1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Homogenity physics homework

  1. Sep 7, 2013 #1
    1. So for today's exam, I was given this equation, and I was required to get the value and units of K:

    T4 = (4K∏4)/g2 - (8d∏4)/g2

    I had to follow an experiment, plot the results, get the gradient. After getting the value of g, I was required to find the value of K, and its units.

    T4 has units s4, d has units m, g is gravitational acceleration, hence I believe it is m.s-2

    2. The problem lies in the fact that I don't believe I was given a homogeneous equation, hence I couldn't provide the units. Any help? Note that the units are enough for me, I managed to solve for K (correctly, I believe).

    Thanks in advance :)
  2. jcsd
  3. Sep 7, 2013 #2
    You can only add or subtract things with same units.
  4. Sep 7, 2013 #3
    So you confirm the equation is not homogeneous?
  5. Sep 7, 2013 #4
    I neither confirm nor deny, I merely point out the obvious. What then is ∏? Or did you mean [itex]\pi[/itex]?
    Last edited: Sep 7, 2013
  6. Sep 7, 2013 #5
    Well, after working it out, the left side is this: T4 has units s4.

    For the K part, I can't work it out without knowing whether it's homogeneous or not. So I go where there's d.

    (8d∏4)/g2 would then become:
    Numerator: m
    Denominator: m2.s-4

    Therefore this part would be s4.m-1

    For homogeneity with addition/subtraction, all parts have to be the same. These two parts aren't the same, evidently. Any confirmation would be very much appreciated, so I could e-mail my invigilators.
  7. Sep 7, 2013 #6
    What is the experiment?
  8. Sep 7, 2013 #7
    That should be a pi :) I couldn't find the symbol :P
  9. Sep 7, 2013 #8
    Nope, it doesn't look homogenous. What's the Experiment?
    You sure d is in meters and not m^2 ?
  10. Sep 7, 2013 #9
    The experiment isn't very important per-se, but for the record, there are two stand clamps, with a wire attached to both, forming a triangle. At the bottom, there's a pendulum, and d is the distance between the two stands. So yeah, definitely in meters. It's also given, but because of copyright issues, I'm afraid I'm not allowed to take a picture of it and upload it.
  11. Sep 7, 2013 #10


    User Avatar

    Staff: Mentor

    Could be a typo in the given equation; I suspect that the 'd' in the second term should have been ##d^2##.

    Even if the second term has a typo, the terms should individually have the same units as the LHS of the equation. So you can still determine what k should be and even repair the typo!
  12. Sep 7, 2013 #11
    While I did as you said, gneill, I couldn't ascertain which was the right one - the 'd' part or the LHS.
  13. Sep 7, 2013 #12


    User Avatar

    Staff: Mentor

    I think you can be pretty certain the LHS would consist of a single variable, and that as missed typos go, dropping a square on a variable in the midst of a term is more likely than dropping a variable on the LHS.
  14. Sep 7, 2013 #13
    Well, I haven't got the derivation of the formula yet but with off-hand information the formula's probably wrong. When you put d=0 the bifilar pendulum becomes a simple pendulum the formula should be reduced to a simple one.
    Making $$k= 4 l^2$$ from the standard formula of time period. Formula's probably missing a square factor on d, will confirm it and let you know.
    Whoops gneil got there before me....
    Last edited: Sep 7, 2013
  15. Sep 7, 2013 #14
    Thanks a lot for your help :) I really appreciate it!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted