Why do we rotate along with the earth's rotation?

[Astaroth]

I've been having a huge debate about this, some people are saying "air resistance" and that just makes zero sense to me, because the moon has no atmosphere and Neil Armstrong rotated along with the moon with no problem whatsoever.

I am thinking it has to have something to do with the properties of a gravity field around a spinning object. Does anyone know?

 

[D H]

The answer is friction. Neither the Earth nor the Moon is coated with teflon.

 

[Astaroth]

But the whole of the Earth's atmosphere rotates along with the earth, why?

 

[D H]

Friction again. Suppose the atmosphere was not rotating at all. The word hurricane doesn't begin the describe what the resulting 1000 mile/hour wind would do. The Earth would quickly spin the lower atmosphere up the Earth rotation rate. The lower atmosphere in turn would spin the upper atmosphere up the Earth rotation rate.

 

[Astaroth]

Why would the Earth spin the lower atmosphere? there is no friction between them.

 

[Xori]

Gases have friction between them.

 

[Astaroth]

is it really enough friction to consistently keep the whole of the atmosphere rotating at 1000mph? that sounds really odd.

 

[Doc Al]

is it really enough friction to consistently keep the whole of the atmosphere rotating at 1000mph? that sounds really odd.

Once the atmosphere (or people!) is rotating along with the earth, friction is not needed to keep it moving with the earth. (You do need gravity, lest we fly off at a tangent.)

When you accelerate in your car, what keeps you going with the car? The car exerts a force on you, of course. Once you have reached your cruising speed, no force is needed to maintain your speed and keep you moving with the car. (Assuming you maintain a constant direction as well as speed.)

 

[HallsofIvy]

In Arthur Clarke's "Fountain's of Paradise", he doesn't have the "space elevator" nearly long enough. He seems to think that you could put the upper end at a satellite in geo-syncronous orbit. In geosynchronous orbit, the centrifugal and gravitational forces of an object orbiting once a day are balanced.

Of course, the gravitational forces on the lines below that are enormous and would pull the thing down. You have to go far enough above geosynchronous orbit so that the net upward forces would over come that.

 

[TVP45]

The Gravity of the Situation

Yes, gravity holds you on. You can do an experiment by setting a cup of very expensive latte on a childs rotating ride at a park. Spin it around and see what happens when there is no gravity.

 

[kurushio95]

I think it's the same thing that let's you toss a ball up and down on a moving vehicle, without the ball streaking to the back. I don't know what that would be called, though.

 

[Doc Al]

Call it inertia.

 

[Astaroth]

Ok I think I've got it pretty much, it's all due to friction and inertia. That brings me to my next question, which is actually where this debate started.

I'm sure many of you are aware that plane journeys from East to West take less time than the same journey from West to East. The classical explanation is that's just the way the wind blows, and of course that is a major factor, but I was wondering if there is actually some effect of breaking free of the Earth's rotation. This would be due to the following:
1. Obviously, not touching the ground anymore.
2. Minimized air resistance due to aerodynamic build, so we can pretty much ignore the atmosphere for a qualitative question (i.e. is there an effect AT ALL?)
3. Again obviously, accelerative force generated by the jet engines.

The plane has inertia as soon as it takes off, but as it begins to push air back with its engines it gradually overcomes the inertia. Due to the massive air drag it actually experiences in the practical situation, it can only manage this to some extent. Does this make any sense?

 

[russ_watters]

By the way - when Neil Armstrong landed on the moon, he had to match the speed of the spacecraft to the rotation rate of the moon (very slow). But once on the moon, Newton's first law keeps you moving with it and the acceleration from the rotation is perpendicular to the rotation direction.

No, the jet stream is entirely responsible for the difference in flight times from east to west.

 

[Astaroth]

By the way - when Neil Armstrong landed on the moon, he had to match the speed of the spacecraft to the rotation rate of the moon (very slow). But once on the moon, Newton's first law keeps you moving with it and the acceleration from the rotation is perpendicular to the rotation direction.

That's very interesting indeed, a good way to convincingly prove the point (however obvious it may be to some).

No, the jet stream is entirely responsible for the difference in flight times from east to west.

Ok, but why? considering the things I mentioned above... I mean you sort of need to address the question of how the inertia is maintained if the friction is much lower and there IS a net force acting against it.

Surely the fact that the atmosphere is much thinner at that altitude means something with regards to this issue!

 

[cesiumfrog]

Wikipedia jet stream. Google coriolis force. Draw diagrams and figure out for yourself what would happen if we made the poles warmer than the equator.

 

[cesiumfrog]

(Wait a sec, my diagram didn't work out... ah..)

plane journeys from East to West take less time than the same journey from West to East.

