Thread: Rotation

Since the earth is clearly exposed to high friction internally and is made up of different consistencies, how does it keep its rotation?
Compare it with spinning a boiled egg as opposed to an unboiled.

Some one please enlighten me!

Originally Posted by Vector Equilibrium

how does it keep its rotation?
)

It keep its rotation due to one of the fundumental physical principe - conservation angular momentum. And no external forse need for keeping its rotation.

External influences like tidal friction has only little impact on the rotation velocity of the Earth. The rotation of the Earth has been slowing down during the last couple of billion years due to the influence of the Sun and the Moon. In particular, the Moon picks up some angular momentum of the Earth's rotation leading to an increase of the Moon's distance. The current rate is only about 3 cm per year. For the Earth's rotation, this means that one day increases by 0.0015 seconds per century of which about half is owed to the tidal interaction with the Moon.

An object like the earth - where there are different levels of viscosity creating friction - how is it, that rotation can be maintained? Shouldn't the core absorb most of the angular momentum, just like when you rotate an unboiled egg?

haven't you already posted a similar thread in the Astronomy part of the forum ? why then do you repost it here ?

unless i get a satisfactory answer to these questions, i will delete this thread

I didn't get a satisfactory answer, probably because i posted it in the wrong forum.
So please, delete the other one instead.

not sure whether this is the right part of the forum either
still, let's see how it goes, and if you feel you don't get a satisfactory reply here, i can try and move it to the physics forum - which tbh you should have done with the first thread, by asking dishmaster as moderator of the astronomy section

just PM me when you feel you want me to make that move

[edit]both threads merged - MR[/edit]

Originally Posted by Vector Equilibrium

Shouldn't the core absorb most of the angular momentum, just like when you rotate an unboiled egg?

Angular momentum is conserved, no matter what. The core does not absorb it. A better question might be why does the raw spinning egg slow down faster. Off the top of my head, I don't know the answer.

Isn't the Earth slowing down in rotation? I'm pretty sure it is. It's just slowly doing so because it's so big, right? Therefore, it can't be maintaining much of anything.

Originally Posted by mormoopid

Isn't the Earth slowing down in rotation? I'm pretty sure it is. It's just slowly doing so because it's so big, right? Therefore, it can't be maintaining much of anything.

Well, as Dishmaster mentioned, the earth is losing some of its angular momentum to the moon, and maybe there are some other minor effects, but the angular momentum overall is being conserved.

The egg has got to be transferring its angular momentum to the earth via friction with the table. I'm thinking the friction with the table is greater because it wobbles around instead of spinning about a single point.

I'm thinking more on the scale of billions of years, not just a day to day thing. It's estimated that in just the Ordovician alone, the standard day was 21 hours as opposed to 24 hours today, based on the slowing of the Earths rotation.

I am not sure, but I think the angular momentum is transferred to another rotation axis. If you spin the raw egg, you'll notice that it does not only stop rotating quickly, it also tries to stand upright because of the chalaza. In a cooked egg, it does not work, because the egg white is solid. Do you think it might work like this?

Originally Posted by mormoopid

I'm thinking more on the scale of billions of years, not just a day to day thing. It's estimated that in just the Ordovician alone, the standard day was 21 hours as opposed to 24 hours today, based on the slowing of the Earths rotation.

Yep, just do the math. 0.0015 seconds per century is 30000 seconds in 2 billion years or 8.3 hours.

I think the difference lies in how the rotation starts. The earth's rotation is the residual angular momentum of the local matter cloud as the solar system formed, while an egg is already in compact form.

When you try to spin the raw egg, the jelly innards lag behind due to inertia, so when you let go the innards elastically pull back against the shell's direction of rotation. So a lot of the initial energy transferred to the egg is stored elastically and when you let go it applies it against the direction of rotation. Kind of like towing a car with a bungee cord? Does that sound plausible? :?

That's exactly my point KALSTER! It's because of internal friction the egg stops. It's true that the egg, not the earth, becomes the problem because of the conservation of angular momentum. It all depends on whether or not you attach the point of reference to the rotating metric. If you attach the point of reference to the rotating metric, which we do(it's a flexible way to get rid of things like torque and coriolis force etc.), only the egg becomes the problem.
Or am I just delirious?

Originally Posted by KALSTER

I think the difference lies in how the rotation starts. The earth's rotation is the residual angular momentum of the local matter cloud as the solar system formed, while an egg is already in compact form.

When you try to spin the raw egg, the jelly innards lag behind due to inertia, so when you let go the innards elastically pull back against the shell's direction of rotation. So a lot of the initial energy transferred to the egg is stored elastically and when you let go it applies it against the direction of rotation. Kind of like towing a car with a bungee cord? Does that sound plausible? :?

It's likely a viscosity issue. When you rotate the egg the only thing transferring the rotation to the the interior part of the egg is the viscosity of the white. When you give the egg a quick spin, there just isn't enough time for the viscosity to transfer the rotation evenly throughout the egg. Thus you get different parts of the egg rotating at different speeds to start out. The viscosity of the fluid white then tries the distribute the rotation speed evenly. The results are that the egg seems to slow down quickly right after giving it the spin as everything "averages" out.

Take an egg, lay it on its side and give it a spin. It will slow down quickly, and settle to a slower more steady rate, only slowing from friction with the surface on which it rests. If you keep tapping the egg with your finger to increase its spin, you can get it spinning at quite a fast rate (by continually taping the egg, you are giving the innards a chance to "catch up" to the shell).

If you quickly stop the egg and then let go, it will start to spin slowly again. Again, stopping the shell of the egg quickly doesn't give the interior time to come to a stop, and when you let go, it starts to average out again, leaving the egg with a spin.

When you try to spin the egg on end, you have to give it a sharp twist in order to get it spinning fast enough to stand on end. But this sharp twist doesn't allow enough momentum to transfer throughout the egg, and when you let go it slows down and falls over.

If you could arrange to spin the egg up to speed slowly, or hold it upright while spinning it for a long enough time to allow the the angular speed to even out through the egg, You most likely could then let go and have a raw egg spinning on its end.

Originally Posted by Vector Equilibrium

That's exactly my point KALSTER! It's because of internal friction the egg stops.

No, no, no. The egg cannot stop itself by internal forces, any more than a car could stop itself without touching the road.

There are two separate conservation principles - conservation of momentum and conservation of energy. Energy can be dissipated internally. Momentum can't.

Janus does make a good point. When you spin the raw egg, it doesn't get up to the same initial speed because the internals lag behind. So it doesn't have the same initial angular momentum as the hard boiled egg. That's probably at least part of the answer.

Well the car doesn’t rotate around its own axis like the egg does; so that comparison would fit more to something like the earth’s movement around the sun.

“When you spin the raw egg, it doesn't get up to the same initial speed because the internals lag behind. So it doesn't have the same initial angular momentum as the hard boiled egg.”

The outer layer does get up to the same speed for a short while, it can’t just instantly slow down to the speed of the lagging core when you release it. So that would suggest internal friction between the different levels of viscosity, slowing it down to the movement speed of the internals.

But yes, the egg ultimately stops due to friction against the floor.

My night shift is over, so time for me to get some sleep (finally). Good night :wink:

Originally Posted by Vector Equilibrium

Well the car doesn’t rotate around its own axis like the egg does; so that comparison would fit more to something like the earth’s movement around the sun.

You are right about that. In one case it is linear momentum and the other case angular momentum. It doesn't matter. There is a conservation law for each.
http://theory.uwinnipeg.ca/physics/rot/node7.html

In the absence of external torque, the angular momentum of a rotating rigid body is conserved

For systems that consist of many rigid bodies and/or particles, the total angular momentum about any axis is the sum of the individual angular momenta. The conservation of angular moment also applies to such systems. In the absence of external forces acting on the system, the total angular momentum of the system remains constant.

"The angular momentum of an isolated system remains constant in both magnitude and direction."

Isolated system:

"An isolated system implies a collection of matter which does not interact with the rest of the universe at all - and as far as we know there are really no such systems. There is no shield against gravity, and the electromagnetic force is infinite in range. But in order to focus on basic principles, it is useful to postulate such a system to clarify the nature of physical laws. In particular, the conservation laws can be presumed to be exact when referring to an isolated system" (which was just said not to exist)

http://hyperphysics.phy-astr.gsu.edu...er.html#isosys