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F-dL Graph Help!

  1. Nov 4, 2009 #1

    Nrt

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    Hi, I need to plot a graph of F=kx for my lab report.

    By measuring x, displacement of spring for every mass (20g, 40g,50g) we found k. Now i need to draw F-x graph.

    I put my F and x data on graph w/o any problems. But when it comes to draw the line of F=kx i get stuck. Using the line's slope i need to find theoritecal k and calculate the error.

    I need to have a graph like this:

    http://phoenix.phys.clemson.edu/tutorials/graph/labgraph.gif

    This is my second lab report so i need help to get started and learn how to draw properly.
     
  2. jcsd
  3. Nov 4, 2009 #2

    Andrew Mason

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    I am not sure where the problem is. The slope of the straight line created by the plotted data IS k.

    AM
     
  4. Nov 4, 2009 #3

    Nrt

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  5. Nov 4, 2009 #4

    Andrew Mason

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    It appears either that force is not proportional to distance or you have some error in your data. Can you draw error bars for each point (representing the accuracy range of your data)? Draw a line that passes close to all points within the range of error for each point.

    AM
     
  6. Nov 4, 2009 #5

    Nrt

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    graph i linked is not the real one. I made it up just to show. But it looks something like that.

    Its normal that there are errors because the x(dL) was measured with a ruler for every mass atached to spring.

    Actually sole purpose of drawing this graph is to see errors and calculate it. So my problem is, i put my data on graph, it looks like in the above link, öow i need my staight line (f=kx) so i can find theoritecal k and calculate the error %. But i dont know how to draw the line. if i give say, 3 to x and f is 3k. k is unknown and the purpose of experiment is to find k. is it the trendline that i need to draw? or is there another way to draw the line?
     
  7. Nov 4, 2009 #6

    Nrt

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  8. Nov 4, 2009 #7

    Andrew Mason

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    By having data points that form a line.

    If your data points do not line up, you might redo the experiment. Try to get more data. You cannot really determine much from three points.

    AM
     
  9. Nov 4, 2009 #8

    Andrew Mason

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    You cannot assume that the spring is a perfect Hooke's law spring. Hooke's law really isn't a law. It is just an approximation that has been observed. Nevertheless, your data should form a reasonably straight line. If not, you should do a few more observations so you have more data points.

    Give us your data and I might be able to add something more to my previous comments.

    AM
     
  10. Nov 4, 2009 #9

    Nrt

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    Thx for help.
     
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