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

jtbell

Hint: "Mentor" = "Moderator" here. Mentors have the power to ban people.

If you insist on using text-speak, you are welcome to find another physics forum that allows you to do so.

JaredJames

The further out you move away from the centre of the earth, the greater the distance you travel and the higher your velocity becomes.

A good example is a geostationary satellite. It is constantly above one point on the surface of the earth, but it's distance travelled and speed is many times greater.

cepheid

I know that this has already been answered, but I felt that I should respond since it was addressed to me:

1. You mean that you thought that we would be psychic enough that we would somehow be able to make (correct) assumptions about your level of physics knowledge, in spite of the fact that you failed to communicate that level clearly? Well, NO, we can't read your mind. All we have to go on when it comes to gauging your physics knowledge are the things that you write. EDIT: and besides, there is no such thing as "a force that gives you a speed of 1005 mph." Any force CAN give you that speed provided you apply it for long enough time. Once again, based on what you have typed, we have no choice but to assume that you don't understand the impulse-momentum theorem.

2. You DON'T have to "run against the direction of the Earth." It's NOT like being on a treadmill, because you are MOVING WITH THE EARTH. In other words, if you don't move, it means you are stationary with respect to the Earth's surface. Even when you are standing still, it appears to an outside observer (who is not rotating with the Earth) that both the Earth and you (on it) are rotating at the same rate. Do you get it now?

Also, read the rest of the thread. You do NOT need to invoke friction at any point in history (not even going back to Earth's formation) to understand why everything that makes up part of the Earth is rotating. Only conservation of angular momentum need be applied.

D H

That's going a bit too far. In the long term, you cannot apply conservation of angular momentum for the simple reason that the Earth's angular momentum has not been constant. the Earth's rotation rate was considerably higher (4-6 times higher!) shortly after the Moon formed compared to its current rate. The atmosphere and oceans are not moving around the Earth at 4-6 times Earth rotation rate precisely because of friction.

cepheid

Thanks. Yeah, that is a fair point. I failed to consider tidal effects.

Edit: but weren't the atmosphere and oceans being torqued on as well?

asdofindia

I thought conservation of angular momentum was applicable only to a rigid body rotating on its own axis. In that case we'd have to be glued to earth's surface from time immemorial.

(But that's what the gravitational force and friction do. Gluing us to the surface)

So, the answer is a complex interplay of conservative and non-conservative forces, and conservation of motion.

Drakkith

I agree with asdofindia. It is a result of several forces over a long period of time. But in the short term, we don't need friction to keep us rotating with the earth.

asdofindia

Imagine a magnetic sphere. Rotating.
Imagine a sticky man on it with iron legs. Let's say he already has acquired the velocity of the point right under his legs.
At every point, the man's velocity wants him to go tangential to the surface, to be thrown away from the sphere.
But the magnetic force pulls him onto the sphere.
(Centrifugal and centripetal forces)
Agreed till now?
The centrifugal force and centripetal forces are exactly opposite and cancel each other. But they've got no component along the surface of the sphere!!
(So there's got to be friction???!!!???!!!)
I don't think I've understood my answer.

Drakkith

What do you mean by this?

asdofindia

See, if we draw a free body diagram. We'd draw an arrow from the man to the centre, calling it centripetal force. And another opposite to it calling it centrifugal force, right?
I thought they'd cancel, but I don't think I've clearly finished that thought process, I made a quick reply...

And of course they wouldn't have any component tangential to the surface.

But a body already moving with a velocity tangential do not need a force to keep it moving along the tangent.
But that's along the tangent...

Oh... I think I'm confused. Let me think for a while...

Drakkith

The force holding the man to your sphere is the magnetic force between his legs and the sphere. This force is greater than the upward force trying to push him away. If it wasn't any small simply movement by the man would send him moving upwards and away.

Because of this, the man is constantly being pulled down towards the sphere while also moving...tangently??...through space around the sphere. The magnetic force on him is similar to the gravitational force on a satellite in orbit. The satellite is constantly falling towards the earth, but also moving, resulting in an orbit.

asdofindia

That makes me wonder whether or not to subtract mv²/r from GMm/r² when calculating the weight of a body

Edit:http://en.wikipedia.org/wiki/Centrif...tious%29#Earth
Appears like there are effects due to centrifugal force

Drakkith

There are most definately effects due to the centrifigal force. They are usually so small as to be irrevelent in day to day stuff. However, for space launches we do calculate that effect as well as the velocity of the earths rotation from the launch site in order to put something into orbit correctly.

asdofindia

Centripetal force, F_g=GMm/r²
Centrifugal force, F_r=mv²/r
So, when v is > v_max where v_max> [tex]\sqrt{GM/r}[/tex] we do fly away!! If it's [tex]\sqrt{2}[/tex] times the v_max in the above equation, it'd escape earth's gravity too (because that'd be 11.2 km/s)
If v/v_max is between 1 and [tex]\sqrt{2}[/tex], it'd fly away and fall down back.
(But luckily on earth, v is approximately 1/17 times v_max)
That's why we're not flying away from earth.

Now, let me do something I've never done before.

Since the centrifugal force on earth is cancelled out by the gravity (289 times stronger), we can safely assume the body to be at rest with respect to the reference frame of earth. (I don't know if that's right, because as I said, I've never done this before) (There'd be no longer any effects that'd be observed due to rotational motion of earth with respect to the space around it)
So, since earth and the body are both at rest (in that inertial frame) there wouldn't be the need of any frictional force.

We have to view this in the inertial frame of earth.

(I have understood my point)

Lsos

I'm not sure what you are asking, but when you are in orbit in space then we can confidently say that you are NOT rotating with the earth. You are moving much, much faster than the earth is spinning.

Now what happens when you re-enter the atmosphere? The atmosphere WILL slow you the fuk back down to it's own speed, and it will use extreme force to do so. The force is so great in fact, that whatever is re-entering must use some kind of heat shield, otherwise it will burn up with a brightness of the sun.

So you see what happens whenever something is not rotating with the earth. The earth WILL slow or speed it up to match its own rotation, and violently so if need be. If you don't believe me go jump out of a moving car, and you'll see first hand how it feels when something is NOT rotating with the earth.

JaredJames

asdofindia centripetal force is not gravity. Your equation above defines gravity not centripetal force.

Note, your equation is virtually constant wherever you are on the planet, which is gravity, but the centripetal force is almost zero at the poles and maximum at the equator.

asdofindia

So the centripetal force is......?