Angular momentum relative to the origin

[JessicaHelena]

Homework Statement

A 2.4 kg particle-like object moves in a plane with velocity components vx = 25 m/s and vy = 80 m/s as it passes through the point with (x, y)coordinates of (3.0, −4.0) m. (Express your answers in vector form.)
(a) What is its angular momentum relative to the origin at this moment?

Homework Equations

L=pxr

The Attempt at a Solution

I did 2.4*the 3x3 matrix
i j k
25 80 0
3 -4 0
and this gave me 2.4*(-100-240)k = -816k.

[FONT=verdana, geneva, sans-serif]When I entered this in, I got this wrong, but without the minus sign (so 816k), the webassign accepted this as correct. However, this bothers me because isn't the correct sign supposed to be negative?

 

[PeroK]

Homework Equations

L=pxr

$\vec{L} = \vec{r} \times \vec{p}$

Surely?

 

[JessicaHelena]

Ah, right... r x p doesn't equal p x r and I memorised the formula the wrong way. Thank you.

 

[PeroK]

Ah, right... r x p doesn't equal p x r and I memorised the formula the wrong way. Thank you.

If you start from a point, the $\vec{r}$ displacement comes first. If you follow this you reach the particle, where you find the momentum $\vec{p}$.

That's how I remember it.