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: Angular momentum about a line

  1. May 9, 2010 #1
    1. The problem statement, all variables and given/known data

    A bird of mass 0.5kg is flying near to a building which is 35.0m tall. Take the direction AWAY FROM THE BUILDING to be the +x direction and take UP to be the +y direction. At some instant in time, the bird has an x velocity of +20m/s and a y velocity of -14m/s. At this same instant the ird is located 8 m from the building and 5 m below the roof (30 m above the ground).

    What is the angular momentum of the bird about the line along the edge of the roof at this instant in time?


    3. The attempt at a solution

    Ok so to start of the answer is (-6.0 kg m2/s) k

    I am not sure how to get it but this is what i tried.

    I used the cross product of this matrix

    ......I........J
    r|....8......(-5)
    F|...20......-14

    (ignore the dots they were the only way i could line up everything like the matrix i drew :)

    so (8)(-14) - (20)(-5)

    = -12 kgm2/s in the k direction

    Can anyone help me with what i am doing wrong or how to do this correctly?

    It was a problem on a test i just took last week and i really wanna know how to get the right answer.

    Thank you
     
  2. jcsd
  3. May 9, 2010 #2

    kuruman

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Angular momentum is r x p. You calculated r x v, i.e. you forgot to multiply by the mass of the bird.
     
  4. May 9, 2010 #3
    Oh yay so i wasnt too far off just a little mistake. Thank you Kuruman :)
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook