# Non-inertial frame

1. Aug 16, 2014

### mikengan

I confuse when should I use non-inertial frame...
is there any keyword or key clues that remind you of using non-inertial frame of reference?

can anyone help explain this with an example?

2. Aug 16, 2014

### dipole

A non-inertial frame is a frame of reference which is accelerating with respect to another frame of reference.

For example, for people standing on the earth we are in a non-inertial frame of reference, because the earth is rotating with respect to the "fixed" stars, and simultaneously moving in a curved orbit around the sun.

If you want a deeper understanding than that, you'll probably have to read up on some general relativity.

3. Aug 16, 2014

### Matterwave

It depends a bit on how you're looking at this problem. Physically, we can tell we are in a non-inertial frame by the appearance of fictitious forces in our frame. For example, here on Earth, we experience the coriolis force and the centrifugal force (for the Earth, it's hard to notice since it's angular velocity is so low). When your car is turning, you feel a centrifugal force even though nothing is pushing on you. This tells you that you're in a non-inertial reference frame.

The difference between a Newtonian non-inertial frame and a General Relativistic non-inertial frame would be which forces we call "fictitious forces". Specifically GR does not treat the gravitational force as a real force, it is more like a "fictitious force".

4. Aug 16, 2014

### A.T.

That would make all reference frames non-inertial.

5. Aug 16, 2014

### dipole

No it wouldn't. Explain how you arrive at that conclusion. The point is, in Newtonian physics (which I assume is the scope the OP is working in) velocities are relative, but acceleration is absolute - it is not correct to say that if I am accelerating on a train and see a person standing at the train station, that from my point of view, that person is in a non-inertial frame of reference and my frame is inertial, because there are no measurable ficticious forces acting on that person, but there are on me. All frames which are inertial with respect to the "fixed stars" are interial relative to eachother, and any frame which is non-inertal with respect to the fixed stars is non-inertial to all others.

6. Aug 16, 2014

### Nathanael

I think what A.T. meant is you should've said:
"A non-inertial frame is a frame of reference which is accelerating with respect to an intertial frame of reference"

(Because inertial reference frames are accelerating with respect to non-inertial reference frames, right?)

7. Aug 17, 2014

### A.T.

Proper acceleration is absolute. But you were taking about coordinate acceleration relative to something:
I guess you mean proper acceleration that an accelerometer on the train would measure?

No. Whether a frame is inertial or not is not a matter of perspective. Proper acceleration is absolute.

If there are fictitious forces in the frame of the train, then we know that the train-frame is non-inertial. So the train passenger cannot say "from my point of view, my frame is inertial", as you did above.