# Homework Help: Update To My Problem On Vectorcalculus

1. Feb 9, 2006

### Lisa...

A particle moves in a circle that is centered at the origin. The particle has position r and angular velocity w. The velocity v is given by:

v = w x r (with x = the cross product).

My question is, when I calculate this crossproduct with

w= (d(theta)/dt) k and
r= x i + y j + z k

it gives:

(d(theta)/dt) * x j - (d(theta)/dt) * yi

Why does this denote the velocity?

2. Feb 9, 2006

### dextercioby

Compute its modulus and see whether you can find

$$|\vec{v}|=\frac{d\theta (t)}{dt} R$$

,where R is the circle's radius.

Daniel.

BTW, you can check whether that vector is always tangent to the circle by dotting it with $\vec{r}$.

3. Feb 9, 2006

### Lisa...

Thanks I get it now :)