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Experiment not working

  1. Feb 1, 2014 #1
    Hey Everybody,

    I have only posted here a few times before, so if you need some context, please look at my past posts. Basically, I don't have access to a lab right now (long story there) and I have a thesis which depends on empirical results. Normally I would have thrown in the towel, but graduating is really important to me and so I thought I would try my hand at obtaining my results through gedanken experiments. To start simple I tried to measure the value of g. I imagined dropping a lead ball from a height of 1 meter to the ground (in a vacuum of course) and timing the fall using the formula \begin{equation}g=\frac{2h}{t^{2}}\end{equation}. So far so good, but I am not obtaining results consistent with other experiments. I ran 3 trials and obtained times of .567 seconds .217 seconds and 1.21 seconds (all results to within .001s). Neglecting the lack of precision, my results are hardly accurate :( and I'm at a loss as to what could be happening. RIght now I am using a stopwatch timer (which I admit is not very accurate), but I don't see where the error comes from! Please help, I can answer any questions about my experiments!
  2. jcsd
  3. Feb 1, 2014 #2


    Staff: Mentor

    How can you have experimental results in a thought experiment?
  4. Feb 1, 2014 #3
    How are you starting and stopping the timing? You really need an electronic stop and start if you don't already have them. You can't expect any useful results timing by eye.

    Edit... oh, it's a thought experiment. In that case, I don't understand any of this.
  5. Feb 1, 2014 #4


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    Staff: Mentor

    According to one of those posts, you're a third-year Ph.D. student, at least of last August, which makes the purpose of the experiment you described rather puzzling, especially if you need it in order to "graduate."
  6. Feb 2, 2014 #5
    This is a kind of question I'd expect from a high school student, not a third year PhD.... something is seriously out of place here.

    Do you have a (bachelors) degree in Physics already?
  7. Feb 2, 2014 #6
    High school students measure g by free fall methods using timing methods described by tfr000 in post three. If all you have is a stop watch you can get reasonble results from pendulum type experiments.
    Last edited: Feb 2, 2014
  8. Feb 2, 2014 #7


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    Staff: Mentor

    Why don't you just think about better results?

    I would think of getting 1.11 s - that would mean I am on the Moon, which would nicely coincide with the vacuum.
  9. Feb 2, 2014 #8
    Maybe your gedank stopwatch is broke
  10. Feb 2, 2014 #9


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    2017 Award

    Staff: Mentor

    If there is a real experiment behind this problem, we'll need more information to be able to say anything.
    In the gedankenexperiment, if your timing is so wrong the method (here: manual operation of a stopwatch) simply does not work.
  11. Feb 4, 2014 #10


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    Science Advisor

    You said you imagined dropping the lead ball. How do you get actual numbers from an imagination?
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