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

In summary, the conversation revolved around the role of friction and inertia in various scenarios, such as the rotation of the Earth and Moon, the movement of objects in a rotating vehicle, and the difference in flight times between east and west. It was determined that friction and inertia are responsible for these phenomena, and that the jet stream plays a significant role in the difference in flight times. The role of coriolis force and the thinning atmosphere at higher altitudes were also discussed.
  • #71
Doc Al said:
I'll accept that for the moment.

Again, a person who is 'at rest' is meaningless. Do you mean a person on Earth who is at rest with respect to earth? If so, then the relative speed is 5 mph, of course.

The force needed to change one's velocity depends on (1) how big a change you want and (2) how fast you want to change.

i mean a person who is in space. if everything was not moving(not at respect to each other but in actual) then he would be at rest with respect to them. now do u get it

finally i am on Earth and moving with the Earth constant speed of 1000 miles. if i move in opposite direction at a speed of x miles per hour and i reach the speed in 1 second then why don't i need a force of which can give me a speed of 1000+x miles in opposite diretion in 1 sec. otherwise i have two different velocities at same time 1000 miles in Earth direction an five miles in opposite direction which is not possible
 
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  • #72
D H said:
Stop using text-speech, singh94. The correct wording is you rather than u, I rather than i, what rather than wat, are rather than r.

Gee i didn kno dat :)
 
  • #73
singh94 said:
finally i am on Earth and moving with the Earth constant speed of 1000 miles. if i move in opposite direction at a speed of x miles per hour and i reach the speed in 1 second then why don't i need a force of which can give me a speed of 1000+x miles in opposite diretion in 1 sec. otherwise i have two different velocities at same time 1000 miles in Earth direction an five miles in opposite direction which is not possible

No, you don't have two velocities.

You are either traveling at 1005mph east or 995mph east. Put simply, you are always traveling east unless you are going faster than 1000mph in a westerly direction.
 
  • #74
singh94 said:
i mean a person who is in space. if everything was not moving(not at respect to each other but in actual) then he would be at rest with respect to them. now do u get it
Just being 'in space' does not determine an object's velocity. It can have any allowable speed.
finally i am on Earth and moving with the Earth constant speed of 1000 miles.
OK.
if i move in opposite direction at a speed of x miles per hour and i reach the speed in 1 second then why don't i need a force of which can give me a speed of 1000+x miles in opposite diretion in 1 sec. otherwise i have two different velocities at same time 1000 miles in Earth direction an five miles in opposite direction which is not possible
Once again, a force is not associated with a velocity, but with a change in velocity. And that's only x mph.

As far as your velocity is concerned, that depends on what you are measuring it with respect to. With respect to the Earth's surface, it would be x mph. With respect to the Earth's center, it would be 1000 + x mph. With respect to the Sun, something else entirely. And so on.
 
  • #75
jarednjames said:
No, you don't have two velocities.

You are either traveling at 1005mph east or 995mph east. Put simply, you are always traveling east unless you are going faster than 1000mph in a westerly direction.
yes u r right.
 
  • #76
singh94 said:
Gee i didn kno dat :)

I didn't know that
singh94 said:
yes u r right.

you are

Really? Asking for trouble after being warned.
 
  • #77
Doc Al said:
Just being 'in space' does not determine an object's velocity. It can have any allowable speed.

OK.

Once again, a force is not associated with a velocity, but with a change in velocity. And that's only x mph.

As far as your velocity is concerned, that depends on what you are measuring it with respect to. With respect to the Earth's surface, it would be x mph. With respect to the Earth's center, it would be 1000 + x mph. With respect to the Sun, something else entirely. And so on.

change in velocity is being 1000miles in east to 5 miles in west. also with Earth's center my v would be zero as the displacement is zero.
 
  • #78
jarednjames said:
I didn't know that


you are

Really? Asking for trouble after being warned.

i don't kno wether it is u jarednjames or the original person but know that u can't force me to stop writing in short forms just cause u don't like it. either u bear with me, or u ignore me but stop trying to bully me OK?
 
  • #79
singh94 said:
change in velocity is being 1000miles in east to 5 miles in west.
Huh? Are you moving 5 mph with respect to the surface or 1005 mph? Kind of makes a difference.

also with Earth's center my v would be zero as the displacement is zero.
Huh? The surface of the Earth is moving with respect to the center at 1000 mph. (Roughly speaking, at the equator.)
 
