# Do things move in uniform motion in real life?

1. Dec 18, 2011

Does this actually happen in outer space? Do things actually move in uniform motion relative to each other?

How would this work for the Earth considering it's rotating?

Can the Earth ever be considered an inertial frame because it's always rotating?

2. Dec 18, 2011

### ghwellsjr

No. Uniform, or inertial, motion means that something has never accelerated--never changed its speed or direction--which means that it has existed forever and "real life" has not existed forever.
No, but we can approximate, so for short periods of time and for short distances, and ignoring the effects of gravity, it works good enough.

3. Dec 18, 2011

I thought an inertial frame cannot be one that is rotating.

Does the symmetry of SR actually exist in space?

Isn't it possible that things have reached a state where they are traveling together uniformly?

4. Dec 18, 2011

### chente12

Hmm.. I remember reading a while back, the book was Einstein’s Theory of Relativity (Born), in which it was said that uniform motion exists if the inertial reference frame is specified. He cleverly uses the example of a fixed ruler, a pencil, and a wheel (imagine a pottery wheel) to get his point across. Imagine the pencil moving along the ruler and the wheel rotating. To the observer fixed on the ruler, the motion seems uniform (straight), but to the one on the rotating disk, the motion is a curved line. Anyone feel free to correct me if i’m misinformed.

5. Dec 18, 2011

### chente12

If you’re looking for a very very correct answer (i don’t know what that means either ) then no, nothing (to my knowledge) ever travels in uniform motion (especially on earth). The earth is spinning, so something that seems to travel in straight lines actually travels in a slightly curved path. But according to the book, we don’t notice this because of the shortness of the paths used compared with the dimensions of the earth (which someone already said). Once again correct me if i’m wrong.

Last edited: Dec 18, 2011
6. Dec 18, 2011

### ghwellsjr

You thought right.
Yes, along with the asymmetry of GR and asymmetries of SR caused by less than perfectly inertially moving bodies.
No, why would you think that?

7. Dec 18, 2011

### ghwellsjr

You have to distinguish between an inertially moving body and an inertial frame. The rotating earth is not an inertial body, but we can invision an inertial frame that contains the non-inertial rotating earth.

8. Dec 18, 2011

### Staff: Mentor

I would phrase this somewhat differently. The Earth as a whole, considered as a single object, is moving inertially, at least to a very good approximation. We know this because we can compute its motion using the assumption that it is moving inertially, i.e., that it is in free fall with its motion determined by nothing but gravity, and we get the right answer to a very good approximation.

However, most *parts* of the Earth are *not* moving inertially; they feel acceleration. This would be true even if the Earth were not rotating, because it is a large gravitating body and most of its parts, as well as objects gravitationally bound to its surface (like us), have forces exerted on them by other parts that push them into non-inertial states of motion. For example, we feel weight standing on the surface of the Earth because that surface pushes us upward, out of the freely falling path we would take if the surface were not there.

The rotation of the Earth, from the standpoint of what parts of it are or are not moving inertially, is only a fairly small correction to the above; it makes the Earth oblate to about one part in 300, and it causes small variations in the acceleration felt at different locations. For example, a person standing at the equator feels a little less weight than a person of identical mass standing at one of the poles.