Earth Spin

[Mintaka]

One thing I find very strange is the fact the earth is spinning but I can't feel it. I know the analogy of the airplane or limousine, and how we don't feel movement when we are insulated from the effects of our movement. I see the effects of our spinning when I watch the sunrise and sunset, the moon and stars slowly shifting their positions during the night, but this happens so slowly it becomes almost an abstract awareness, not something I feel physically, which is what I find strange. The sky is very dark where I live, opposite the Moroccan coast near Gibraltar, having recently moved here after 6 years in Moscow, a city which moves so fast you don't notice anything except the feet that are tripping you up in front. The other night I was sitting in my chair looking out at the sky through the frame of the window, and I had a strange sensation I have never had before, of actually sensing the movement of the planet, by unfixing my gaze and looking at the sky almost without focussing on anything. I had a very strange sensation of gently rolling or 'drifting', as if on an ocean. No, I wasn't drunk and don't do drugs.

 

I'd be interested to hear if anyone else has ever experienced this when looking at the night sky. Does no one else find it strange that we don't physically feel any effects as hurtle around the sun? And why doesn't the atmosphere strip away from the earth as we do? Is the air also held by gravity?

 

I know we don't sense our spinning because the atmosphere spins with us and we can only sense it in relation to distant objects (ie very slowly), but if, say, there was a small moon situated very close to the edge of our atmosphere, but not in it, just outside, would we sense the earth spinning past it quickly, because we are close to it?

[Tormod]

I know we don't sense our spinning because the atmosphere spins with us and we can only sense it in relation to distant objects (ie very slowly), but if, say, there was a small moon situated very close to the edge of our atmosphere, but not in it, just outside, would we sense the earth spinning past it quickly, because we are close to it?

 

It would probably be just like it is with man-made satellites - you would see it pass, rather than sensing that you are passing it.

[Mintaka]

It would probably be just like it is with man-made satellites - you would see it pass, rather than sensing that you are passing it.

 

But I thought man-made satellites flew through the upper atmosphere at about 10km/sec? That they were still in our atmosphere?

I meant a moon that didn't move ;-)

[sanctus]

There are both kinds of man-made satellites. Geo-stationnary ones (always above the same spot of earth or still in the atmosphere as you put it) (usually telecomunication, I think) and the non-geostationnary ones (like these)

[Tormod]

Mintaka, geostationary satellites orbit at 36000 km above the ground, which is far beyond the atmosphere. Those do not move as seen from our perspective on the ground, but they do travel very fast. You probably would not feel as if you were moving if you were sitting on one of those, either, since you would be hovering above the same spot of ground all the time.

[sanctus]

I always thought that there was no clear definition as to where the atmosphere ends, that the higher you go the more it rarefies...so is there such a definition?

[Qfwfq]

But I thought man-made satellites flew through the upper atmosphere at about 10km/sec? That they were still in our atmosphere?

If they were in the atmosphere, they would need propulsion else they wouldn't remain up very long at all.

 

I meant a moon that didn't move

We'd see it drifting from east to west, though it would take less than sun, moon or stars to get from horizon to horizon. At an altitude of 300 km over the equator, it would seem to do it in a bit less than 2 h 18 min.

 

If it is something that fills your whole sky, it might induce vection: the sense of being in motion due to contrary motion of a large portion of surrroundings, like when a train of bus just outside the window starts to move. If it's just a puny thing up in the sky it doesn't have much of an effect. We tend to go by the dominant part of our surroundings.

[Mintaka]

Thanks, interesting answers, I like the thing about Vectral motion.

 

But if the earth is hurtling around the sun at such high speed, why doesn't the earth's atmosphere get stripped away like a tablecloth flying off a plate in the wind? Is it because outer space is a vacuum and therefore things fall through it without any 'resistance' or friction? Does the speed of things which get ejected into outer space depend on their initial propulsion speed or momentum, or is their a constant velocity for objects in a vacuum? Is it possible that something could simply be 'stuck' there motionless forever?

[modest]

I know we don't sense our spinning because the atmosphere spins with us and we can only sense it in relation to distant objects (ie very slowly), but if, say, there was a small moon situated very close to the edge of our atmosphere, but not in it, just outside, would we sense the earth spinning past it quickly, because we are close to it?

 

No, distance is not a factor.

 

If you were passing the earth in a spaceship going hundreds of thousands of kilometers per hour (relative to the earth), you still wouldn't 'feel' the motion.

 

You can't feel speed.

 

Going slow relative to something feels the same as going really, really fast relative to it—no matter how close you are to it and no matter how big it is. What people 'feel' is change in speed, or acceleration. Like... if you are in a car going 40 kph with your eyes closed you don't feel the speed, but when the car changes speed and accelerates to 80 kph even with your eyes closed you can tell that you're accelerating. If you then stay at a constant 80 kph it feels the same as when you were going a constant 40 kph and the same as zero kph (assuming the road is level and smooth).

 

And why doesn't the atmosphere strip away from the earth as we do? Is the air also held by gravity?

 

Yes, the atmosphere is held to the earth just like people because of gravity.

 

One thing I find very strange is the fact the earth is spinning but I can't feel it.

 

The earth is rotating around its axis, which means we are spinning around or rotating (so long as we are not on the pole). Rotation is "non-inertial" which means it can be felt. The 'force' that you can feel when you are rotating is called the centrifugal force. The force is outward, away from the center of rotation.

 

You might imagine a small weight tied to the end of a string. If you spin the string around—over your head let's say—then the weight is forced away from the center causing the string to be stretched tight. The weight feels as if it is being forced away from the center of rotation.

 

We, on the earth, are in the same situation. We are being forced away from the earth's axis (its center of rotation). The question is, do we feel it? Let's take the example of a person on earth's equator. Gravity forces the person toward the center of the earth and the centrifugal force forces them away from the center of the earth. The force of each can be calculated. Really quickly...

 

Force of gravity = G•M₁•M₂/r² = 587.3 Newtons

Centrifugal force = M₂•v²/r = 2.02 Newtons

 

A person on the equator that is 60 kilograms is 'forced' toward earth's center with 587.3 Newtons (a Newton is a measure of force) and is 'forced' away from earth's center with 2.02 Newtons. These two forces are added (or, subtracted, in this specific case) so that all the person feels is a force in one direction. The strength of the force is (587.3 - 2.02 =) 585.28 Newtons towards the earth. If the earth were not spinning then the person would feel 587.3 Newtons towards the earth.

 

So, the spinning is, in a sense, felt.

 

With the earth spinning, a person on the equator might have a weight scale they are standing on read 60 kilograms. The same scale, calibrated the same way, would read 60.204 kilograms if the earth were not rotating. The person feels 0.34% lighter because the equator is spinning 464 meters per second around earth's center.

 

If the earth were spinning even faster then the person would feel even lighter, and if the earth were spinning any more than about 17 times its current speed then the centrifugal force would overcome gravity and things would fly off the equator away from earth's axis and towards outer space.

 

But if the earth is hurtling around the sun at such high speed, why doesn't the earth's atmosphere get stripped away like a tablecloth flying off a plate in the wind? Is it because outer space is a vacuum and therefore things fall through it without any 'resistance' or friction?

 

That is exactly correct. Things moving around on earth's surface are subject to air resistance. They are moving relative to the air which they are moving through. As the earth moves through space around the sun there is no significant solar atmosphere—so earth's atmosphere doesn't feel the motion in a way that a person might intuitively think.

 

Does the speed of things which get ejected into outer space depend on their initial propulsion speed or momentum, or is their a constant velocity for objects in a vacuum? Is it possible that something could simply be 'stuck' there motionless forever?

 

The speed of something orbiting the sun depends on its distance from the sun. Closer objects (like Mercury) orbit faster and distant objects (like Pluto) orbit slower. If an object in space had no orbital velocity then it would fall toward the sun and into the sun. Every orbit is a balance between the centrifugal force which pushes things away from the center because of velocity and the gravitational force which pulls things toward the center. If something is in a stable, circular orbit then the centrifugal force is equal to the gravitational force and the object in orbit stays a constant distance away from the object it is orbiting.

