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!

I Why is "dr" not used for moment of inertia calculations?

  1. Dec 1, 2016 #1
    When calculating the moment of inertia for a rotating object, why is "dr" not incorporated into the integral? However, dm is part of the integral, isn't the distance from the axis of rotation changing as well? Hence shouldn't the integral involve both dm and dr?
     
  2. jcsd
  3. Dec 1, 2016 #2

    Nugatory

    User Avatar

    Staff: Mentor

    Can you show us an example of an moment of inertia calculation that you're wondering about? The simpler and more symmetrical it is, the better.
     
  4. Dec 1, 2016 #3

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    It IS taken into consideration.

    dm = ρ dV

    So for, say, a spherical mass distribution, you get

    dm = ρ r2 sinθ dr dθ dφ

    There's your "dr" there!

    Zz.
     
  5. Dec 1, 2016 #4

    jtbell

    User Avatar

    Staff: Mentor

    However, watch out for the different kinds of "r"s. The "r" that matters in a moment of inertia calculation (dI = dm⋅r2) is the perpendicular distance from the axis of rotation, whereas the "r" in spherical coordinates is the distance from the origin (usually the center of a spherical distribution).
     
  6. Dec 1, 2016 #5
    Thanks for all your responses.

    So generally, I = Σmiri2 = ∫r2dm
    Why is the r2 left as a constant? Isn't it also changing depending on where the dm is located?

    I asked this question to a physics major, and he said I should just take it as it is because I only know up to single variable calculus. He said, the integral is a multivariable integral involving dr as well. Not sure if he's correct though.
     
    Last edited: Dec 1, 2016
  7. Dec 1, 2016 #6

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Did you even read my post?!

    Oy vey.

    Zz.
     
  8. Dec 1, 2016 #7
    Oh, I don't know how I skipped your post.
    I don't if I'm correct though, but, isn't that multivariable calculus? I'm still taking single variable at the moment. So I guess, dr is taken into consideration, but, I should just take the general formula as it is given?
     
    Last edited: Dec 1, 2016
  9. Dec 1, 2016 #8

    Student100

    User Avatar
    Education Advisor
    Gold Member

    Because the prereq for the course probably didn't include multivariate calculus. Just asking why it's left constant doesn't mean a whole lot without context, but generally in these courses where they try to keep things simple they will use some sort of symmetry or constraints to avoid math students haven't had yet.

    You'll see this a lot in E&M.
     
  10. Dec 1, 2016 #9
    The course just requires calc 2, or a coreq in calc 2. Currently I'm taking calc 2, so I guess I should just take this general formula as is?

    However, I do plan to take mechanics, which requires multivariable, I guess that's where the proof for r2 will be explained?

    That reminds me, my professor showed the class a differential equation for harmonic motion, and the answer to it. Since it's a certain class of differential equations and the class hasn't done differential equations, he said we should just take the answer for what it is. I guess it's the same for this case?
     
    Last edited: Dec 1, 2016
  11. Dec 1, 2016 #10

    jtbell

    User Avatar

    Staff: Mentor

    I second Nugatory's request. It would prevent a lot of confusing guessing if you could give us a specific example of the kind of calculation you're thinking of.
     
  12. Dec 1, 2016 #11
    A simple one would be a uniform rod. The method would involve ∫r2dm. However, I was confused as to why r2 is left as a constant. Since r is changing compared to where dm is, so shouldn't r also be represented as dr?

    But, Student100 told me to just take the equation for what it is, since it involves higher level math (multi-variable), which I haven't taken yet.
     
  13. Dec 1, 2016 #12

    Student100

    User Avatar
    Education Advisor
    Gold Member

    It's not quite the same, since moment of inertia for things like thin rigid body rods with fixed axis of rotations that are highly symmetrical (##dm = \rho dl## ... etc.) are fully accessible.

    The differential equation for simple harmonic motion should also be accessible, but is easier with auxiliary equations which you probably haven't see yet either.

    Edit: Tried to clean it up, think I'm confusing myself here.

    Errr, uniform thin rod is perfectly accessible. So in that case, you're looking a specific example of a fixed axis of rotation with a rigid body.

    The moment of inertia when we constrain ourselves to rigid bodies around fixed axis's of rotation depends on the mass distribution of in the body with respect to the axis of rotation. ## I = \int r^2 dm## where ##dm = \rho dV## and r is the perpendicular distance to the axis of rotation.

    For a uniformly thin rigid rod with mass M and length L you have get $$I = \int r^2dm$$ where if we choose our coordinate system so that the rod lays in the x axis with the fixed axis of rotation around the origin at the middle of the rod it then becomes $$I = \int_{-L/2}^{L/2} x^2 dm$$ With ##dm = \frac{M}{L} dx## so $$I = \frac{M}{L} \int_{-L/2}^{L/2} x^2 dx$$

    Basically, you're integrating over r. r (as the location of the axis of rotation) is also a constant (not changing, stays at origin) in this case. You can have other cases were r changes with time, etc, etc. Maybe it would make more sense to call the distance from the axis of rotation another variable.
     
    Last edited: Dec 1, 2016
  14. Dec 1, 2016 #13
    Okay, I think I found the solution to my dilemma. Since, the distance from each point mass is r distance away, hence, r is changing, but, the mass at each point is the same for a uniform rod. So we represent the small changes in mass by dm, so we can replace it with a density formula to represent the change in r?
     
  15. Dec 1, 2016 #14

    Student100

    User Avatar
    Education Advisor
    Gold Member

    r isn't changing, the distance the mass elements are from r is.
     
  16. Dec 2, 2016 #15
    Oh, I thought r represented the distance between the mass element and the axis of rotation. Thus, r should be different for every mass element.
     
  17. Dec 2, 2016 #16

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

  18. Dec 2, 2016 #17
    I actually read that page and it just added to my dilemma. From the diagram the distance for each xi is different, just as how the masses are different. So why is x represented as a constant in the integral?

    The same goes for r2. Like Student100 mentioned, r is a constant and not the distance from the axis of rotation for each element of mass. I thought of r as x is show in the diagram. Except that origin point is the axis of rotation.

    I'm just confusing myself now, I've had a lot of different responses.
     
  19. Dec 2, 2016 #18

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    I still don't understand why you keep thinking that the coordinate variable is a "constant". I will say it once again, this time with emphasis, so that you pay attention:

    LOOK AT THE EXPRESSION FOR dm!!

    For the rod, dm can be written in terms of the linear mass density and..... get this.... dx!

    The position of each mass element is contained in dm, because it has a "length" factor! So you are integrating the mass element over x!

    This is no different than when I showed you what happened with a spherical mass distribution earlier where "dr" is actually part of dm!

    You need to sit down and figure out why this is not sinking in for you even after you've been told of this several times on here.

    Zz.
     
  20. Dec 2, 2016 #19
    I've been receiving different responses that have been confusing me. I should've mentioned, my notion of using calculus may have been wrong. I forgot that dx represents a minute change, and the distance x is basically broken down to small elements of dx using the linear mass formula, as dm correlates with the width of dx.
     
  21. Dec 2, 2016 #20
    Check out this link: http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html and scroll down to Moment of Inertia Examples. For a rigid body, we consider the body to be made up of small elements, dm, at distance r from the rotation axis. Each element contributes r2dm to the moment of inertia, which his found by integrating over all the elements: ∫r2dm. If all elements are not at the same distance from the axis, then as ZapperZ showed by example, the expression for dm will contain dr.
     
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: Why is "dr" not used for moment of inertia calculations?
Loading...