# Angular momentum of particle in xy plane

## Homework Statement

Okay so, particle of mass 4kg moves in positive x direction with v=8m/s along line y=4m. Find angular momentum relative to point (x,y) = (1m, 10m)

## Homework Equations

Angular momentum L=mrxv

## The Attempt at a Solution

Basically, I set my velocity vector = (8i+0j)m/s and my position vector = (i -6j)m. I determined this position vector from the point (1,10) to the position of the velocity vector(0,4) and then just used the equation 4[(8i+0j)x(i-6j)] and came up with the answer -192 kg*m^2/s.

So my question I guess is did I determine my position and velocity vectors correctly and did is this how the angular momentum equation should be done?
Thanks!

## Answers and Replies

tiny-tim
Science Advisor
Homework Helper
welcome to pf!

hi barz1! welcome to pf! Angular momentum L=mrxv

Basically, I set my velocity vector = (8i+0j)m/s and my position vector = (i -6j)m. I determined this position vector from the point (1,10) to the position of the velocity vector(0,4) and then just used the equation 4[(8i+0j)x(i-6j)] and came up with the answer -192 kg*m^2/s.

yes that's fine , except you used vxr instead of rxv (personally, i'd have done an easy question like this much less formally … i'd say that the perpendicular from P to L is obviously -6j, so the angular momentum is 4 times -6j x 8i, = 192 i x j = 192 )

Hey thanks a lot tiny-tim ill remember your recommendation for test time.