# Special Relativty and Inertial Reference Frames

1. Oct 13, 2006

### SudanBlack

Hi - I've just started having lectures on special relativity at uni. We were talking about inertial reference frames and how these can be characterised by the facts that:
1) They move relative to one another with constant velocity, and
2) Newton's laws operate in inertial reference frames.

Now, we pressume that Newton's laws are true on earth. However, this is not travelling at a constant velocity, since there is a centripetal acceleration towards the sun. So how can we still class the earth as an inertial reference frame?

2. Oct 13, 2006

### ZapperZ

Staff Emeritus
The "non-inertial" component is VERY weak under most circumstance. Try it yourself.

Sit on a rotating platform and put a ball on the platform. Now, try the same thing with our earth. Put a ball on your desk and see if you observe the same effect. No? Then for all practical purposes, you do not detect this centripetal component due to earth's orbit around the sun.

Zz.

3. Oct 13, 2006

### Meir Achuz

Your point (2) is not true in SR.
The rotation of the Earth is a more important non-inertial effect than its acceleration. The rotation causes hurricanes, etc. For table top experiments, the Earth can be considered a reasonable inertial frame if the equivalence principle of GR is not applied to g.

4. Oct 13, 2006

### MeJennifer

I think that both statements are incorrect, or at least they are incomplete.

We can certainly have two inertial reference frames that move relative to each other where the velocity is not constant. This is that case in a gravitational field. And at relativistic speeds it is not Newton's laws but GR that operates in inertial reference frames.

Seems to me a bit of a straw man argument, since who claims it is?
A spinning sphere is obviously not an inertial reference frame.

Last edited: Oct 13, 2006