Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Setting up Integrals

  1. Sep 17, 2005 #1
    Can anyone reccomend a website, or text, where there are step by step examples of setting up area and volume integrals where your looking for forces. I need to be able to do set ups for situations similar to the following :find the gravitational field vector on surface point of a cylinder or find or gravational force on a point mass from a disk.

    Really get confused with this-- especially when it comes to putting the pieces in terms of each other?
    Seems my calc. book just has me evalulate integrals and there's a big leap from my freshman/softmore physics text, where I hardly did any calc., to my junior level text.

  2. jcsd
  3. Sep 17, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Last edited by a moderator: Apr 21, 2017
  4. Sep 17, 2005 #3
    Peliminary inspection of these sites shows a lot about techniques for solving integrals but not a lot on setting one up for the situations I tried to describe. Maybe what I'm looking for is a good mechanics text. The one I have has examples in it that I can't follow, not because my integration is rusty, but because my text starts with a simple enough relation and the next line is the result of a triple integration.

    Example of Hw problem:

    Calculate the gravitational field vector due to a homogeneous cylinder at exterior points on the axis of the cylinder.
    Only because the problem states that the result is to be found by computing the force directly; start with g = F/m

    g = -GMrhat/ r^2

    I can get that symmetry gives us that there is only force in z direction, and choosing a reference point on the z axis and pick an arbitrary point on the surface of mass dm- where dm = rhodV . The point dm connects to the (0, 0, z) reference point with a radial line and makes an angle with the z axis that we can put into the integral as the magnitude of dgz, so what goes into the integral is cos(angle)--where cos(angle) =(zo-z)^2/((sqrt r^2 + (zo -z)^2)).

    Now, there's rhodV = dm = rhodr rdangle dz

    Somehow, I have in my notes the final integral, skipping the 3 limits of integration resloves itself into, bringing rho outside, rho///drdangledz(zo -z)/(((zo -z)^2 + r^2))^3/2)). Even if there's an error in my notes, I'm stuck on the set up.

    I can't figure out why the top term is no longer squared. Looking back at the origional formula; gz = -Grho(integral)cosangle/r^2.
    I've drawn a triangle on my picture connecting the z axis across to the surface point, to the radial line, and back to the point zo. Now, I'm confused-- I've labled the radius of the cylinder R and the radial vector from reference point to zo is labled little r. To evaluate the integrand I need to get r in terms of R ? Just stuck!

    See, what I need are some examples with some intermediate steps in setting up these types of problems.

  5. Sep 17, 2005 #4
    Solved it

    ok it was simple and I apologize to anyone who tried to read my post. The (zo -z)^2 term never belonged there- Just lack of sleep r in terms of R easy.
    No biggie-- hard part, I know is evaluating the result of the set up but looks like integration by parts--
    Still, need practice, and worked out examples would be of a lot of help.

    Reccomendations, advice, appreciated. :biggrin:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook