40; m2 = 2 9. Click on Weight Data at the bottom of the same window. 10. Click OK. 11. Open your lab-manual on page 34 and follow the procedure of Performing a Weighted Least-Square Fit starting on step 5. 12. If you get a bad fit or a “singular coefficient matrix” message you need to start over from step 1 and change the m1 and m2 values you used in step 6. Play around with the numbers (multiplying them or dividing them by 10) until you get a nice fit like the one on page 59. 13. Make sure your errors for m1 and m2 are NOT bigger than your base values for m1 and m2....
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