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    Physics lab

    Pendulum lab

    inital height of spring:11.5cm

    m=2000g d=8cm
    m=3000g d=6cm
    m=2500g d=7cm

    I need to calculate C....I'm not exactly sure how to do that...
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  2. #2
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    Quote Originally Posted by johett View Post
    Pendulum lab

    inital height of spring:11.5cm

    m=2000g d=8cm
    m=3000g d=6cm
    m=2500g d=7cm

    I need to calculate C....I'm not exactly sure how to do that...
    Maybe there's a mindreader cruising by who can shed light on what the symbol C means. Barring that, perhaps you could provide more information - enough in fact for someone not familiar with your pendulem lab or your experiment - to understand the question.

    When posting a problem, you've got to ask yourself a question (and it's not "do I feel lucky?"):

    "Have I posted enough information so that someone else with no prior familiarity with the problem will understand it well enough to provide an answer?"
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    Quote Originally Posted by johett View Post
    Pendulum lab

    inital height of spring:11.5cm

    m=2000g d=8cm
    m=3000g d=6cm
    m=2500g d=7cm

    I need to calculate C....I'm not exactly sure how to do that...
    I've done some channeling and have seen that you need to find the spring constant.

    This is probably best done graphically, but I'm too lazy to use Excel right now. The other way is to calculate a k for each data point and then get the average and standard error.

    The equation you are trying to fit is
    F = k(x - x_0)

    Since you have a hanging mass F = w = mg. So
    mg = k(x - x_0)

    Now, x - x_0 is the amount the spring has stretched from its equilibrium length. Given what the data look like (more clairvoyance) I would guess that the 11.5 cm is the end of the spring with no weight on it and the d values are the position the mass hangs at. Thus the spring will have stretched a distance of 11.5~cm - d, so this is the x - x_0.

    So finally:
    mg = k(0.115 - d)
    (To keep the units consistent I am using d in meters and m in kilograms.)

    I get k = 560.000, 534.545, 544.444 N/m (respectively). So the experimental value for k will be k = 550 \pm 7.4 ~ N/m.

    -Dan
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    Quote Originally Posted by topsquark the psychic View Post
    I've done some channeling and have seen that you need to find the spring constant.
    [snip]
    Yes, I'd actually guessed that myself but felt there was a point to be made .....
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    Quote Originally Posted by johett View Post
    Pendulum lab

    inital height of spring:11.5cm

    m=2000g d=8cm
    m=3000g d=6cm
    m=2500g d=7cm

    I need to calculate [edit: the spring constant k]....I'm not exactly sure how to do that...
    Quote Originally Posted by topsquark View Post
    [snip]

    I get k = 560.000, 534.545, 544.444 N/m (respectively). So the experimental value for k will be k = 550 \pm 7.4 ~ N/m.

    -Dan
    Not really maths but ..... I think there's scope for valuable comment on significant figures and rounding in this sort of problem .....

    The least significant data has 1 sig fig .... you can't give an answer more accurate than the least accurate data ... so an answer correct to 1 sig fig would be the technically correct answer to give .....

    Personally, I'd be inclined to state the answer as 550 \pm 10 N/m ......

    All comment welcome.
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  6. #6
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by mr fantastic View Post
    Not really maths but ..... I think there's scope for valuable comment on significant figures and rounding in this sort of problem .....

    The least significant data has 1 sig fig .... you can't give an answer more accurate than the least accurate data ... so an answer correct to 1 sig fig would be the technically correct answer to give .....

    Personally, I'd be inclined to state the answer as 550 \pm 10 N/m ......

    All comment welcome.
    I never know how many digits to keep when I'm doing a standard error problem, so I just keep the number of sig digs the problem has.

    We actually probably have two sig digs in the "d" measurement since any reasonable meter stick is measured out in mm. Likewise I would imagine the mass measurements are correct to 4 places.

    However, the student did not report the numbers that way. So I must have done a bit more ESP than I mentioned earlier.

    -Dan
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