Is it possible to block g's, when traveling at a high velocity?

[Woodchuck2000]

Originally posted by: liquid51
how do you figure silver? They maintain a constant orbital velocity, and as a result, feel no inertial forces. Velocity and acceleration are two different things.

They are constantly accelerating towards the earth's centroid and hence do feel a force. It's not easily observed however, because relative to the equipment aroung them they are in free-fall.

Astronauts are not truly weightless - there will always be a gravitational force acting on them hence they will always have a weight.

Silverpig - how do you propose that astronauts are constantly accelerating if there's no force on them? You can't have one without the other; in this case Newton's Second Law definitely applies.

 

[DrPizza]

Originally posted by: Woodchuck2000

Originally posted by: liquid51
how do you figure silver? They maintain a constant orbital velocity, and as a result, feel no inertial forces. Velocity and acceleration are two different things.

They are constantly accelerating towards the earth's centroid and hence do feel a force. It's not easily observed however, because relative to the equipment aroung them they are in free-fall.

Astronauts are not truly weightless - there will always be a gravitational force acting on them hence they will always have a weight.

Silverpig - how do you propose that astronauts are constantly accelerating if there's no force on them? You can't have one without the other; in this case Newton's Second Law definitely applies.

And, Newton's first law also applies. If there wasn't a force on them, they'd continue travelling in a straight line, rather than in a circle. What causes the orbit to be circular (or elliptical)? Force of gravity. As the space shuttle moves forward, it is falling closer to the earth. However, because the earth is round, its surface is falling away as well.

Go to an amusement park. Get on a ride, typically called "free fall" where you're raised to a height in some seat or other contraption, and just dropped. You feel "weightless" because everything around you is accelerating toward the ground at the same rate that you are. You are not pushing down against the seat and the seat is not pushing up against you. If it weren't for the shoulder straps or seat belt, a moron could push against the back of his chair, propelling his body forward so that it's no longer above the chair. But, until the chair starts braking at the bottom, the person will be directly in front of the chair, falling at the same rate (falling with a speed increasing at 9.8 m/s each second or falling with a speed increasing at a rate of 32 feet per second each second.

 

[Woodchuck2000]

True! Free-fall simply means that a = g. Technically you're not in free-fall in a theme park because of the air resitance (thus giving a = g - kv^2, or a close approximation thereto).

Astronauts are in free-fall - their centripetal acceleration is equal in magnitude to thei acceleration due to gravity, assuming that they're in a stable orbit.

 

[MrDudeMan]

Originally posted by: DrPizza

Originally posted by: Woodchuck2000

Originally posted by: liquid51
how do you figure silver? They maintain a constant orbital velocity, and as a result, feel no inertial forces. Velocity and acceleration are two different things.

They are constantly accelerating towards the earth's centroid and hence do feel a force. It's not easily observed however, because relative to the equipment aroung them they are in free-fall.

Astronauts are not truly weightless - there will always be a gravitational force acting on them hence they will always have a weight.

Silverpig - how do you propose that astronauts are constantly accelerating if there's no force on them? You can't have one without the other; in this case Newton's Second Law definitely applies.

And, Newton's first law also applies. If there wasn't a force on them, they'd continue travelling in a straight line, rather than in a circle. What causes the orbit to be circular (or elliptical)? Force of gravity. As the space shuttle moves forward, it is falling closer to the earth. However, because the earth is round, its surface is falling away as well.

Go to an amusement park. Get on a ride, typically called "free fall" where you're raised to a height in some seat or other contraption, and just dropped. You feel "weightless" because everything around you is accelerating toward the ground at the same rate that you are. You are not pushing down against the seat and the seat is not pushing up against you. If it weren't for the shoulder straps or seat belt, a moron could push against the back of his chair, propelling his body forward so that it's no longer above the chair. But, until the chair starts braking at the bottom, the person will be directly in front of the chair, falling at the same rate (falling with a speed increasing at 9.8 m/s each second or falling with a speed increasing at a rate of 32 feet per second each second.

i agree that this is what silverpig was driving at

 

[Woodchuck2000]

Originally posted by: Bigsm00th

i agree that this is what silverpig was driving at

But that statement entirely contradicts what silverpig was saying - he's suggesting that astronauts can constantly accelerate without a force acting on them with is patently wrong.

 

[DrPizza]

Originally posted by: Woodchuck2000

Originally posted by: Bigsm00th

i agree that this is what silverpig was driving at

But that statement entirely contradicts what silverpig was saying - he's suggesting that astronauts can constantly accelerate without a force acting on them with is patently wrong.

No, I don't think that's what silverpig meant.
He said they don't *feel* a force. (as in, they don't perceive a force) I'm fairly certain silverpig would agree that a force does act on them if there's acceleration. I said earlier in this thread, and I'll say it again, our bodies are horrible at detecting velocity or acceleration. We can detect changes in acceleration, but that's about it, unless we use visual stimuli to aid our interpretation. If we were in a sealed compartment, we would be unable to detect if we were simply floating in some region of space where the net gravitational force was zero (if such points exist), or if we were accelerating at 100 m/s toward some massive planet (due to a gravitational force of 10g's acting on us) Either way, we would feel like we were in free-fall. We'd have that sensation that we call "weightlessness" Furthermore, if we did have a window in that capsule, we wouldn't be able to tell the difference between if the planet was accelerating toward us, or if we were accelerating toward the planet. Compare this to the car at a redlight that rolls backward a little bit and you push down on your brakes harder, thinking you're moving forward.

 

[Woodchuck2000]

Originally posted by: DrPizza

Originally posted by: Woodchuck2000

Originally posted by: Bigsm00th

i agree that this is what silverpig was driving at

But that statement entirely contradicts what silverpig was saying - he's suggesting that astronauts can constantly accelerate without a force acting on them with is patently wrong.

No, I don't think that's what silverpig meant.
He said they don't *feel* a force. (as in, they don't perceive a force) I'm fairly certain silverpig would agree that a force does act on them if there's acceleration. I said earlier in this thread, and I'll say it again, our bodies are horrible at detecting velocity or acceleration. We can detect changes in acceleration, but that's about it, unless we use visual stimuli to aid our interpretation. If we were in a sealed compartment, we would be unable to detect if we were simply floating in some region of space where the net gravitational force was zero (if such points exist), or if we were accelerating at 100 m/s toward some massive planet (due to a gravitational force of 10g's acting on us) Either way, we would feel like we were in free-fall. We'd have that sensation that we call "weightlessness" Furthermore, if we did have a window in that capsule, we wouldn't be able to tell the difference between if the planet was accelerating toward us, or if we were accelerating toward the planet. Compare this to the car at a redlight that rolls backward a little bit and you push down on your brakes harder, thinking you're moving forward.

If you were subjected to a force of 10g, believe me, you'd know about it. Our bodies are really quite good at detecting forces - our brains can be tricked to think we're moving but generally not to think we're accelerating. We have no way of detecting velocity other than observation relative to other bodies. We can feel forces however.

In a geostationary orbit, the acceleration due to gravity is only about 0.21ms^-2 (assuming an angular velocity of 3kms^-1 and an orbital radius of about 42x10^6m). This is equivalent to going from 0 to 1mph in about two and a half seconds and without a convenient nearby reference it's going to go unnoticed.

 

[silverpig]

Originally posted by: Woodchuck2000

Originally posted by: Bigsm00th

i agree that this is what silverpig was driving at

But that statement entirely contradicts what silverpig was saying - he's suggesting that astronauts can constantly accelerate without a force acting on them with is patently wrong.

I said they don't FEEL a force. There is a force on them, but they don't feel it.

As to how to accelerate them without them feeling any g-forces... well imagine if the earth was out in open space, not orbiting the sun. We'd be stuck to it by gravity still... a force of mg. Now, imagine someone standing at the south pole, and then imagine some kind of force acting on the earth, that accelerates it along it's normal rotational axis in a northerly direction with an acceleration of 1 g. The earth would move away from you, you would be accelerated by the earth's gravity as well. You would be in a constant free-fall state at the surface of the earth, weightless, and constantly accelerating.

 

[silverpig]

Originally posted by: Woodchuck2000

Originally posted by: DrPizza

Originally posted by: Woodchuck2000

Originally posted by: Bigsm00th

i agree that this is what silverpig was driving at

But that statement entirely contradicts what silverpig was saying - he's suggesting that astronauts can constantly accelerate without a force acting on them with is patently wrong.

No, I don't think that's what silverpig meant.
He said they don't *feel* a force. (as in, they don't perceive a force) I'm fairly certain silverpig would agree that a force does act on them if there's acceleration. I said earlier in this thread, and I'll say it again, our bodies are horrible at detecting velocity or acceleration. We can detect changes in acceleration, but that's about it, unless we use visual stimuli to aid our interpretation. If we were in a sealed compartment, we would be unable to detect if we were simply floating in some region of space where the net gravitational force was zero (if such points exist), or if we were accelerating at 100 m/s toward some massive planet (due to a gravitational force of 10g's acting on us) Either way, we would feel like we were in free-fall. We'd have that sensation that we call "weightlessness" Furthermore, if we did have a window in that capsule, we wouldn't be able to tell the difference between if the planet was accelerating toward us, or if we were accelerating toward the planet. Compare this to the car at a redlight that rolls backward a little bit and you push down on your brakes harder, thinking you're moving forward.

If you were subjected to a force of 10g, believe me, you'd know about it. Our bodies are really quite good at detecting forces - our brains can be tricked to think we're moving but generally not to think we're accelerating. We have no way of detecting velocity other than observation relative to other bodies. We can feel forces however.

In a geostationary orbit, the acceleration due to gravity is only about 0.21ms^-2 (assuming an angular velocity of 3kms^-1 and an orbital radius of about 42x10^6m). This is equivalent to going from 0 to 1mph in about two and a half seconds and without a convenient nearby reference it's going to go unnoticed.

DrPizza was probably talking about the weak equivalence principle... Gravity = acceleration.

And if you were subjected to a force of 10g, it depends on how the force was applied. If you were in a rocket at 10g, yeah you'd feel it, but if you were accelerating through space, freely falling towards a planet with a gravitational constant of 10g, you wouldn't notice.

 

[Matthias99]

Originally posted by: Woodchuck2000

Originally posted by: DrPizza
If we were in a sealed compartment, we would be unable to detect if we were simply floating in some region of space where the net gravitational force was zero (if such points exist), or if we were accelerating at 100 m/s toward some massive planet (due to a gravitational force of 10g's acting on us) Either way, we would feel like we were in free-fall. We'd have that sensation that we call "weightlessness"

If you were subjected to a force of 10g, believe me, you'd know about it. Our bodies are really quite good at detecting forces - our brains can be tricked to think we're moving but generally not to think we're accelerating. We have no way of detecting velocity other than observation relative to other bodies. We can feel forces however.

DrPizza is talking about a different situation -- one where your frame of reference is accelerating at the same speed you are. In this case, you couldn't detect the acceleration. However, I'm pretty sure that an accelerometer in the closed capsule would also register "0" here, since the entire frame of reference is accelerating (it would correctly tell you that you are not accelerating relative to the capsule). It has been a while since I've taken physics, though, so maybe I'm misunderstanding part of the situation.

Now, if someone strapped a rocket engine onto the side of the capsule and started firing it, you'd feel that, since the capsule would be accelerating relative to you. However, if the capsule didn't have any windows, it would be impossible for you to tell whether the acceleration was being caused due to something pushing the capsule, or the capsule being held still relative to a gravitational field.

 

[Woodchuck2000]

Originally posted by: silverpig

Originally posted by: Woodchuck2000

Originally posted by: Bigsm00th

i agree that this is what silverpig was driving at

But that statement entirely contradicts what silverpig was saying - he's suggesting that astronauts can constantly accelerate without a force acting on them with is patently wrong.

I said they don't FEEL a force. There is a force on them, but they don't feel it.

As to how to accelerate them without them feeling any g-forces... well imagine if the earth was out in open space, not orbiting the sun. We'd be stuck to it by gravity still... a force of mg. Now, imagine someone standing at the south pole, and then imagine some kind of force acting on the earth, that accelerates it along it's normal rotational axis in a northerly direction with an acceleration of 1 g. The earth would move away from you, you would be accelerated by the earth's gravity as well. You would be in a constant free-fall state at the surface of the earth, weightless, and constantly accelerating.

