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Homework Help: Pendulum in thermal balance

  1. Nov 24, 2014 #1
    1. The problem statement, all variables and given/known data
    We have a pendulum in thermal balance with the surroundings - no damping. We measure the speed of the pendulum. The accuracy of each speed measurement is known. How does the accuracy of the speed change, if we decide also measure the position of the pendulum with accuracy 10% of the amplitude?

    2. Relevant equations
    Just anything you can think of.

    3. The attempt at a solution
    Believe me, I would be more than happy to show anything. It has been quite some time since I first came across this problem but I still have no idea how to continue.
  2. jcsd
  3. Nov 24, 2014 #2


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    Is this a Heisenberg Uncertainty Principle oriented problem?
  4. Nov 24, 2014 #3
    Not necessary but it could also be included.
    The topic I am dealing with is "Measurement and measuring systems in Physics". The goal is more to simply use all of the knowledge he have, if solving this problem includes using Heisenberg Principle, than so be it.
  5. Nov 26, 2014 #4
    You could try writing an expression for the pendulum's velocity in terms of its position along its trajectory. Then find the uncertainty in velocity due to the uncertainty of position by taking a differential. I'm assuming it's not a Heisenberg uncertainty problem since quantum effects would be negligible (unless your pendulum is very very small).
  6. Nov 27, 2014 #5


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    What do you measure ? The speed of a pendulum ? That's a function of time. Or the period ? That depends on the amplitude.
    See e.g. here, appendix.
  7. Nov 27, 2014 #6
    I assumed the instantaneous speed could be measured with a photogate of some kind (with some uncertainty in itself) but if there is also uncertainty in the pendulum's position when the speed is measured, it would add additional error to the speed measurement.

    Yes the speed is a function of time but so is the angle (thus position), so you can eliminate the time variable. It is possible I'm misinterpreting something.
  8. Nov 27, 2014 #7
    I am almost 100% sure this is not the case. Of course each measurement of the velocity has it's well defined uncertainty BUT if I can (at the same time) measure also it's position, that this should improve my result not make the error even bigger.

    The first case, measuring the velocity only and having no idea of pendulum's position, gives me a certain value of it's velocity.

    However measuring the position of the pendulum and using some basic math, I improve my knowledge of the system and what exactly is happening, therefore if I combine both measurements (as the problem states) the error should reduce. Or am I wrong?
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