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

JaredJames

Works for me.

cepheid

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

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

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

Yes.

D H

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 [tex]\Phi[/tex], 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

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

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

Really? At rest with respect to what?

JaredJames

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

singh94

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

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.