And you'd feel a force of 1g accelerating you constantly in that situation. You would indeed be weightless in that there'd be no reaction force between you and the surface of the earth, but you're still under 1g of acceleration however you look at it.

That's no different from being in free space, accelerating at 1g.

 

[Woodchuck2000]

Originally posted by: silverpig

Originally posted by: Woodchuck2000

Originally posted by: DrPizza

Originally posted by: Woodchuck2000

Originally posted by: Bigsm00th

i agree that this is what silverpig was driving at

But that statement entirely contradicts what silverpig was saying - he's suggesting that astronauts can constantly accelerate without a force acting on them with is patently wrong.

No, I don't think that's what silverpig meant.
He said they don't *feel* a force. (as in, they don't perceive a force) I'm fairly certain silverpig would agree that a force does act on them if there's acceleration. I said earlier in this thread, and I'll say it again, our bodies are horrible at detecting velocity or acceleration. We can detect changes in acceleration, but that's about it, unless we use visual stimuli to aid our interpretation. If we were in a sealed compartment, we would be unable to detect if we were simply floating in some region of space where the net gravitational force was zero (if such points exist), or if we were accelerating at 100 m/s toward some massive planet (due to a gravitational force of 10g's acting on us) Either way, we would feel like we were in free-fall. We'd have that sensation that we call "weightlessness" Furthermore, if we did have a window in that capsule, we wouldn't be able to tell the difference between if the planet was accelerating toward us, or if we were accelerating toward the planet. Compare this to the car at a redlight that rolls backward a little bit and you push down on your brakes harder, thinking you're moving forward.

If you were subjected to a force of 10g, believe me, you'd know about it. Our bodies are really quite good at detecting forces - our brains can be tricked to think we're moving but generally not to think we're accelerating. We have no way of detecting velocity other than observation relative to other bodies. We can feel forces however.

In a geostationary orbit, the acceleration due to gravity is only about 0.21ms^-2 (assuming an angular velocity of 3kms^-1 and an orbital radius of about 42x10^6m). This is equivalent to going from 0 to 1mph in about two and a half seconds and without a convenient nearby reference it's going to go unnoticed.

DrPizza was probably talking about the weak equivalence principle... Gravity = acceleration.

And if you were subjected to a force of 10g, it depends on how the force was applied. If you were in a rocket at 10g, yeah you'd feel it, but if you were accelerating through space, freely falling towards a planet with a gravitational constant of 10g, you wouldn't notice.

How do you expect to apply an acceleration of 10g to a human without it being noticed? If you are subjected to an acceleration of 10g, the force required will cause damage to you. You will feel it.

 

[Woodchuck2000]

Originally posted by: Matthias99

Originally posted by: Woodchuck2000

Originally posted by: DrPizza
If we were in a sealed compartment, we would be unable to detect if we were simply floating in some region of space where the net gravitational force was zero (if such points exist), or if we were accelerating at 100 m/s toward some massive planet (due to a gravitational force of 10g's acting on us) Either way, we would feel like we were in free-fall. We'd have that sensation that we call "weightlessness"

If you were subjected to a force of 10g, believe me, you'd know about it. Our bodies are really quite good at detecting forces - our brains can be tricked to think we're moving but generally not to think we're accelerating. We have no way of detecting velocity other than observation relative to other bodies. We can feel forces however.

DrPizza is talking about a different situation -- one where your frame of reference is accelerating at the same speed you are. In this case, you couldn't detect the acceleration. However, I'm pretty sure that an accelerometer in the closed capsule would also register "0" here, since the entire frame of reference is accelerating (it would correctly tell you that you are not accelerating relative to the capsule). It has been a while since I've taken physics, though, so maybe I'm misunderstanding part of the situation.

Now, if someone strapped a rocket engine onto the side of the capsule and started firing it, you'd feel that, since the capsule would be accelerating relative to you. However, if the capsule didn't have any windows, it would be impossible for you to tell whether the acceleration was being caused due to something pushing the capsule, or the capsule being held still relative to a gravitational field.

