# The surface of the Earth as a reference frame

1. Mar 10, 2015

At about 05.30, Dr. Lewin makes a small calculation to find the centripetal acceleration one experiences due to Earth's spin (at the equator). He then says that because of how small the magnitude of this acceleration is, we could consider the Earth's surface to be an inertial reference frame (approximately). My question is, what if the centripetal acceleration were much larger? Wouldn't the frame be (approximately) inertial as well? For instance, two people standing somewhere near the equator would have the same acceleration relative to the centre of the Earth, and so they would be stationary relative to one another. Is there something I'm not getting right? I'm actually quite confused about the definition of an inertial reference frame; "a frame that is not accelerating", not accelerating relative to what?

Last edited: Mar 10, 2015
2. Mar 10, 2015

### PeroK

I think he means that the variation in centripetal acceleration across the globe is small enough to be ignored. Although, I don't see how that makes the Earth an Inertial reference frame.

Unless, of course, you consider gravity a real force. That makes sense.

Last edited: Mar 10, 2015
3. Mar 10, 2015

### Quantum Defect

In a rotating frame (accelerating) you will begin to see "mysterious" forces that require an explanation. If you and a friend tried to play catch with a ball on a quickly rotating platform, you would see odd, curving trajectories of the ball -- the origin of these forces is due to the fact that you are on a rotating frame. You and your friend can play catch with a ball on the surface of the earth, without compensating for these forces.

When I took a classical mechanics course in college, the professor remarked that although things like the Coriolis force are small (at our scael) for us on the earth, they give rise to measurable effects at our scale. The example he provided was the fact that N-S train tracks show different patterns of wear, when compared with E-W train tracks.