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Homework Help: Centripetal Force Lab of string and mass

  1. Feb 25, 2010 #1
    1. The problem statement, all variables and given/known data
    I had to do a lab, the diagram of which is shown. ucm_app.gif

    The radius (r) of the string was 50.0 cm, and our first mass was 29.0 g. The time for 20 trials was 15.75 s. We repeated the process with five other masses and recorded all the same information. Then, we had to plot the weight (F) of the masses versus the frequency and find the slope of the line of best fit. Our slope came out to be 3.83 (according to Excel).
    We also have to use this information to find the mass of the rubber stopper. My attempt at the solution is below but I'm not sure if I did it right. Also, someone else in my class told me that the mass of the stopper should be close to the slope of the line of best fit, but mine was way off. Is that true?

    2. Relevant equations
    F = m4π2r/T2

    3. The attempt at a solution
    F = m4π2r/T2
    0.284N = m(4)(π2)(.5m)/(.62s)
    m = 0.0089 kg
  2. jcsd
  3. Feb 26, 2010 #2
    Interesting experiment (how did you manage to keep the mass on the bottom from oscillating up and down??). Looking at your calculations, there was a tiny error regarding Period and Frequency. Frequency is 1/T where T = 15.75 s for your trial. But you divided by this number (1/15.75) instead of multiplying it! Perhaps you should reevaluate your calculations a bit and then try it out again. I can tell you now that the mass of the stopper will not be close to the line of the best fit. It will differ by a factor of 4pi^2r (As you have unknowingly indicated in your equation).

    Also, you should try to plot against f^2 and not f (frequency)

    Best of luck!
    Last edited: Feb 26, 2010
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