That makes no sense whatsoever in terms of physics. If you're in a sealed capsule which is accelerating, you must be accelerating at the same rate. The capsule will be exerting a force on you which you will feel.

In all of these situations, just consider the observer. Regardless of the reference frame, or anything else, for the observer to accelerate they must be subjected to a force. There is no way round this.

In your case, the accelerometer would report the same acceleration as the capsule. If it didn't, there would soon be a difference in velocity and the accelerometer would leave the capsule behind.

 

[Matthias99]

Originally posted by: Woodchuck2000

Originally posted by: Matthias99

Originally posted by: Woodchuck2000

Originally posted by: DrPizza
If we were in a sealed compartment, we would be unable to detect if we were simply floating in some region of space where the net gravitational force was zero (if such points exist), or if we were accelerating at 100 m/s toward some massive planet (due to a gravitational force of 10g's acting on us) Either way, we would feel like we were in free-fall. We'd have that sensation that we call "weightlessness"

If you were subjected to a force of 10g, believe me, you'd know about it. Our bodies are really quite good at detecting forces - our brains can be tricked to think we're moving but generally not to think we're accelerating. We have no way of detecting velocity other than observation relative to other bodies. We can feel forces however.

DrPizza is talking about a different situation -- one where your frame of reference is accelerating at the same speed you are. In this case, you couldn't detect the acceleration. However, I'm pretty sure that an accelerometer in the closed capsule would also register "0" here, since the entire frame of reference is accelerating (it would correctly tell you that you are not accelerating relative to the capsule). It has been a while since I've taken physics, though, so maybe I'm misunderstanding part of the situation.

Now, if someone strapped a rocket engine onto the side of the capsule and started firing it, you'd feel that, since the capsule would be accelerating relative to you. However, if the capsule didn't have any windows, it would be impossible for you to tell whether the acceleration was being caused due to something pushing the capsule, or the capsule being held still relative to a gravitational field.

That makes no sense whatsoever in terms of physics. If you're in a sealed capsule which is accelerating, you must be accelerating at the same rate. The capsule will be exerting a force on you which you will feel.

In the case being described (where both you and the capsule are being affected by the same gravitational field), you would be accelerating at exactly the same rate as the capsule. The capsule would *not* be exerting any force on you. You would feel "weightless".

In all of these situations, just consider the observer. Regardless of the reference frame, or anything else, for the observer to accelerate they must be subjected to a force. There is no way round this.

In your case, the accelerometer would report the same acceleration as the capsule. If it didn't, there would soon be a difference in velocity and the accelerometer would leave the capsule behind.

I'm just not seeing how you would measure such an acceleration. You have to measure against some reference frame (since acceleration/velocity are not absolute), and the only reference frame you have is accelerating at the same speed you are.

 

[silverpig]

Originally posted by: Woodchuck2000And you'd feel a force of 1g accelerating you constantly in that situation. You would indeed be weightless in that there'd be no reaction force between you and the surface of the earth, but you're still under 1g of acceleration however you look at it.

That's no different from being in free space, accelerating at 1g.

There would definitely be a force yes, but without anything pushing back on you, you wouldn't notice. Hence the reason astronauts seem to float weightless. They don't feel as though they are being accelerated. When I say "feel" a force, I mean that they are able to notice it physically.

 

[silverpig]

Originally posted by: Woodchuck2000How do you expect to apply an acceleration of 10g to a human without it being noticed? If you are subjected to an acceleration of 10g, the force required will cause damage to you. You will feel it.

Not necessarily. Create a planet where the gravitational constant is 10g. Hang a person 100 km above the surface (they'd FEEL 10g heavy here actually), and then drop them. Immediately they would feel weightless, would be accelerated at 10g but they wouldn't be damaged until they hit the surface.

Being accelerated by a rocket at 10g is definitely felt though. There is a difference here. If you are standing, the rocket only pushes on the soles of your feet, which in turn push on the rest of your body. The rocket doesn't directly accelerated your head, it is up to the internal forces inside your body to accelerate that. This is where you feel the pressure of acceleration and would incur injury. When you free fall at 10g above a massive body, it pulls on all the particles of your body equally (assuming a relatively smooth potential gradient of course). The fact that the force acts on your whole body at once means you don't feel it.