  • #80
singh94 said:
i don't kno wether it is u jarednjames or the original person but know that u can't force me to stop writing in short forms just cause u don't like it. either u bear with me, or u ignore me but stop trying to bully me OK?

Forum rules say you can't, you agreed to them on signing up.
 
  • #81
singh94 said:
i don't kno wether it is u jarednjames or the original person but know that u can't force me to stop writing in short forms just cause u don't like it. either u bear with me, or u ignore me but stop trying to bully me OK?
Think twice about continuing to violate our rules.
 
  • #82
jarednjames said:
Forum rules say you can't, you agreed to them on signing up.

allright then if that is the case ill stop imediately but i may make some slips here or there.
 
  • #83
Doc Al said:
Think twice about continuing to violate our rules.

i APOLOGISE TO DOC AL JADERNJAMES AND ALL WHO TOLD ME TO STOP WRITING IN SHORT FORMS.
SORRY
 
  • #84
Doc Al said:
Think twice about continuing to violate our rules.
the Earth is rotating the center may not move from its position but it is also rotating if u take the Earth and shrink it to the size of the center of the Earth then it would still rotate right? so the position of a point on Earth's surface to the center remains same.
 
  • #85
singh94 said:
the Earth is rotating the center may not move from its position but it is also rotating if u take the Earth and shrink it to the size of the center of the Earth then it would still rotate right? so the position of a point on Earth's surface to the center remains same.
No. Imagine a wheel spinning on its axle. The axle is fixed--its speed is zero (with respect to some frame). But the rim of the wheel is moving with respect to that frame. Same with the earth.

(Don't confuse rotational speed with translational speed.)
 
  • #86
singh94 said:
u can't force me to stop writing in short forms

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.
 
  • #87
singh94 said:
the Earth is rotating the center may not move from its position but it is also rotating if u take the Earth and shrink it to the size of the center of the Earth then it would still rotate right? so the position of a point on Earth's surface to the center remains same.

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 traveled and speed is many times greater.
 
  • #88
singh94 said:
i thought u people would be and mature enough to understand that I am just saying that do i need to exert a force which gives me a speed of 1005 miles so that i can run against the direction of the earth.also i know what r the units of force velocity etc. which concern physics thank u very much.

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.
 
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  • #89
cepheid said:
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.
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.
 
  • #90
D H said:
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.

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?
 
  • #91
cepheid said:
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.

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.
 
  • #92
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.
 
  • #93
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.
 
  • #94
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!

What do you mean by this?
 
  • #95
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...
 
  • #96
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.
 
  • #98
There are most definitely 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 Earth's rotation from the launch site in order to put something into orbit correctly.
 
  • #99
Centripetal force, Fg=GMm/r2
Centrifugal force, Fr=mv2/r
So, when v is > vmax where vmax> [tex]\sqrt{GM/r}[/tex] we do fly away! If it's [tex]\sqrt{2}[/tex] times the vmax in the above equation, it'd escape Earth's gravity too (because that'd be 11.2 km/s)
If v/vmax 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 vmax)
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 canceled 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)
 
  • #100
singh94 said:
i mean a person who is in space. if everything was not moving(not at respect to each other but in actual) then he would be at rest with respect to them. now do u get it

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.
 
  • #101
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.
 
  • #102
jarednjames said:
asdofindia centripetal force is not gravity. Your equation above defines gravity not centripetal force.

At the poles there is virtually no centrifugal forces compared to at the equator.
So the centripetal force is...?
 
  • #103
asdofindia said:
So the centripetal force is...?

Centripetal force occurs due to a body moving on a curved path.

Gravity occurs simply because a body has mass.

We have both on earth, one is down to the mass, the other the rotation.

Gravity, aside from the ellipsoid shape of the Earth is relatively constant all over the planet.

Centripetal force is at its maximum at the equator and tends to zero towards the poles.
 
  • #104
jarednjames said:
Centripetal force is at its maximum at the equator and tends to zero towards the poles.
yeah, i was talking at the equator.
 
  • #105
asdofindia said:
yeah, i was talking at the equator.

Doesn't matter.

You are describing gravity as centripetal force which is incorrect.
 

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