# Inertial and Non-Inertial Reference Frames

1. Apr 8, 2015

### SQLMAN

Hi All
Physics newbie here......

Just a quick question regarding Inertial and Non-Inertial Reference Frames.
From what I understand:
Inertial = One that obeys Newton's Law of Inertia. Moves at constant velocity in one direction
Non-Inertial: One that accelerates.
Right?
So, I read somewhere that there is no possible way to have an Inertial reference frame on planet earth because earth rotates - Is this correct?
I figured (with my limited understanding) that because it's rotating at a constant velocity, it's inertial?
Where am I going wrong here?

Thanks

2. Apr 8, 2015

### PWiz

You experience the Sun's gravitational force on Earth. You also experience the Coriolis effect and an apparent "centrifugal" force due to rotation of the Earth. These apparent forces (also called fictitious forces) must be accounted for while calculations are being made, so the Earth is not an inertial frame of reference as these "external" forces act any observer on the planet (although in most cases they are negligible, and we can approximate the Earth to an inertial reference frame).

3. Apr 8, 2015

### A.T.

No. Rotating frames are not inertial, even at constant angular velocity:

http://en.wikipedia.org/wiki/Rotating_reference_frame

But for local and short experiments the effects of the Earth's rotation are often negligible compared to the effects you are interested in, so it's a valid approximation.

4. Apr 8, 2015

### stevendaryl

Staff Emeritus
I think what's clearer is to talk about inertial vs. non-inertial coordinate systems, rather than frames. An inertial coordinate system is one where a free particle (one not acted on by any nongravitational force) travels in "straight lines". That is, if you plot $x$ versus $t$ on graph paper, you get a straight line (and similarly for $y$ and $z$). In contrast, a noninertial coordinate system is one where the graph of the position of a particle as a function of time is a curve, rather than a straight line.

For example, in a rotating coordinate system, if you plot $x$ versus $t$, you don't get a straight line.[/QUOTE]