It's sort of like how we can be sitting in 15 psi of air pressure right now. 15 psi is a LOT of pressure when you think about it. Say you'r 72 inches tall and 18 inches wide. That's 1296 sq inches of area on the front of your body. That translates into almost 20 000 pounds of force pushing on the front of you right now. Think about that. There is a dumptruck sitting on your chest right now. But how do we not notice it and how do we survive? Well that air pressure pushes out as well as in; it pushes on our entire body equally, and thus it is safe. It's not a direct analogy, but because the force acts on everything equally, we don't notice it.

 

[liquid51]

True about gravitational forces constantly pulling astronauts toward Earth, even if it is imperceptible in there relative 0g environment. However, we stray from the main topic of the thread's discussion. Just as silverpig described above, true gravitational forces and g forces due to acceleration are two different forces which result in similar "feelings". A constant acceleration will eventually feel identical to the force of gravity, and vise versa: those rapidly fading (or faded) virtual reality rides, using gravity to somewhat effectively simulate acceleration. Likewise, a sudden stop from a free fall will feel the same as a sudden acceleration of the same magnitude.

The original topic of the thread was a discussion of shedding g forces. I said that astronauts are not accelerating, simply maintaining orbital velocity, because that is what is required to negate the gravitational pull of the earth. If you could remove all friction, a coin would spiral the donation drain infinitely, provided that tangential velocity was equivalent to 1g. Because forces are acting apon the astronauts in a perfect balance, they feel nothing. Therefore, because we are speaking reletively, reletive to the astronauts bodies, there is no force. They are not currently being smashed or ripped apart. Those are the magnitudes of force which place limits on our rates of acceleration.

 

[Kibbo86]

Here's the answer:

Talk to starfleet, get yourself a set of inertial dampeners. I'm not sure if you need a deflector array with that or not.

 

[DrPizza]

Originally posted by: liquid51
The original topic of the thread was a discussion of shedding g forces. I said that astronauts are not accelerating, simply maintaining orbital velocity, because that is what is required to negate the gravitational pull of the earth. Because forces are acting apon the astronauts in a perfect balance, they feel nothing. Therefore, because we are speaking reletively, reletive to the astronauts bodies, there is no force. They are not currently being smashed or ripped apart. Those are the magnitudes of force which place limits on our rates of acceleration.

liquid51
Velocity is a vector; it has magnitude AND direction.
Simply because the astronauts are changing direction, their velocity is changing.
Change in velocity is called acceleration.
The astronauts are.... (drum roll) accelerating.
There is no room for debate on this. Circular motion is not possible without acceleration.
Acceleration is the result of unbalanced forces, so the forces are, in fact, not balanced.

Furthermore, if you do F=Gmm/r^2 for the orbit of astronauts, you may be surprised to find out how high their acceleration actually is.

Question: If a high diver is standing on top of a 100 foot cliff, does the earth exert a force on him? Yes. If the high diver jumps over the cliff, does the earth exert a force on him? Yes. If it didn't, he would float in mid-air. The force (gravitational) accelerates him downward. Now, if someone drove a car off a cliff, does the frame of the car magically block the gravitational force acting on the driver? Of course not. The force of attraction between the earth and car, and between the earth and driver cause both to accelerate at the same rate toward the earth (9.8m/s^2). When you're in an amusement park ride that experiences "0 g's", is it because somehow Disney has figured out how to turn gravity on and off (by pushing a button??) No, it's because you and the vehicle are in freefall. (Although, if you have a forward velocity, freefall will follow a parabolic path; much the same way a rock thrown upward at an angle falls back to the earth along the path of a parabola (ignoring air resistance.)

Now, the astronauts are in fact accelerating, somewhere around 7 or 8 m/s^2. (It's been a year since I had students calculate the gravitational force acting at that distance) This acceleration is free-fall toward the center of the earth. There *IS* a force acting on those astronauts. However, they are accelerating toward the earth in free fall *at the same rate their spacecraft is accelerating toward the earth.* Thus, they're in the equivalent of riding on a rollercoaster that experiences "0 g's" Now, on the roller coaster, if you're moving forward at 20 m/s relative to the ground, by the time your altitude has changed from 45 meters to 0 meters, (about 3 seconds in freefall), you will have moved forward about 60 meters, relative to the ground, following a parabolic path. However, the astronauts are accelerating just a little bit slower. BUT, they're moving so fast that by the time they've fallen 45 meters in altitude, they've moved far enough forward that the curvature of the earth causes the land to have dropped away by 45 meters as well.

