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Q & A: inertia

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Most recent answer: 08/27/2008
Q:
Since the orbital speed of earth is 30Km/s, how come the things on the earth's surface don't feel a thing about the speedy movement? (In other words, why the atmosphere of earth provides such a good shield while the air around a bullet train cannot).
- Terry Hung 55
Fremont, CA, USA
A:
Hi Terry- The answer to this is closely related to your other question. There is no way to feel velocity. Saying what the Earth's 'velocity' is doesn't even mean anything. It is meaningful, within the context of Newtonian physics, to say how much the Earth is accelerating. However, that acceleration is very small compared to what you experience in, say, a moving car, and thus is hard to feel. The air has nothing to do with it.

Why then does a bullet train feel so much air resistance? Why doesn't the air just stay at rest from the train's point of view? The problem is that the air is also in contact with the much larger Earth. Since air molecules frequently touch the Earth, they tend to stay at rest from the Earth's point of view, which is quite different from the train's point of view.

Mike W.

p.s. The questions you are asking are very good. They were among those which most troubled philosophers around the time Aristotelian physics was replaced with Newtonian physics.

(published on 08/27/2008)

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