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Homework Help: Simple Spring Constant Lab not adding up

  1. Nov 9, 2006 #1
    For some reason, my spring constants are all screwy. an example

    Masses (Newtons) .049 .098 .196 .294 .490
    Displacement (kg) .006 .019 .035 .055 .090

    and the line i got was Mass=.188d-.001

    doesnt that mean that the spring constant is .188 N/m? that doesnt make any sense though, because i did 4 other springs and the stiffest one had a spring constant of .067. what am i doing wrong?
  2. jcsd
  3. Nov 9, 2006 #2
    Displacement is not Kg.

    your intercept is wrong.The intercept should be the natural length of the spring, so it can't be negative.

    You should redo the experiment.
  4. Nov 9, 2006 #3
    right, i meant meters...not kilograms. typo.

    as far as the y intercept being off, i set it equal to zero, so the original length would be zero. besides, those are millimeters, and theres something big wrong with the calculations...
  5. Nov 9, 2006 #4
    Doubling your mass (such as from .049 to .098), should NOT more than double your displacement (which, respectively was .006 and then .019).

    Also, those spring constants seem extraordinarily small. You are saying that using your stiffest spring, it only takes an increase of .067 N to increase the displacement by a meter. Do you remember your equations correctly? (hell, the units of a spring constant give it away)
  6. Nov 9, 2006 #5
    i know, thats the problem. i have no idea why my constants are so small. here, the original data collected was:

    5g 10g 20g 30g 50g
    0.6cm 1.9 3.5 5.5 9.0

    i divided all the grams by 1000 to get the weight in kilos, then multiplied them by 9.8 to get the mass in Newton.
    i divided all lengths by 100 to get the number in meters...

    im sure my problem was in my conversion, because it simply cant be anything else. any ideas?
  7. Nov 9, 2006 #6


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    Just looking at your line, I'd say you've fitted the inverse line:
    you seem to have written d = 0.188 M - 0.01 instead of M = 0.188 d - 0.01.

    Fill in your data in your formula and you'll see: d is roughly about 1/5th of M, numerically.
  8. Nov 9, 2006 #7
    Umm, dividing (.005kg * 9.8m/s^2) / (.006m) results in 8.167. So are you dividing wrong or something? (obviously this isn't the average, but im just showing that it works out for the first data point).
  9. Nov 11, 2006 #8
    vanesch, you were right. my axises (axes?) were reversed. thanks for the help guys
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