# Homework Help: Vector product question in cylindrical coordinates

1. Apr 18, 2012

### GarethB

I am trying to work the following problem;
A rigid body is rotating about a fixed axis with a constant angular velocity ω. Take ω to lie entirely on th z-axis. Express r in cylindrical coordinates, and calculate;
a) v=ω × r
b)∇ × v
The answer to (a) is v=ψωρ and (b) is ∇ × v = 2ω

Firstly, I am not sure if we need r=[ρcosψ, psinψ, z] or simply [p, ψ, z]. Secondly, are they saying that ω is simply [0, 0, ω]? if so then I don't get the result so I am doing something wrong. Any help? I am sure that tackling this problem will deepen my understanding of curvilinear coordinates.

2. Apr 18, 2012

### Eng1

the first expression ist good ... r=[ρcosψ, psinψ, z] ...keep going with that ;)
next describe omega as a vector ... and calc. velocity with cross product :>

3. Apr 18, 2012

### I like Serena

I think the answer to (a) should be: $\boldsymbol{\vec v}={\boldsymbol{\hat \psi}}\omega\rho$, where $\boldsymbol{\hat \psi}$ is the local unit vector in the ψ direction.

Similarly the answer to (b) should be: $\nabla \times \boldsymbol{\vec v}=2 \boldsymbol{\vec \omega}$, where $\boldsymbol{\vec \omega}$ is the vector in the z direction with length $\omega$.

I expect that they want you to express r as [ρcosψ, ρsinψ, z], so you can calculate the curl in cartesian coordinates.
(Alternatively, you could simply use r=[ρ, ψ, z] and use the relevant formula from: http://en.wikipedia.org/wiki/Del_in_cylindrical_and_spherical_coordinates)

And yes, they are saying that $\boldsymbol{\vec \omega}$ is simply [0, 0, ω].

You say you didn't get the same result they do, so what did you try?

4. Apr 19, 2012

### GarethB

I get the answer you describe for part (a) (that v=[0,ρω,0] which seems the obvious answer) if I use r=[ρ,0,0] not with r=[pcosψ,ρsinψ,0] is ω=[0,0,ω] unless I am really dumb and can't comput vector products (which is likely the problem b.t.w!) I am guessing that if I use the cartesian representation for r then ω has to be re-expressed in some way to resolve this.
The answer that v=[0,ρω,0] seems to give a curl of 2ω in one of the components I can't remember which (making use of the cylindrical coordinate version of curl).
Regarding the problem for (a) I guess there is a typo in the textbook?

5. Apr 19, 2012

### I like Serena

Not a typo, but I'm missing vector notations for things that are vectors and, in particularly, unit vectors.

The challenge would be in the curl in cylindrical coordinates.
Can you express v in cartesian coordinates (x,y,z)?
Then take the curl, followed by a conversion to cylindrical coordinates?

The curl of 2ω is in the z-direction, which is the same direction as ω points.
That is: $2ω\boldsymbol{\hat z}=2\boldsymbol{\vec \omega}$, where $\boldsymbol{\hat z}$ is the unit vector in the z-direction.

Last edited: Apr 19, 2012
6. Sep 12, 2012

### Smx_Drummerboy

Can be deleted

Last edited: Sep 12, 2012
7. Sep 12, 2012

### Smx_Drummerboy

So I know this problem is old, but it is cut & paste one of my HW problems with which I am having a lot of trouble. If I need to start a new thread I can, my apologies if I've broken a rule, I looked and didn't see one.

Anyways,

a) I have been letting:

$\vec{r}=[\rho*cos(\psi),\rho*sin(\psi),0]$

$\vec{\omega}=[0,0,\omega]$

Then I get $\vec{v}=\vec{\omega}\times\vec{r}$ $=[-\rho\omega*sin(\psi),\rho\omega*cos(\psi),0]$

If I have performed this correctly, then I don't see how I get to the asnswer: $[0,\omega\rho,0]$?

If $\psi=0$ then I get the answer, but I think that is just a coincidence. Does anyone see anything wrong with my logic.