Now, I think a few people are confused about the g-forces a body can stand; how it is that the body can't stand them. Try this: Cut about a 1/4 inch hole in the bottom of a 2-liter bottle. Cover it with your hand and fill the bottle with water. Leave the cap off.
Now, when you remove your hand, what happens to the water? Answer (captain obvious) - it runs out the hole in the bottom.
Now, suppose we cover the hole with some sort of thin membrane. Just strong enough so that it won't break... right on the verge of breaking though. Now, get in the worst elevator you've ever been in (one that accelerates so fast at the beginning that you think you're stomach is in your feet.) Oh no. The membrane is going to break. Think of it as the elevator is applying a force to the bottle (causing the acceleration). Since you're accelerating upward, the bottle is experiencing more than 1 "g" of force, and the membrane breaks. This could represent your blood vessels, or whatever tissue you want in your body, not to mention your blood rushing from your brain to your feet.

Now, the fun part. Find a nice 10 story building where they'll let you throw stuff off the roof or out an upper floor window. Fill the 2 liter bottle with water, keep the hole covered with your hand and keep the cap off. Now, throw the 2-liter bottle out the window (obviously, letting go of the hole.) You'll notice that as the bottle accelerates toward the earth that NO water is coming out the top or out the hole in the bottom. That's because gravity is pulling on the bottle and water at the same time and causing both to accelerate at the same rate. Back to the elevator. The elevator was applying the force to the bottle (but not to the water) The bottle was applying the force to the water. Likewise (Newton's 3rd law) the water was applying a force to the bottle. It is in this case where the human body can only stand so much force. Not in the case of freefall where all the parts of the body are being pulled on to accelerate at the same rate. (Silverpig or someone else above noted that the gravitational field can't be changing rapidly as in the case of black-holes. In the case of black holes, if you fell in feet first, the difference in the force acting on your feet and the force acting on your head is so large, that it would rip you in half)

One more attempt at an analogy. Imagine 2 large toy cars, connected by a piece of thread. Push on just the car in front, causing it to accelerate, and the thread is going to break. However, if you push on each of them, causing each to accelerate at the same rate, the thread isn't going to break, regardless of the acceleration. You could accelerate both cars at 100m/s^2, and as long as they start at the exact same time and accelerate at the same rate, they're going to stay the same distance apart and the thread isn't going to break. Such is the case of a human in free-fall. They can accelerate at any rate, and as long as each little piece of their body is accelerating at the same rate, they're not going to break (like the thread.) But, just push on one of the cars, and the thread breaks. Put a person an elevator and accelerate them vertically at a very high rate, by just pushing against their feet, and their head is going to wind up at their feet (call it inertia if you will.)

I hope that makes some sense of it for you.

 

[DrPizza]

Whao, talk about your "definitely not a nef" post!

 

[silverpig]

Originally posted by: liquid51
True about gravitational forces constantly pulling astronauts toward Earth, even if it is imperceptible in there relative 0g environment. However, we stray from the main topic of the thread's discussion. Just as silverpig described above, true gravitational forces and g forces due to acceleration are two different forces which result in similar "feelings". A constant acceleration will eventually feel identical to the force of gravity, and vise versa: those rapidly fading (or faded) virtual reality rides, using gravity to somewhat effectively simulate acceleration. Likewise, a sudden stop from a free fall will feel the same as a sudden acceleration of the same magnitude.

The original topic of the thread was a discussion of shedding g forces. I said that astronauts are not accelerating, simply maintaining orbital velocity, because that is what is required to negate the gravitational pull of the earth. If you could remove all friction, a coin would spiral the donation drain infinitely, provided that tangential velocity was equivalent to 1g. Because forces are acting apon the astronauts in a perfect balance, they feel nothing. Therefore, because we are speaking reletively, reletive to the astronauts bodies, there is no force. They are not currently being smashed or ripped apart. Those are the magnitudes of force which place limits on our rates of acceleration.

