# Coriolis expression - Question about sign

1. Dec 17, 2012

### Curious2013

1. The problem statement, all variables and given/known data

Dear all

I have a question concerning the Coriolis acceleration expression. I learnt it as Ac = -2ω x v, where ω is the vector which indicates the rotation axis direction of Earth and v the velocity of a body that I want to check the Coriolis effect on.

My question: where the minus sign comes from? As far as I understand, it depends on what reference frame I use (inertial or non inertial - the minus comes from the latter, like the Earth, in my conception). Am I correct?

2. Relevant equations

3. The attempt at a solution

2. Dec 17, 2012

### tiny-tim

Welcome to PF!

Hi Curious2013! Welcome to PF!
There's no Coriolis force in an inertial frame.

(same as centrifugal force)

3. Dec 17, 2012

### TheEtherWind

The sign is solely due to the choice of coordinates. Here's a site that derives it as positive:

http://www.nws.noaa.gov/om/wind/deriv.shtml [Broken]

Last edited by a moderator: May 6, 2017
4. Dec 17, 2012

### tiny-tim

no, it derives it as negative

Coriolis force exists only in the Earth's frame (the rotating frame),

so in that frame (d2R/dt2)Earth is on one side of the equation, and everything else on the other side …

the Coriolis force then does have a negative sign