 

~modest

[Qfwfq]

But if the earth is hurtling around the sun at such high speed, why doesn't the earth's atmosphere get stripped away like a tablecloth flying off a plate in the wind? Is it because outer space is a vacuum and therefore things fall through it without any 'resistance' or friction?

Of course, there is no such a wind. Not, anyway, one due to the motion of Earth; there is a flux of particles from the Sun called the solar wind and it isn't able to strip away the atmosphere.

 

Does the speed of things which get ejected into outer space depend on their initial propulsion speed or momentum, or is their a constant velocity for objects in a vacuum?

In lack of friction or other forces, velocity remains constant.

 

Is it possible that something could simply be 'stuck' there motionless forever?

Motionless or constant motion, it's the same thing. It just depends on what you refer position to. When you're not looking out the train window and the tracks are giving a perfectly smooth ride, you'd say you're just sitting there. Pull the blind up and you see the hedges whizzing past you... no, you know damn well it's the other way around. I mean, according to common sense, because that's the only reason you say it isn't the whole countryside whizzing along. And yet, it is whizzing along west to east. But it's all whizzing even more along the direction of terrestrial orbit (25 km per second). Even more, the whole thing is whizzing through the galaxy. But the galazy is also whizzing along through the other ones...

[Mintaka]

No, distance is not a factor.

 

If you were passing the earth in a spaceship going hundreds of thousands of kilometers per hour (relative to the earth), you still wouldn't 'feel' the motion.

 

You can't feel speed.

 

Going slow relative to something feels the same as going really, really fast relative to it—no matter how close you are to it and no matter how big it is. What people 'feel' is change in speed, or acceleration. Like... if you are in a car going 40 kph with your eyes closed you don't feel the speed, but when the car changes speed and accelerates to 80 kph even with your eyes closed you can tell that you're accelerating. If you then stay at a constant 80 kph it feels the same as when you were going a constant 40 kph and the same as zero kph (assuming the road is level and smooth).

 

Is it really true to say I can't 'feel speed'? When I fly around a rollercoaster, I feel the wind against my face, pushing my cheeks out. Would a spaceman, disconnected from his safety line, and falling into the vacuum of space, would he not 'feel' any sensation of moving at all? This is fascinating and horrific, to be moving through space forver but not even to be aware that you are moving.

 

 

Yes, the atmosphere is held to the earth just like people because of gravity.

 

 

 

The earth is rotating around its axis, which means we are spinning around or rotating (so long as we are not on the pole). Rotation is "non-inertial" which means it can be felt. The 'force' that you can feel when you are rotating is called the centrifugal force. The force is outward, away from the center of rotation.

 

You might imagine a small weight tied to the end of a string. If you spin the string around—over your head let's say—then the weight is forced away from the center causing the string to be stretched tight. The weight feels as if it is being forced away from the center of rotation.

 

We, on the earth, are in the same situation. We are being forced away from the earth's axis (its center of rotation). The question is, do we feel it? Let's take the example of a person on earth's equator. Gravity forces the person toward the center of the earth and the centrifugal force forces them away from the center of the earth. The force of each can be calculated. Really quickly...

 

Force of gravity = G•M₁•M₂/r² = 587.3 Newtons

Centrifugal force = M₂•v²/r = 2.02 Newtons

 

A person on the equator that is 60 kilograms is 'forced' toward earth's center with 587.3 Newtons (a Newton is a measure of force) and is 'forced' away from earth's center with 2.02 Newtons. These two forces are added (or, subtracted, in this specific case) so that all the person feels is a force in one direction. The strength of the force is (587.3 - 2.02 =) 585.28 Newtons towards the earth. If the earth were not spinning then the person would feel 587.3 Newtons towards the earth.

 

So, the spinning is, in a sense, felt.

I understand what you say about the spinning having an effect on weight. But my looking up at the night sky through my window and'sensing movement' of the planet, is this possible? I did feel it. A sensation of moving. Has anyone ever heard about this?

