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!

Precession: Deriation of formula

  1. Dec 15, 2007 #1
    Please see this link first: http://en.wikipedia.org/wiki/Precession#Classical_.28Newtonian.29

    How do they get to the formula
    [tex]\textbf{wp}[/tex] = [tex]\frac{Q}{I*w}[/tex]

    I also note that I*w = L (angular momentum).

    How do they get to that equation? I've thought and though, I just don't know how. I really hope that someone can give me a link to a deriation of it, or perhaps help me themselves.

    Thanks in advance.
     
  2. jcsd
  3. Dec 15, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

  4. Dec 15, 2007 #3
    Wow! Thank you so much.

    Just one thing though: In equation (4), the d0, what is that? Change in angle? And where does that equation (4) come from? Never seen it before? Why is that true? :s
     
  5. Dec 15, 2007 #4
    I think I get it now actually. It is because the change as well as angle are inifitsmally small... :) Right? :D
     
  6. Dec 15, 2007 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Yes, that's it. And because the torque is perpendicular to the angular momentum.

    Now that I looked it over more carefully, I'm not happy with that derivation that I linked. It seems a bit sloppy. I'll post my own version in a bit.
     
  7. Dec 15, 2007 #6
    Oh thank you so much! :eek: It's deeply appreciated.
     
  8. Dec 15, 2007 #7

    Doc Al

    User Avatar

    Staff: Mentor

    I was just about to post my own derivation, when I found this on hyperphysics (one of my favorite educational sites--I highly recommend it): Precession of Spinning Top

    This is almost exactly what I would have written, so it saves me the trouble! If you have questions about this derivation, let me know.
     
  9. Dec 15, 2007 #8
    Thank you once more. What I need is acutally a combination of the two. The first is good because it fits the precession of a bicycle wheel (which is what I need to explain), and the next is good because of the smart derivation.

    I've tried to write it down, how I think I need it. But I'm not sure, if I did any errors... I would very much like if you could just quickly look it through?
    Here's the link: http://peecee.dk/?id=85095 (danish upload site: Click "Download fil").
    At the moment it hasn't got an illustration, I will make one however.

    Thanks in advance, once again. :smile:
     
  10. Dec 15, 2007 #9
    Oh, and a last question: Is this good argumentation:
    If I call the angle v. Can I say this:

    "It is obvious that
    [tex]sin(v) = \frac{\Delta L}{L}[/tex]​
    For infinitsimal small angles sin(v)=v, so that:
    [tex]dv = \frac{dL}{L}[/tex]​
    since the angle is small when delta L is small."

    Thanks in advance.
     
    Last edited: Dec 16, 2007
  11. Dec 16, 2007 #10

    Doc Al

    User Avatar

    Staff: Mentor

    It looks OK to me. Note that your "r" is the moment arm--I assume your bicycle wheel is oriented perpendicular to the vertical?

    It is certainly true that for small angles [itex]\sin\theta \approx \theta[/itex], as long as the angle is in radians. But you can also argue directly that as [itex]\Delta L[/itex] becomes small it more closely approximates the arc length of a circle, thus [itex]\Delta L/L[/itex] becomes the radian measure of the angle.
     
  12. Dec 16, 2007 #11
    Thank you once more! :) I think I get it now. :D
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Precession: Deriation of formula
  1. Gyroscopic precession (Replies: 6)

  2. Precession question (Replies: 2)

  3. Earth's precession (Replies: 8)

  4. Gyroscopic precession (Replies: 7)

  5. Gyroscope precession (Replies: 3)

Loading...