# Is it possible that the gravity that keeps our feet planted on the Earth is..

What do you not understand about trying to isolate a component?
I don't think you've addressed my question.

I don't think you've addressed my question.

You didn't ask a question. You paraphrased my question with a question mark after it and then answered it yourself.

What is this coincidence?

Where projectiles from the surface of the Earth would go if there were no gravity? Well, if we switched off universal gravity, the projectiles would just fly off into deep space, provided there's no atmospheric resistance.
What do you not understand about trying to isolate a component?
I don't think you've addressed my question.
You didn't ask a question. You paraphrased my question with a question mark after it and then answered it yourself.

Ok, I see it now.

Answering that question would require a really long time. The best thing I could do at this time of night is direct you to look at the way any point on the surface of the Earth moves relative to the way the whole earth moves. Then do the same for the moon. Then the other planets. You might notice the same pattern that I've noticed.

Right, but then there is curved space time to make it come back to the surface.

Exactly. Which is another way of saying "gravity."

one other thing of note is that if you throw something off the Earth the direction of its path around the Sun or Galaxy, it is accelerating in that direction while what you have thrown is not.

No, it isn't.

Exactly. Which is another way of saying "gravity."

No it isn't. Where does the acceleration come from?

No, it isn't.

Yes it is. If you had a grid on the plane of the Earth's orbit around the sun and plotted a point once a day and then shifted them all to 0 on the X axis, you would notice that the space between the dots on the Y axis increases as it gets closer to crossing past the Sun and then they decrease again. That's acceleration. Besides, you can't have circular motion without acceleration.

Not really where I was heading with that.

Think of if the Earth was rotating fast enough to sling you out into the air, but not fast enough for you to escape its gravity. What do you think would happen?

Assuming no atmosphere and a perfectly spherical earth, you would be in orbit right at the earth's surface. Not sure what point you are trying to make.

Assuming no atmosphere and a perfectly spherical earth, you would be in orbit right at the earth's surface. Not sure what point you are trying to make.

I didn't mean escape all of Earth's gravity. I meant that it didn't throw me hard enough that I would clear the Earth. The arc wouldn't be large enough for me to not collide with the Earth again.

Edit:

http://www.physicsclassroom.com/mmedia/vectors/sat.cfm - Here is a page explaining what I mean a little bit.

If the Earth rotated so fast that it slung your body into the air whenever it made contact with the surface and went around the curve, you wouldn't weigh any different, but you would impact the ground with more force than you were thrown with.

Now think about the centrifugal force of the Earth's rotation as it is in reality. Does it really make you lighter?

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No it isn't. Where does the acceleration come from?

Gravity.

Yes it is. If you had a grid on the plane of the Earth's orbit around the sun and plotted a point once a day and then shifted them all to 0 on the X axis, you would notice that the space between the dots on the Y axis increases as it gets closer to crossing past the Sun and then they decrease again. That's acceleration.

Correct! Caused by gravity. Take gravity away and everything goes in straight lines.

Besides, you can't have circular motion without acceleration.

Also correct. Caused by gravity in the case of orbits, or by mechanical means in the case of rotating structures.

If the Earth rotated so fast that it slung your body into the air whenever it made contact with the surface and went around the curve, you wouldn't weigh any different, but you would impact the ground with more force than you were thrown with.

No, your weight would be zero; otherwise you would not be "slung into the air." (or, in this case, "slung into the vacuum.") You would also never re-impact the ground (barring running into a mountain or something.) You would be in orbit.

Now think about the centrifugal force of the Earth's rotation as it is in reality. Does it really make you lighter?

Yes, it does, and it's measurable.

No, your weight would be zero; otherwise you would not be "slung into the air." (or, in this case, "slung into the vacuum.") You would also never re-impact the ground (barring running into a mountain or something.) You would be in orbit.

Can you explain why I would never re-impact the ground?

Edit: I suppose it would be because the Earth would have to be rotating fast enough to negate all of the force of gravity on my body, which would put me into an orbit around it. Which is basically what the atmosphere does, right?

But, like you said, I would re-impact the Earth again if I ran into a mountain or something.

Now, since the Earth doesn't rotate that fast, wouldn't you say that any amount that the Earth's rotation would push me outwards would cause more force on my body when gravity accelerates me back towards the surface? If not, can you explain why?

Yes, it does, and it's measurable.

How do they measure it?

I didn't mean escape all of Earth's gravity. I meant that it didn't throw me hard enough that I would clear the Earth. The arc wouldn't be large enough for me to not collide with the Earth again.

Edit:

http://www.physicsclassroom.com/mmedia/vectors/sat.cfm - Here is a page explaining what I mean a little bit.

If the Earth rotated so fast that it slung your body into the air whenever it made contact with the surface and went around the curve, you wouldn't weigh any different, but you would impact the ground with more force than you were thrown with.

Now think about the centrifugal force of the Earth's rotation as it is in reality. Does it really make you lighter?

I am assuming for arguments sake you start from a position on the equator. Once the earth and you on its surface have reached a critical speed, you become weightless at the earth's surface, and being weightless you would not have any friction between you and the earth. The earth could double its speed or come to a stop or reverse direction and it would not affect your orbit, which would continue at the surface.

I am assuming for arguments sake you start from a position on the equator. Once the earth and you on its surface have reached a critical speed, you become weightless at the earth's surface, and being weightless you would not have any friction between you and the earth. The earth could double its speed or come to a stop or reverse direction and it would not affect your orbit, which would continue at the surface.

