1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Need help proving MoI is negligible

  1. Apr 24, 2014 #1
    I did an experiment to measure the spring constant of a torsion spring (lab set-up pictured here:
    eckqbA5.png

    I have no problems calculating anything, but in the calculations, we assumed the inertia of the spring and axis to be negligibly small and it worked nicely. In the lab report I need to be able to prove this claim but I have no idea how to do it without taking the entire apparatus apart and measuring everything, which I'm not allowed to do anyways.

    How do I go about proving that only the masses (2, 3, and 4 in the picture) provide non-negligible contributions to the moment of inertia of the system?

    In case it helps, the total inertia calculated is:
    [itex]\frac{1}{12}(M_{rod}L^{2}+M_{cyl}[6r_{o}^2+6r_{i}^2+2h^2+24d^2][/itex]
     
  2. jcsd
  3. Apr 24, 2014 #2
    My current idea is to use our data for the equation of motion of this thing without the masses so ONLY that negligible inertia is there and use our calculated spring constant and angular frequency to calculate what the inertia actually is. But using something we calculated by ignoring inertia to calculate the ignored inertia seems circular to me.
     
  4. Apr 24, 2014 #3

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    You should be able to show the MOI of the shaft is negligible by estimating its dimensions and density. You don't need to calculate it to 6 decimal places just to show it doesn't matter.

    For the torsion spring, I guess the outer end is fixed so it doesn't rotate.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Need help proving MoI is negligible
  1. Need help (Replies: 3)

Loading...