Haven't you got that completely backwards?

 

[Astaroth]

(Wait a sec, my diagram didn't work out.. ahh...)
Haven't you got that completely backwards?

Nope.

Are you implying i could have got it somewhat backwards? :P

 

[Astaroth]

Ok never mind I worked it out with some help from someone. Basically since the airplane just pushes air back in order to propel itself forward, the inertia is always taken into account and never disappears, so anything that happens in the frame of reference of the atmosphere that actually depends on the atmosphere will maintain its inertia as normal.

However, I would assume that a space shuttle would not, if it was using, say, its ion thruster within the Earth's atmosphere. Not that it would, just theoretically speaking, since its propulsion would not depend on the atmosphere, it would escape its inertia.

 

[cesiumfrog]

"Nope." No reasoning nor explanation? I've no patience for people who are just obstinately wrong.

EDIT: Ah, I see you've edited your statement. "with help from someone" indeed.
Alas, saying "escape its inertia" does not indicate that you have understood the previous posts here.

 

[Astaroth]

Uhm, what exactly was I wrong about? Am I missing something here, doesn't the Earth rotate towards the east?

Also, "escape its inertia" is just figurative, i mean something like "get over its inertia".

There is no reason to be condescending, by the way. I am just asking questions.

 

[russ_watters]

Ok, but why? considering the things I mentioned above... I mean you sort of need to address the question of how the inertia is maintained if the friction is much lower...

Inertia is resistance to acceleration. It is a property of mass. It isn't "maintained" (or "escaped"). Maybe you mean momentum?

...and there IS a net force acting against it.

What force? (hint: there is no force, and the coriolis effect is something different)

Ok never mind I worked it out with some help from someone. Basically since the airplane just pushes air back in order to propel itself forward, the inertia is always taken into account and never disappears, so anything that happens in the frame of reference of the atmosphere that actually depends on the atmosphere will maintain its inertia as normal.

Yes.

And actually, yeah, I didn't notice, but a plane does fly west to east faster than east to west (in the northern hemisphere).

 

[Astaroth]

So, wait, does the Earth rotate into the east or the west? Because I'm actually quite certain that as far as flight times go, east to west is shorter, actually by almost an hour from london to new york.

 

[russ_watters]

No, flight times are shorter from west to east because of the jet stream, but yes the Earth rotates from west to east.

The jet stream is caused by the coriolis effect and convection.

Jet streams can be explained as follows. In general, winds are strongest just under the tropopause (except during tornadoes, hurricanes or other anomalous situations). If two air masses of different temperatures meet, the resulting pressure difference (which causes wind) is highest along the interface. The wind does not flow directly from the hot to the cold area, but is deflected by the Coriolis effect and flows along the boundary of the two air masses.

I'm not sure about flight times from New York to London, because they might take great circle routes.

The location of the jet stream is extremely important for airlines. In the United States and Canada, for example, the time needed to fly east across the continent can be decreased by about 30 minutes if an airplane can fly with the jet stream, or increased by more than that amount if it must fly west against it. On longer intercontinental flights, the difference is even greater, it is faster and cheaper (by flying the pressure pattern) or flying eastbound along with the jet stream and flying around the jet stream going west bound, than taking the shorter great circle route between two points.

http://en.wikipedia.org/wiki/Jet_stream

 

[Astaroth]

Sorry but I am almost certain that Westward is quicker, and a quick google will give you tons of proof (even though I actually experience this several times a year flying from home to uni and back).

Just one example:
http://www.aerospaceweb.org/question/dynamics/q0027.shtml

 

[HallsofIvy]

Sorry but I am almost certain that Westward is quicker, and a quick google will give you tons of proof (even though I actually experience this several times a year flying from home to uni and back).

Just one example:
http://www.aerospaceweb.org/question/dynamics/q0027.shtml

? The very example you cite:

When we fly west over the Pacific Ocean (e.g. California to Bombay) it takes about 22 hours to reach the destination, but it only takes 17 hours back.

says exactly the opposite: that it takes less time flying eastward (Bombay to California- 17 hours) than westward (California to Bombay-22 hours)!

I notice you also asked several time whether the Earth rotates to the east or to the west (it rotates to the east). Perhaps you just have difficulty distinguishing east from west. Were you under the impression that flying from Bombay to California was flying west?

Since the Earth is a sphere you could fly west from Bombay and get to California- but that's the long way!

 

[Astaroth]

Ah, see, I missed the "over the Pacific" part. Sorry about that. That explains a lot then. Sorry about that.

