Curvature of Space-time and gravity.

[Coggeh]

So according to Einsteins theory of relativity gravity is caused by large objects such as the sun bending space and time and any object in that bend feels the pull of that gravity.

If thats the case, then when your in a space ship within this bending, then why don't you still feel the pull of gravity?
Im confused...

[leftace53]

If thats the case, then when your in a space ship within this bending, then why don't you still feel the pull of gravity?
Im confused...

What do you mean don't still feel the pull of gravity? Are you talking about the weightless-ness inside a space craft (like with all the peeps floating about)?

The thing to keep in mind is that gravity is the weakest of the forces (the big thing in particle physics right now is the process of trying to figure out what gives things mass, and why it is so weak). So the astronauts inside the space ship do have gravity acting upon them, but a space ship doesn't have much mass (compared to anything in the solar system really), so while it does bend spacetime, it doesn't do it to the extent of anything with a comprehendable gravitational field.

Its sort of the same concept as to why people don't just stick to each other, because we don't weigh enough to effect spacetime to the extent that we would be able to feel the gravitational forces around people. (I'll reference to that one family guy episode where peter became so fat that things orbited around him)

[Coggeh]

What do you mean don't still feel the pull of gravity? Are you talking about the weightless-ness inside a space craft (like with all the peeps floating about)?

The thing to keep in mind is that gravity is the weakest of the forces (the big thing in particle physics right now is the process of trying to figure out what gives things mass, and why it is so weak). So the astronauts inside the space ship do have gravity acting upon them, but a space ship doesn't have much mass (compared to anything in the solar system really), so while it does bend spacetime, it doesn't do it to the extent of anything with a comprehendable gravitational field.

Its sort of the same concept as to why people don't just stick to each other, because we don't weigh enough to effect spacetime to the extent that we would be able to feel the gravitational forces around people. (I'll reference to that one family guy episode where peter became so fat that things orbited around him)
lol.
Thanks, fair explains it, just occured to me and it confused me .

[Q]

Also, spacecraft like the space shuttle or the international space station are in a constant fall towards the Earth (which is an orbit around Earth), which creates the effect of weightlessness.

[mikelepore]

Sorry, but leftace53's answer is wrong, and Q's answer is right.

Every object in orbit is in free fall. It has the right amount of forward velocity so that the distance that it falls toward the earth is equal to the distance that the surface of the earth curves away from it, therefore it stays at the same altitude above the surface even though it's always falling.

The terms "weightless" and "no gravity" are incorrect, but NASA and the media allowed those terms to become customary because they are quicker to say than the correct term, "apparent weightlessness." With the spaceship, and the passengers and objects inside the spaceship, all falling at the same rate, the passengers and objects will appear to be floating around relative to the walls that surround them.

That much is true using either Newton's or Einstein's model, so Einstein's expanation of gravity in terms of spacetime curvature isn't needed to answer the question of why orbiting spaceships produce apparent weightlessness.

Also, to ask "why don't you feel the pull of gravity" isn't quite right either. People never feel gravity. If you're in the process of falling off a cliff, what you feel is the sensation of being weightless. However, if you're standing on the earth, and you're tired of standing up, you're feet hurt, etc., what you are feeling is the reaction force of the earth pushing upward on the bottom of your feet. When you're lying down on a mattress, you can't feel gravity pulling you down to the mattress, but you feel the reaction force of the mattress pushing upward on you.

[leftace53]

My bad, misunderstood the question. *facepalm*

[x371322]

To address the object in orbit, try to imagine taking a ball on a string and spinning it round and round. The ball is the object in orbit, and your hand is the earth. Now the ball's own speed naturally pulls it away from your hand. This can be demonstrated by letting go of the string, and watching the ball go flying across the room (just as a satellite would do if the earth suddenly lost it's gravity). On the other side of the equation, the ball is being pulled toward your hand (the free fall of the satellite) as you spin the string. These two forces balance each other, to keep the ball spinning 'round your hand.

If you could sit inside that ball, you would feel weightless. And as already pointed out above, this is not because there's no gravity around, but because you're in a constant free fall toward the hand (earth). It's like how the Zero G plane rides work. They fly up, and up, and up, and then they fall. Since you're falling with the plane, and inside the plane (so you don't feel the air resistance), you feel like you're floating (something I'd love to do someday by the way).

Here's a good link explaining this better than I can.
http://science.nasa.gov/science-news/science-at-nasa/fluid/zero-gplane/

[RED DAVE]

Or, as we used to say during the 60s:

GRAVITY SUCKS!

RED DAVE

[mikelepore]

These two forces balance each other, to keep the ball spinning 'round your hand.