 

 

 

That is exactly correct. Things moving around on earth's surface are subject to air resistance. They are moving relative to the air which they are moving through. As the earth moves through space around the sun there is no significant solar atmosphere—so earth's atmosphere doesn't feel the motion in a way that a person might intuitively think.

 

 

This is again mind-blowing. Our planet moving through the solar system but our awareness and sensations limited only to our tiny experience of it underneath the atmosphere. I find it hard to imagine the idea of air as a physical thing with physical properties, and resistance, whereas it is much easier with, say, water. We tend to think of air as 'nothing'. We instinctively take air for granted, knowing it is essential for life. The idea that just beyond this tiny layer above us, there is a vacuum, in which we cannot even breathe, is a chilling thought. The idea that most of the universe is completely hostile to what we are and that we exist like a tiny colony of amoeba in a miniscule pool of air.

 

 

The speed of something orbiting the sun depends on its distance from the sun. Closer objects (like Mercury) orbit faster and distant objects (like Pluto) orbit slower. If an object in space had no orbital velocity then it would fall toward the sun and into the sun. Every orbit is a balance between the centrifugal force which pushes things away from the center because of velocity and the gravitational force which pulls things toward the center. If something is in a stable, circular orbit then the centrifugal force is equal to the gravitational force and the object in orbit stays a constant distance away from the object it is orbiting.

 

Is it possible that our orbit in relation to the sun could change very slightly, spelling the end of us? Could the sun suddenly exert more gravitational pull , meaning we get sucked in, or less, meaning we fall out of it's orbit and into space?

 

 

~modest

[Mintaka]

 

 

Im sorry, I haven't worked out how to reply with green box quotes like you do.

[Mintaka]

Of course, there is no such a wind. Not, anyway, one due to the motion of Earth; there is a flux of particles from the Sun called the solar wind and it isn't able to strip away the atmosphere.

 

In lack of friction or other forces, velocity remains constant.

 

Motionless or constant motion, it's the same thing. It just depends on what you refer position to. When you're not looking out the train window and the tracks are giving a perfectly smooth ride, you'd say you're just sitting there. Pull the blind up and you see the hedges whizzing past you... no, you know damn well it's the other way around. I mean, according to common sense, because that's the only reason you say it isn't the whole countryside whizzing along. And yet, it is whizzing along west to east. But it's all whizzing even more along the direction of terrestrial orbit (25 km per second). Even more, the whole thing is whizzing through the galaxy. But the galazy is also whizzing along through the other ones...

 

Thank you. I think we are probably lucky we don't sense all the things that are happening around us. Even thinking about the distances involved is enough to drive one insane. To think that the first Voyager left earth in the 1970's, has been travelling at a speed of about 40,000 mph , and STILL hasn't even reached the edge of our own Solar System, never mind got anywhere close to the nearest star, is incredible. Since i started thinking of these quesitons recently, I have changed my philosophy on life. I no longer kill the fly that lands on my table. Because I am no larger or more significant than him in the scale of things. I think this maye be one of the most valuable results of asrtronomy and exploration of the universe. It might teach us to be more aware of how insignificant we are, and to be more humble in the way we live and treat the planet. I think all polictcians should be forced to go up to a space station and look down at the earth for a while. They might find a new respect for it if they sense that they aren't the centre of it.

[Qfwfq]

I think this maye be one of the most valuable results of asrtronomy and exploration of the universe. It might teach us to be more aware of how insignificant we are, and to be more humble in the way we live and treat the planet. I think all polictcians should be forced to go up to a space station and look down at the earth for a while. They might find a new respect for it if they sense that they aren't the centre of it.

Sure! :)

 

http://video.google.com/videoplay?docid=2588435272514121795#

[Mintaka]

Sure! :)

 

http://video.google.com/videoplay?docid=2588435272514121795#

 

 

 

Thank you so much for this my friend. I have never seen it before, and it touched me very deeply.

[modest]

Is it really true to say I can't 'feel speed'?

 

Absolutely, yup.