 

[singh94]

The answer is friction. Neither the Earth nor the Moon is coated with teflon.

but the maximum friction can only be uN u=coeff of friction and N=normal reaction or weight. then the tiniest particles and on hilltops with significant heights, there should be some exceptions. Please explain to me. THANKS.

 

[cjl]

but the maximum friction can only be uN u=coeff of friction and N=normal reaction or weight. then the tiniest particles and on hilltops with significant heights, there should be some exceptions. Please explain to me. THANKS.

The frictional force doesn't need to be large, it just needs to exist. The smaller the force, the longer it would take to establish equilibrium, but it will always happen.

 

[asdofindia]

but the maximum friction can only be uN u=coeff of friction and N=normal reaction or weight. then the tiniest particles and on hilltops with significant heights, there should be some exceptions. Please explain to me. THANKS.

But the mass of such a particle would be even smaller. For the same coefficient of friction, the mass wouldn't matter at all.
And coefficient of friction can never be 0. In that case we would build a perpetual machine.

 

[Drakkith]

Remember that we are NOT talking about the Earth suddenly moving from a standstill. The Earth and everything on it has been rotating for billions of years. Because of that friction has littled to do with it. Gravity pulls you to the ground but you still have a velocity through space, that doesn't change.

 

[russ_watters]

I don't really like the friction answer. Standing still on my floor right now, there is no friction at all between my feet and the floor. What keeps me from slamming into the west wall of my house at 1000mph isn't friction, it's inertia (Newton's first law).

 

[JaredJames]

I don't really like the friction answer. Standing still on my floor right now, there is no friction at all between my feet and the floor. What keeps me from slamming into the west wall of my house at 1000mph isn't friction, it's inertia (Newton's first law).

Wouldn't the friction answer only apply to initial acceleration / final deceleration of the air (assuming the Earth was to stop rotating somehow)?

 

[Drakkith]

Wouldn't the friction answer only apply to initial acceleration / final deceleration of the air (assuming the Earth was to stop rotating somehow)?

That's what I'm guessing, like my own post said.

 

[JaredJames]

That's what I'm guessing, like my own post said.

Works for me.

 

[cepheid]

That's what I'm guessing, like my own post said.

Yeah I think the post of yours that you refer to had the right idea. Better to talk about the "beginning." I think the conventional model is that you can think of Earth as having formed from some initial "clump" or overdensity in the protoplanetary disk, and that that clump had some initial angular momentum (it was spinning -- all of it). Higher density things settled toward the centre and lower density things stayed on top, but basically all of it was spinning (both what we now call atmosphere and the solid part) since the beginning.

What do you guys think?

 

[russ_watters]

...or if the atmosphere somehow came to be on earth, not rotating with earth, friction would have "stopped" it.

 

[cepheid]

...or if the atmosphere somehow came to be on earth, not rotating with earth, friction would have "stopped" it.

I don't understand your remark. Can you elaborate?

 

[russ_watters]

It's a hypothetical and the only way "friction" could possibly be the answer to "what makes the atmosphere rotate with the earth?" It isn't reality, though.

 

[cepheid]

It's a hypothetical and the only way "friction" could possibly be the answer to "what makes the atmosphere rotate with the earth?" It isn't reality, though.

Are you saying that if the Earth initially didn't have an atmosphere, and then started to "accrete" gas to create one, that that gas, if initially not rotating with the Earth, would be "spun up" by friction with the planet until such time as it was "co-rotating?" If so, then I agree. That would be the answer to why the atmosphere rotates with the solid body in this hypothetical scenario.

 

[RedX]

The equator spins faster, so if you walk north or south from the equator, friction should slow you down until you're at the right speed for your latitude. When you hit the poles, you will no longer be spinning. So if you multiply the angular velocity of the Earth (easy to do as it spins one revolution a sidereal day) by the radius of the Earth (easy to do also as the angle the North Star makes with the northern horizon dips by 1 degree if you drive 70 miles south, so circumference of the Earth is 360*70 miles), you'll get your change in velocity if you move from the equator to the north pole. If I didn't screw up the numbers, you are spinning 465 meters per second at the equator (which also means at the equator you're lighter by 3/1000 of your weight at poles, due to the centrifugal force pushing you up so that the ground doesn't have to). So you lose a lot of kinetic energy when you walk to the North pole, equal to .5*(your mass) 465^2. Friction steals your kinetic energy and gives it to the Earth I guess, making it spin faster.

 

[russ_watters]

Are you saying that if the Earth initially didn't have an atmosphere, and then started to "accrete" gas to create one, that that gas, if initially not rotating with the Earth, would be "spun up" by friction with the planet until such time as it was "co-rotating?"

