Determining angular momentum of a point mass.

[howsockgothap]

Homework Statement

The velocity of a 2.00 kg body moving in the xy plane is given by v=(7.00i-2.00j) m/s. Its position vector is r=(2.00I-1.00j) m. Calculate its angular momentum (magnitude and direction) about the origin (kgm²/s). Express your answer in unit vector notation.

Homework Equations

L=rp
p=mv
..
L=Iω?

The Attempt at a Solution

I attempted to use L=rp=rmv and then substituted in my conditions. I got (28.0i + 4.00j) kgm²/s but the answer is apparently 6.00k. Where am I going wrong?

 

[Rick88]

You are dealing with vectors, not scalars.
Therefore the product rv should really be r x v. (x being the vector cross product).

A 2 dimensional cross product can be written as
r x v = r₁ v₂ - r₂ v₁ , where 1 and 2 simply denote the components of the vector: 1 for the i component, 2 for the j.

R.