That makes sense. Which seems to be pretty much what happens to the atmosphere. What about objects that don't fly off, but are accelerated towards the surface again?

Edit: I suppose it would be because the Earth would have to be rotating fast enough to negate all of the force of gravity on my body, which would put me into an orbit around it. Which is basically what the atmosphere does, right?

No, not right. Gravity holds the atmosphere. No gravity, no atmosphere. The weight of the atmosphere from gravity is 14.7 psi at sea level.

No, not right. Gravity holds the atmosphere. No gravity, no atmosphere. The weight of the atmosphere from gravity is 14.7 psi at sea level.

Right, but wouldn't it be closer to the Earth if it wasn't rotating?

Right, but wouldn't it be closer to the Earth if it wasn't rotating?
A little tiny fraction that would be nigh impossible to calculate. Someone may have attempted it already but I haven't seen it.

That makes sense. Which seems to be pretty much what happens to the atmosphere. What about objects that don't fly off, but are accelerated towards the surface again?

Not quite. The earth at its normal rotation rate is not spinning nearly fast enough to reduce the amount of force due to gravity on anything lying on its surface, including the atmosphere. The atmosphere is simply less dense than water so sits on top.

Nothing flies off. By increasing the rotation of the earth gradually you get to a point where the centrifugal force increases to equal that of gravity at which point you are in orbit at the earth's surface. If the earth continues spinning at this critical speed your position relative to the point on the equator from where you started simply remains below your feet. All objects at the equator that are not tied down will become weightless, whether massive boulders or feathers.

Can you explain why I would never re-impact the ground?

Because you would be in orbit. You would fall towards the Earth; the Earth would recede (since it is curved) at the same rate.

Edit: I suppose it would be because the Earth would have to be rotating fast enough to negate all of the force of gravity on my body, which would put me into an orbit around it. Which is basically what the atmosphere does, right?
No. The atmosphere does not orbit the Earth.

Now, since the Earth doesn't rotate that fast, wouldn't you say that any amount that the Earth's rotation would push me outwards would cause more force on my body when gravity accelerates me back towards the surface?
No, it would cause LESS force.

If not, can you explain why?
Because gravity accelerates you towards the center of the Earth; centrifugal force accelerates you AWAY from the axis of the Earth's rotation.

Not quite. The earth at its normal rotation rate is not spinning nearly fast enough to reduce the amount of force due to gravity on anything lying on its surface, including the atmosphere. The atmosphere is simply less dense than water so sits on top.

Nothing flies off. By increasing the rotation of the earth gradually you get to a point where the centrifugal force increases to equal that of gravity at which point you are in orbit at the earth's surface. If the earth continues spinning at this critical speed your position relative to the point on the equator from where you started simply remains below your feet. All objects at the equator that are not tied down will become weightless, whether massive boulders or feathers.

If the whole Earth were rotating that fast, would it not just grow in size and decrease in density? I mean, it wouldn't just be objects on its surface that would move outward from its center. Everything would. Lots of bad things would happen.

Because you would be in orbit. You would fall towards the Earth; the Earth would recede (since it is curved) at the same rate.

That makes sense, but only if my body was the only thing affected by the centrifugal force. Am I wrong in thinking that less dense objects and substances would be pushed, by the (apparent) centrifugal force, outward further than more dense objects and substances?

It's kind of difficult for me to determine this because it isn't the same as centrifugal force caused by an object tied to a string being swung around. Mainly because the further you get from the center of the Earth, the weaker the pull of gravity. Does Hooke's Law apply?

I think objects would only be pushed outward if there were something "beneath" them to push. I also think that, at some point, anything that was being compressed (like gasses and water) will eventually expand to whatever form it would have in a vacuum (what is this called?). Without the pressure of it trying to expand, anything around it would not move as much because it will not be pushed away by the expansion. Eventually, everything would be uncompressed. (side note: I'm pretty sure that this make things like magma cool down. For that matter, it might make water cool down, which would make it more dense again. Interesting.)

Then you've got the centrifugal force of the Earth revolving around the Sun, Galaxy etc. That would push things away from the center of that circular motion. (Another side note: Since centrifugal force is fictitious, how does it travel through an object? I assume that in the case of the Earth's revolution and a body on it's surface, the force would be applied to an object by whatever mass was on the opposite side of it, relative to the center of the rotation.)

Since force travels at the speed of sound through a medium, It would take longer than one full rotation of the Earth to propagate through the atmosphere to the center of the planet. Since no point on the surface of the Earth is directly between the center of the planet and the center of the Sun all of the time, I think this force would appear to come in waves.

I could go on and on with these ideas, but I think this is enough to get you pondering these points. Since my knowledge of physics is severely limited compared to yours, would you care to take into account the things I mentioned and answer my questions again?

An important question: If an object moving in an orbit is as though it is moving inertially in a straight line, why would there be any centrifugal force?

No. The atmosphere does not orbit the Earth.

I'll buy that, but I'm also wondering what speed an air molecule would have to be moving in order to orbit the Earth at a distance of, say, 40km from the surface of the Earth with no other atmosphere.

No, it would cause LESS force.

Because gravity accelerates you towards the center of the Earth; centrifugal force accelerates you AWAY from the axis of the Earth's rotation.

I understand this, but the surface of the Earth can't accelerate you away any more than it can accelerate the ground beneath you away. If it is accelerating the ground beneath you away with greater force, then why would there not be more force between your body and the surface as the centrifugal force propagates through you?