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Earth Spinning

 
Ranch Hand
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Just had a discussion with a guy in a bar. We were trying to work the answer to the following:

The earth rotates at 1000MPH (or whatever it is). If you take off in a plane and travel in the same direction at the same speed why will you not be flying over the same point on th ground?

I know this site contains the finest minds in the world so I know I will find the answer here...

We discussed momentum and air pressurre and drag but got nowhere.

What do you reckon?
 
Sheriff
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You mean, if you take off opposite to the direction the earth is rotating? The key point is that you, the airplane before it "starts moving", and the air around you (unless there is wind) is all travelling at the same speed that the Earth is rotating. You have that momentum working for you.

It's not a meaningless effect though. The Earth's rotation is why rockets usually take off to the east. It also affects the wind, which is why it is generally faster flying east than west.
 
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Um, isn't that more or less what some satalites do? for example, Direct TV satalites (however they communicate) have to be in a fixed location otherwise you'd lose reception for most of the day when your dish went out of the line of sight. I don't think it's as big a mystery as you might think.
 
Rick Beaver
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Satellites maintain a geostationary orbit by travelling at the same speed as the earth rotates, but the same cannot be said for aircraft. If an aircraft takes off in the opposite direction as the earth rotates then the ground speed should surely be the speed of the aircraft plus the rotation speed of the earth?
 
Greg Charles
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Newton's First law: Objects in motion tend to stay in motion, and objects at rest tend to stay at rest, unless an outside force acts on them.



This means that the airplane taking off west is still travelling east at 1000 mph, as is the air around it. (Here's a simple experiment to prove that. Walk outside. Do you get hit in the face by 1000 mph wind? No? It shows the air is moving at the same speed as you and the ground you are standing on.) From takeoff, the plane measures its speed relative to the air and the ground, which are moving at roughly the same speed and in the same direction. If it measured progress relative to something fixed like the sun or stars, it would, in fact, seem to be slowing down. If it could get up to 1000 mph (ground speed), it would even seem to be stopped relative to the stars.

The atmosphere rotates at about the same speed as the earth for the same reason that satellites do: the earth's gravity holds them in place. The net forces that cause that to happen are a bit harder to comprehend though.
 
Wanderer
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I'm not sure whether Henry is responding to Rick or Greg, so I can't tell how his thoughts align. So I'll just ignore that post for now; no offense. I think Greg's first post covered the original question well enough anyway. But now there are some further clarifications to be made...

[Rick]: Satellites maintain a geostationary orbit by travelling at the same speed as the earth rotates

Where speed must mean angular velocity, not distance divided by time, right?

[Greg]: The atmosphere rotates at about the same speed as the earth for the same reason that satellites do: the earth's gravity holds them in place. The net forces that cause that to happen are a bit harder to comprehend though.

Hm, the first sentence seems misleading, but the last sentence implies you're well aware there's more to the story. I would argue that geostaionary satellites only travel at the same angular velocity as the earth's rotation because humans have intentionally put them in that particular orbit. It's entirely possible for satellites to travel in other orbits which are not geostationary, nor even necessarily geosynchronous. We humans just have more use for satellites that are either stationary (from our perspective) or at least moving on a regular schedule. As for the atmosphere, though - the reason it's synched up with the earth is that it's in contact with the ground, and the air molecules keep bouncing against the ground and against each other in such a way that they average out the overall velocity. No human design goes into this; it just happens.
 
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I know this site contains the finest minds in the world so I know I will find the answer here...

In fact, the answer was here, in MD, before you even asked the question: https://coderanch.com/t/40976/md/Physics-Airtravel
 
Rick Beaver
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Aha - enlightenment. So the plane maintains its initial speed (same as earth) because no external forces act to slow it down. Which means, relative to the earth the plane may be going 400MPH but to an independant observer the plane speed would be the earths rotation speed offset by up to +-400MPH.

Is this it?

Thanks...
 
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Originally posted by Rick Beaver:
Just had a discussion with a guy in a bar. We were trying to work the answer to the following:



Seems to be a long hangover.
 
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Wait a second, Earth actually rotates?!!
 
lowercase baba
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The air moving at the same speed as the earth is what causes the spin of hurricanes (to my understanding). the air at the equator is moving faster than the air at the poles.

now, create a low pressure system - a great big vacuum that is sucking in air - somewhat north of the equator. the air being sucked in from south is moving really fast. the air coming in from the north is moving really slow. so, the southern half of the storm starts moving east, and the northern half starts moving west, causing the spinning of the hurricanes.

in the southern hemisphere, they will spin in the opposite direction, for the same reason
 
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I remember my physics teacher saying something about an "Inertial frame of reference" or some such before I dozed off.

The orbits of satellites - whether they are geosynchronous or not have something to do with the concept of "escape velocity" and Keplar's Laws of Planetary motion (if I can remember correctly).
 
Sheriff
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If anyone needs the Earth's rotation stopped, just let me know. (I found the emergency brake lever.)
 
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Marc,
Were u a part of the "earth-saving" mission in the movie "The Core"??
 
marc weber
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Originally posted by Ganpi Srinivasan:
Marc,
Were u a part of the "earth-saving" mission in the movie "The Core"??


The first rule of Fight Club is...
 
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Originally posted by Ganpi Srinivasan:
Marc,
Were u a part of the "earth-saving" mission in the movie "The Core"??

Was anyone else really disappointed by that film? I know I'm a bit of a sucker for films with a sciency theme to them, but I was hoping that this one would be quite interesting. Instead it was the biggest load of rubbish ever made.

Special suits which can withstand the huge pressure inside the Earth and still allow people to walk around in them? Lasers used to blast the magma out of the way? Visible beams of gamma rays? A human built device capable of changing the rotation of the core? Bah!
 
Trailboss
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The mainland U.S. is about 3000 miles across and we have three times zones. So I would guess that the earth spins at about 1000 miles per hour "up here". It must spin much faster at the equator and slower near the poles.
 
Sheriff
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Originally posted by Paul Wheaton:
The mainland U.S. is about 3000 miles across and we have three times zones.



Er, four: Eastern, Central, Mountain and Pacific.
 
paul wheaton
Trailboss
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3 is the new 4!
 
Jim Yingst
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The Earth's equatorial radius is 3963 miles, so the circumference is 24900 miles, and points on the equator travel at 1038 mph. To get the speed at other locations, just multiply by the cosine of the latitude. At the southern tip of Florida, that's 25° and 940 mph. For most of the western border with Canada, that's 49° and 681 mph. In between, it's... in between.
 
Rancher
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This thread motivated me to bring back to life an old favorite applet of mine: Martin Minow's SunSphere, which shows the rotating earth along with its current spheres of light and darkness.
 
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