Homework Help: Angular momentum about a line

1. May 9, 2010

mybrohshi5

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. May 9, 2010

kuruman

Angular momentum is r x p. You calculated r x v, i.e. you forgot to multiply by the mass of the bird.

3. May 9, 2010

mybrohshi5

Oh yay so i wasnt too far off just a little mistake. Thank you Kuruman :)