You're definitely on the right track with the whole ball-on-the-string analogy there except for the part about forces in balance. The tension in the string is the one and only force acting on the ball, just as gravity is the one and only force acting on a satellite. As you already pointed out, the inertia tends to make the mass fly off on a tangent line, and it would fly off along that tangent if the force were to suddenly disappear. That tangent line is the direction of the instantaneous velocity. The force (which is unbalanced, not balanced) continuously changes the direction of the velocity vector, but it doesn't change the magnitude of the velocity vector. Therefore this behavior is a continuous acceleration (change in the velocity) even though the speed is constant.

Some other famous examples of this centripetal acceleration are:

* the role of friction when a car drives around a curve on an unbanked road.

* the role of the normal force exerted by the road when a car drives around a curve on a banked road.

* the horizontal component of the lift force on the wing when an airplane makes a banked circular turn.

* the circular or spiral path that a charged particle follows after it enters a regions where there is a magnetic field.

* the electrostatic force exerted on orbital electrons by the atomic nucleus, particularly in the simplified Bohr model of the atom.

The same thing is happening in any circular motion, say, a rotating wheel, but the centripetal force is harder to see in those cases. In the case of a wheel, instead of a single string that makes the source and the direction of the force obvious to see, we have a whole array of atomic or molecular bonds, with million of zigzag forces adding up to one effective force.

Or, as we used to say during the 60s:

GRAVITY SUCKS!



The version I'm used to seeing on the dormitory bathroom walls is:
There is no gravity. The world just sucks.

[Coggeh]

Sorry, but leftace53's answer is wrong, and Q's answer is right.

Every object in orbit is in free fall. It has the right amount of forward velocity so that the distance that it falls toward the earth is equal to the distance that the surface of the earth curves away from it, therefore it stays at the same altitude above the surface even though it's always falling.

The terms "weightless" and "no gravity" are incorrect, but NASA and the media allowed those terms to become customary because they are quicker to say than the correct term, "apparent weightlessness." With the spaceship, and the passengers and objects inside the spaceship, all falling at the same rate, the passengers and objects will appear to be floating around relative to the walls that surround them.
I'm confused, how are rockets and stuff in constant free fall? are planets in constant free fall? i can't wrap my head around it :(

And you do feel the pull of gravity, if you didn't when you jumped you wouldn't come back down. I certainly would link the sensation of falling from a high building not with weightlessness but gravity is a prick and is pulling me to the floor :(i want to fly god damn it !

[mikelepore]

I'm confused, how are rockets and stuff in constant free fall? are planets in constant free fall? i can't wrap my head around it :(

Free fall means that the only force acting on a projectile is gravity. A rocket is not in free fall while it is burning fuel. But if the rocket gets into an orbit around the earth, and the engine is turned off, then it is in free fall. Yes, planets and moons are also in free fall.

Newton was the first to describe how an orbit works. He imagined a cannon that can shoot a cannon ball with a perfectly selected speed, so that, every time the ball falls distance x, the curved surface of the earth recedes away from it by the same distance x. Therefore the ball would move in a circular path around the earth forever at a constant height (neglecting air resistance).

This is how all satellites are placed into orbit. For every altitude there is only one right speed.

http://en.wikipedia.org/wiki/Newton's_cannonball



And you do feel the pull of gravity, if you didn't when you jumped you wouldn't come back down. I certainly would link the sensation of falling from a high building not with weightlessness but gravity is a prick and is pulling me to the floor :(i want to fly god damn it !

You mean when you are falling you can see accelerated motion happening. But how do you feel a force? Suppose I pulled your arm out of your shoulder joint. You would feel it because you arm was pulled while the shoulder was NOT pulled, causing them to separate. But when you are falling, every atom of your body is accelerating at the same rate and staying in constant positions relative to each other, so there is no relative motion in any part of you to stimulate any nerves. (You may feel wind blowing on you, but that's another subject.)

[Q]

Or, as we used to say during the 60s:

GRAVITY SUCKS!

RED DAVE

No, vacuums do.

[chegitz guevara]

Actually, Mike, it's very easy to feel the pull of gravity.

Simply hold your arm out. You will feel gravity pulling against your muscles.

When you're very tired, you can feel it in your neck and shoulders, etc.

[mikelepore]

chegitz, I'm getting too semantical, but I meant to emphasize the distinction between the earth's gravity and the reactions to it, and the redirections of those many reaction forces. Suppose I hold out my arm until it hurts. The same gravity is pulling on every particle of the body, therefore the force that I feel can't be coming directly from the pull by the earth. The sensation must be coming from the forces that some parts of the body exert on the other parts of the body, in reaction to the various parts being supported unequally against gravity. I feel forces among bone-tendon-muscle-tendon-bone. I don't feel the earth. To check this reasoning, consider what would happen if no part of me were supported against gravity, although the same gravity is present: the whole body would just fall through space -- but while I'm falling there would be no more sensation that my arm hurts.