# Big deal that we speed along on the surface of the Earth

1. Feb 19, 2010

### silvanet

I often read or hear people making a big deal that we speed along on the surface of the Earth at about 1,000 miles an hour - well, at least on the equator. In Miami, were only "speeding" along at about 900 mph; in Chicago, they're slower, only around 775 mph; and in Anchorage, even slower, only about 485. Nearer the poles, persons are relatively moving like snails. Is there any significance to that speed as far as human experience goes? Or is it largely irrelevant. Everybody just feels like they're standing still, and for almost all their imaginable earthly activities it makes no difference. I mean other than if the sun were to instantly vanish they would go flying off the earth's surface at different velocities?

How significant is 445 meters per second to the fastest men on earth who run 10 meters per second? Would whether they were running with or against the Earth's rotation make a slight difference on their times, like running with or against the rotation on a running machine? Would that difference be actually noticeable in Anchorage, or Reykjavic? Of course runners run same course, so no advantage; but, I mean relative to an individual runner. I would guess that it does make a difference, when you're traveling at speeds faster than sound, whether you're moving in the direction of Earth's rotation or against it.

I know this may sound like a stupid question to someone who has advanced understanding of physical motion. I did study physics in high school and a couple of courses in college, both with calculus, but only one with a lab. My daughter is taking a college intro physics course and I'm trying to help her. These thoughts just started coming up in my own mind as we worked on centripetal acceleration and angular velocity. I hope this is an appropriate post. Just wondering, and I'd appreciate comments from people who know the subject.

Thanks

2. Feb 19, 2010

### kanato

Re: speed

It's largely irrelevant. The centripetal acceleration required to stay on the surface of the earth at the equator is tiny compared to the amount of acceleration gravity provides for people.

Velocities are always relative. The question for someone running on the surface of the earth is what is his velocity relative to the surface of the earth, or relative to the atmosphere which provides air resistance to his movement. For these situations the rotational velocity of the surface of the earth won't matter at all. Now if you want to fire ballistic missiles or fly a jet a long distance over the earth in a short period of time, then one has to account for the motion of the earth by computing the effect of the Coriolis force. But for more basic human experiences, it's not relevant.