 

The scientific principle would be called Galilean invariance, which wiki puts concisely:

 

Galileo Galilei first described this principle in 1632 in his Dialogue Concerning the Two Chief World Systems using the example of a ship travelling at constant velocity, without rocking, on a smooth sea; any observer doing experiments below the deck would not be able to tell whether the ship was moving or stationary.

 

When I fly around a rollercoaster, I feel the wind against my face, pushing my cheeks out.

 

Right, if something hits you like air blowing on your face, then no doubt you can feel it. You might think of it this way... wind blowing in your face while you stand still on the ground feels the same as traveling at a constant speed and hitting the wind.

 

The other thing you feel on a roller coaster is acceleration when you change velocity (a change of direction or speed). You can definitely feel acceleration, and roller coasters are designed with just that in mind.

 

Would a spaceman, disconnected from his safety line, and falling into the vacuum of space, would he not 'feel' any sensation of moving at all?

 

Correct, he would not. Even if he were going nearly the speed of light, so long as his speed is constant and he doesn't change direction, he would feel weightless with no sensation of movement at all as long as nothing is bumping into him.

 

This is again mind-blowing. Our planet moving through the solar system but our awareness and sensations limited only to our tiny experience of it underneath the atmosphere. I find it hard to imagine the idea of air as a physical thing with physical properties, and resistance, whereas it is much easier with, say, water. We tend to think of air as 'nothing'. We instinctively take air for granted, knowing it is essential for life.[/size]

 

Yeah, definitely.

 

I think people intuitively think that an object's natural state is rest. If you throw something or drop something it seems to want to come to a state of rest. But, it isn't a natural tendency for things to do that so much as it is a result of friction and air resistance. Newton's first law of motion, I think you would find interesting, is paraphrased "an object at rest will want to stay at rest and an object in motion will want to stay in motion".

 

Our environment on earth fools us into thinking that things naturally want to slow down. But, it is only air and ground friction and resistance that makes this happen. Satellites in orbit, for example, have no air or ground resistance so they just keep going.

 

here's a cool link:

 

http://www.physicsclassroom.com/class/newtlaws/u2l1a.cfm

 

Very cool stuff

 

~modest

[Mintaka]

Absolutely, yup.

 

The scientific principle would be called Galilean invariance, which wiki puts concisely:

 

 

 

 

 

Right, if something hits you like air blowing on your face, then no doubt you can feel it. You might think of it this way... wind blowing in your face while you stand still on the ground feels the same as traveling at a constant speed and hitting the wind.

 

The other thing you feel on a roller coaster is acceleration when you change velocity (a change of direction or speed). You can definitely feel acceleration, and roller coasters are designed with just that in mind.

 

 

 

Correct, he would not. Even if he were going nearly the speed of light, so long as his speed is constant and he doesn't change direction, he would feel weightless with no sensation of movement at all as long as nothing is bumping into him.

 

 

 

Yeah, definitely.

 

I think people intuitively think that an object's natural state is rest. If you throw something or drop something it seems to want to come to a state of rest. But, it isn't a natural tendency for things to do that so much as it is a result of friction and air resistance. Newton's first law of motion, I think you would find interesting, is paraphrased "an object at rest will want to stay at rest and an object in motion will want to stay in motion".

 

Our environment on earth fools us into thinking that things naturally want to slow down. But, it is only air and ground friction and resistance that makes this happen. Satellites in orbit, for example, have no air or ground resistance so they just keep going.

 

here's a cool link:

 

http://www.physicsclassroom.com/class/newtlaws/u2l1a.cfm

 

Very cool stuff

 

~modest

 

 

 

 

really interesting link , thanks, I have bookmarked it.

 

I see what you say about speed. We only feel it in relation to our surrounding environment, objects our eyes can see moving past us quickly, sudden changes in direction and through acceleration or deceleration.

 

One of the most sickening experiences I have had was on a fairground centrifugal machine which 'spun everything outwards'. I have never been so green or felt so ill. I wonder why as a child I had no problems with swings or roundabouts, or fairground rides, but as an adult I cannot stand any sudden changes in my body state or motion. I get sick very easily now on all these things which only gave me excitment as a child. Perhaps this has to do with poor cirulation or maybe children sense the world in a different way which adults lose.