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

by Astaroth
Tags: earth, rotate, rotation
 Mentor P: 21,999 ....or if the atmosphere somehow came to be on earth, not rotating with earth, friction would have "stopped" it.
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 Quote by russ_watters ....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?
 Mentor P: 21,999 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.
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 Quote by 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.
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.
 P: 969 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.
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 Quote by cepheid 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.
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 Quote by russ_watters 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.
 P: 969 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.
 P: 768 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.
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 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.
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 Quote by 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.
Yes...... but we come into existence when we are born so we have to be at rest first before we start moving with inertia.
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 Quote by singh94 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?
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 Quote by singh94 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?
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 Quote by cjl 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
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 Quote by jarednjames Interesting, are you proposing that your mothers eggs, fathers sperm, mothers womb etc aren't rotating with the earth?
I thought that would be the reply but i wasnt sure so any ways tell me atleast one thing must have come into existence...... so atlest one thing would have required friction or some force.
so we come back to the same point..... how did the first of the things to come into being start rotating with earth/ ans that and your theory according to me is foolproof.
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 Quote by singh94 if the frictional force is constant ...... and the earth's speed be constant, how can we attain equilibrium?
It's not constant. (Have you read this thread from the beginning?)
 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?
What do you mean by 'real speed'? Speed is relative.
 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
A speed is not a force. And if you are moving at a constant velocity, the net force on you is zero. (Of course, on earth you'll be centripetally accelerating.)
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 Quote by singh94 I thought that would be the reply but i wasnt sure so any ways tell me atleast one thing must have come into existence...... so atlest one thing would have required friction or some force. so we come back to the same point..... how did the first of the things to come into being start rotating with earth/ ans that and your theory according to me is foolproof.
What first things to come into being?

Your father and mother are rotating with the earth, when you grow in your mother you are also rotating. The molecules you are made from have always been rotating with the earth.

It is only when you go back to the origins of earth that this argument is relevant. At which point, frictional forces come into play and are what set them in motion.

You are arguing that a human life being created is some mystical process and the molecules, particles, atoms etc "pop" into existence at that point and need to be accelerated. This is non-sense. You are made from materials already existing on the planet, moving with the planet.
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 Quote by Doc Al It's not constant. (Have you read this thread from the beginning?) What do you mean by 'real speed'? Speed is relative. A speed is not a force. And if you are moving at a constant velocity, the net force on you is zero. (Of course, on earth you'll be centripetally accelerating.)
i know a speed is not a force but to aquire a speed that i didnt posses initially i neeeda force also const velocity doesnt need force but aquiring a velocity which didnt exist before needs one.
By real i mean to a person on earth who can see me. Also while at it........ a person who is in space out of earths atmosphere.

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