Yes.

 

[D H]

I don't really like the friction answer.

You are correct in that friction is not the right answer in the case of someone standing still with respect to the rotating Earth. What keeps some who is standing still rotating with the Earth are inertia, gravity, and the normal force.

A hidden part of the problem in this thread is that some posters appear to have an Aristotelean view of physics, that a force needs to be continually applied to an object to keep the object in motion. That of course is not the case. A force needs to be applied to change an object's motion.

From the perspective of a non-rotating observer moving alongside the Earth, a person standing still on the surface of the Earth is undergoing uniform circular motion. A net force is needed to maintain that circular motion. This net force is normal to and directed towards the Earth's rotation axis. The forces acting on this person are gravitation, directed downward, and the normal force, directed upwards. Due to the Earth's non-spherical shape the angle between these forces is not quite 180 degree. The net sum of these two forces is exactly equal to the net force needed to make the person keep following that uniform circular motion.

 

[RedX]

Actually, now that I think about it, friction is actually what keeps us in our place on the earth. Just imagine a hoop that's placed vertically, rotating around a vertical axis through it's center (so imagine \Phi, with the hoop "O" spinning around the axis "I"). If you have a bead that slides on the hoop, then the centrifugal force will naturally push the bead to the midpoint of the hoop. The only thing that will oppose that is friction. We are ignoring gravity. So without friction, everyone would slide towards the equator!

Of course gravity is what keeps us glued to the earth. But if the question is why do we rotate along with the Earth and not why we are glued to the earth, then friction is important.

 

[Lsos]

You can try to not rotate with the earth. Go drive a car, plane, bike, or even go swimming. When you do that, you are not rotating along with the earth, but slightly faster or slower (depending which way you're going). You'll notice that this always takes energy. The faster you go, the more energy it takes. And if you stop inputting energy, friction will bring you back to the speed of the earth. Jump out of a moving car if you don't believe me! (don't really do that though :) )

This works the same way with the atmosphere. A jet liner requires a constant input of tens of thousands of horsepower to move at a different speed than the air around it...it goes to reason that if you keep a jet liner still but move the air over it, you need at least the same power input. Now imagine this effect, but working over the entire surface of the earth. The tendency is always for everything to go the same speed as the earth. The larger the difference, the stronger this tendency. Friction is no small matter, even with air.

Inertia also does its part to even things out, but if it was just inertia at play than the car who's driver dies will keep on driving. The wind will keep on blowing. The river will keep on flowing. The landslide will keep on sliding. Eventually things would be just flying around every which way at every possible speed. Of course inertia is the reason the Earth itself is rotating at the particular speed that it's rotating...which in turn drags everything along with it.

Naturally gravity has it's role as there'd be no friction (and indeed, no earth) if not for gravity holding everything together.

So, in simplest terms, the answer is friction, inertia, and gravity.

 

[Drakkith]

A jet liner requires a constant input of tens of thousands of horsepower to move at a different speed than the air around it...it goes to reason that if you keep a jet liner still but move the air over it, you need at least the same power input.

Sure. The wind is powered by, ultimately, the sun creating a temperature difference between places here on earth. In general through, inertia is why everything is rotating with the earth, not friction. Note, we are talking about generalized objects here. Obviously a car going down the road is using work to do it. Friction and compression are what stop a car if the driver suddenly was incapacitated.

 

[singh94]

Remember that we are NOT talking about the Earth suddenly moving from a standstill. The Earth and everything on it has been rotating for billions of years. Because of that friction has littled to do with it. Gravity pulls you to the ground but you still have a velocity through space, that doesn't change.

Yes... but we come into existence when we are born so we have to be at rest first before we start moving with inertia.

 

[Doc Al]

Yes... but we come into existence when we are born so we have to be at rest first before we start moving with inertia.

Really? At rest with respect to what?

 

[JaredJames]

Yes... but we come into existence when we are born so we have to be at rest first before we start moving with inertia.

Interesting, are you proposing that your mothers eggs, fathers sperm, mothers womb etc aren't rotating with the earth?

 

[singh94]

The frictional force doesn't need to be large, it just needs to exist. The smaller the force, the longer it would take to establish equilibrium, but it will always happen.

if the frictional force is constant ... and the Earth's speed be constant, how can we attain equilibrium?
Another doubt... so we are moving with a speed of 1000miles per hourin west to est direction... if i start runnning in the east to west direction at 5 miles per hour then what should be my real speed... 995miles in the west to east direction or what?
do i need to exert a force of 1005 miles per hour because if not then i am running in 2 opposite directions at the same time which is definitely not possible