Actually, according to general relativity, gravitational forces and forces due to acceleration are one and the same.

 

[zmaster]

But, if te answer to the original question is no, could someone explain to me how we feel no different traveling at 900km/h in a jet to traveling 50ks an hour in a car at a highway. I think that would be a more suitable analogy

 

[DrPizza]

Originally posted by: zmaster
But, if te answer to the original question is no, could someone explain to me how we feel no different traveling at 900km/h in a jet to traveling 50ks an hour in a car at a highway. I think that would be a more suitable analogy

because in both instances, you're not accelerating. You're maintaining a constant speed. However, if you go around an unbanked corner while maintaining the same speed, you're going to feel a force accelerating you around the turn. (either a change in speed, or a change in direction (or both) imply an unbalanced force and thus, acceleration)

In both instances, if you accelerated from rest to those speeds in the same amount of time, you would feel a force. Furthermore, since your acceleration is much greater in the case of the jet (assuming equal times, the acceleration would be 18 times higher), you would feel a much greater force against you.

 

[Woodchuck2000]

Originally posted by: silverpig

Originally posted by: liquid51
True about gravitational forces constantly pulling astronauts toward Earth, even if it is imperceptible in there relative 0g environment. However, we stray from the main topic of the thread's discussion. Just as silverpig described above, true gravitational forces and g forces due to acceleration are two different forces which result in similar "feelings". A constant acceleration will eventually feel identical to the force of gravity, and vise versa: those rapidly fading (or faded) virtual reality rides, using gravity to somewhat effectively simulate acceleration. Likewise, a sudden stop from a free fall will feel the same as a sudden acceleration of the same magnitude.

The original topic of the thread was a discussion of shedding g forces. I said that astronauts are not accelerating, simply maintaining orbital velocity, because that is what is required to negate the gravitational pull of the earth. If you could remove all friction, a coin would spiral the donation drain infinitely, provided that tangential velocity was equivalent to 1g. Because forces are acting apon the astronauts in a perfect balance, they feel nothing. Therefore, because we are speaking reletively, reletive to the astronauts bodies, there is no force. They are not currently being smashed or ripped apart. Those are the magnitudes of force which place limits on our rates of acceleration.

Actually, according to general relativity, gravitational forces and forces due to acceleration are one and the same.

At the risk of heckling your semantics, that's not what general relativity says. They are equivalent. The effects are identical - the mechanisms however, are entirely different.

 

[silverpig]

Originally posted by: Woodchuck2000

Originally posted by: silverpig

Originally posted by: liquid51
True about gravitational forces constantly pulling astronauts toward Earth, even if it is imperceptible in there relative 0g environment. However, we stray from the main topic of the thread's discussion. Just as silverpig described above, true gravitational forces and g forces due to acceleration are two different forces which result in similar "feelings". A constant acceleration will eventually feel identical to the force of gravity, and vise versa: those rapidly fading (or faded) virtual reality rides, using gravity to somewhat effectively simulate acceleration. Likewise, a sudden stop from a free fall will feel the same as a sudden acceleration of the same magnitude.

The original topic of the thread was a discussion of shedding g forces. I said that astronauts are not accelerating, simply maintaining orbital velocity, because that is what is required to negate the gravitational pull of the earth. If you could remove all friction, a coin would spiral the donation drain infinitely, provided that tangential velocity was equivalent to 1g. Because forces are acting apon the astronauts in a perfect balance, they feel nothing. Therefore, because we are speaking reletively, reletive to the astronauts bodies, there is no force. They are not currently being smashed or ripped apart. Those are the magnitudes of force which place limits on our rates of acceleration.

Actually, according to general relativity, gravitational forces and forces due to acceleration are one and the same.

At the risk of heckling your semantics, that's not what general relativity says. They are equivalent. The effects are identical - the mechanisms however, are entirely different.

You're travelling a curved worldline in both cases. Whether it be a straight line through curved spacetime or a curved line through flat spacetime is just a matter of gauge. They are gauge invariant though AFAIK.