Why Do Astronauts Feel Weightless in Orbit?

[Naty1]

My prior post was poorly organized...where I originally said:

The astronauts 'appear' weightless because they ARE actually weightless. Have you ever seen pictures??...they' float' inside a space station and so does, say, a tool they release. Things maintain their relative positions inside...like in free fall because it IS freefall. Everything nearby floats because there are no net forces. An accelerometer shows no acceleration.

[In the context of General Relativity gravitation is space-time curvature and a body in free fall has no force acting on it as it moves along a geodesic...a particular type curve in spacetime.]

It should have appeared like this:

[The last sentence of the first paragraph should have been within the parenthesis as follows:

..The astronauts 'appear' weightless because they ARE actually weightless. Have you ever seen pictures??...they' float' inside a space station and so does, say, a tool they release. Things maintain their relative positions inside...like in free fall because it IS freefall.

[In the context of General Relativity gravitation is space-time curvature and a body in free fall has no force acting on it as it moves along a geodesic...a particular type curve in spacetime. Everything nearby floats because there are no net forces. An accelerometer shows no acceleration. ]

The point [from GR not the Newtonian perspective] I was trying to make is briefly covered in the Wikipedia Link I posted:

Relativity
To a modern physicist working with Einstein's general theory of relativity, the situation is even more complicated than is suggested above. Einstein's theory suggests that it actually is valid to consider that objects in inertial motion (such as falling in an elevator, or in a parabola in an airplane, or orbiting a planet) can indeed be considered to experience a local loss of the gravitational field in their rest frame. Thus, in the point of view (or frame) of the astronaut or orbiting ship, there actually is nearly-zero proper acceleration (the acceleration felt locally), just as would be the case far out in space, away from any mass. It is thus valid to consider that most of the gravitational field in such situations is actually absent from the point of view of the falling observer...

http://en.wikipedia.org/wiki/Weightlessness#Relativity


and

... Accelerometers, can only detect g-force i.e. weight2 (= mass x proper acceleration) They cannot detect free fall.

I think the overall Wikipedia article is rather good.

 

[A.T.]

If they are at a constant speed and not changing altitude which way do you propose they are accelerating?

An object in uniform circular motion undergoes centripetal acceleration towards the center.
http://en.wikipedia.org/wiki/Circular_motion#Acceleration

If the acceleration was zero, it would move in a straight line.
http://en.wikipedia.org/wiki/Newton's_laws_of_motion#Newton.27s_1st_Law

This is why I chose to explain this with Newtonian physics...

That's great, but first you should learn Newtonian physics.

 

[p1l0t]

An object in uniform circular motion undergoes centripetal acceleration towards the center.
http://en.wikipedia.org/wiki/Circular_motion#Acceleration

If the acceleration was zero, it would move in a straight line.
http://en.wikipedia.org/wiki/Newton's_laws_of_motion#Newton.27s_1st_LawThat's great, but first you should learn Newtonian physics.

How does it accelerate towards the center while not actually losing any altitude? That doesn't make any sense at all. I can see if the forces are in equilibrium but that's microgravity. If they were actually accelerating it would take a considerable force to move something more like it does in Earth. They are weightless because of the lack of acceleration.

 

[A.T.]

How does it accelerate towards the center while not actually losing any altitude?

Did you check Newton's 1st Law? Does the astronaut move in a straight line? No? Then it is accelerating.

That doesn't make any sense at all.

It makes perfect sense:
http://en.wikipedia.org/wiki/Centripetal_force#Uniform_circular_motion

I can see if the forces are in equilibrium but that's microgravity.

Microgravity has nothing to do with it. It refers to negligible tidal effects.

 

[WannabeNewton]

How does it accelerate towards the center while not actually losing any altitude?...They are weightless because of the lack of acceleration.

At a given instant the centripetal force points towards the center along a certain radial direction but at the very next instant it changes to an infinitesimally close radial direction.

They are not weightless because of the lack of acceleration. Gravity is obviously acting on the shuttle and the astronaut. If you are so sure that there is no net force on the two then prove it.

 

[p1l0t]

I'm not saying that there is not gravity in orbit. What I am saying is that in a free fall situation they do not FEEL the effect of gravity. To the astronaut it is microgravity. They may not be truly floating in space but they could hold an accelerometer and it will read zero. Why? Because they aren't accelerating. They may have their tangential velocity opposing the inward acceleration but either way they are not accelerating overall. How else would they continue to freefall without colliding with the Earth?

 

[Bandersnatch]

O.k., I think I understand the gist of the problem here.

P1lot seems to be thinking in terms of a rotating reference frame zeroed at Earth's centre and with angular velocity matching that of the astronaut's ship.

Since this is a non-inertial reference frame, the force of gravity is exactly canceled by the emergent centrifugal force. His acceleration is 0 and his velocity is 0.

The problems with this choice of ref.frame are as follow:
It is not the ref.frame chosen by the OP.
In this frame, the astronaut remains at all times stationary, so we cannot talk about circular motion.
The force cancelling gravity is fictitious. I.e., it dissapears in inertial reference frames.

 

[WannabeNewton]

Because they aren't accelerating. They may have their tangential velocity opposing the inward acceleration but either way they are not accelerating overall. How else would they continue to freefall without colliding with the Earth?

First of all the tangential velocity of a circular orbit is orthogonal to the acceleration so it doesn't "oppose" it. Secondly, free fall doesn't literally mean you fall like you jumped off a building. It just means the only force on you is the gravitational force.

 

[p1l0t]

O.k., I think I understand the gist of the problem here.

P1lot seems to be thinking in terms of a rotating reference frame zeroed at Earth's centre and with angular velocity matching that of the astronaut's ship.

Since this is a non-inertial reference frame, the force of gravity is exactly canceled by the emergent centrifugal force. His acceleration is 0 and his velocity is 0.

The problems with this choice of ref.frame are as follow:
It is not the ref.frame chosen by the OP.
In this frame, the astronaut remains at all times stationary, so we cannot talk about circular motion.
The force cancelling gravity is fictitious. I.e., it dissapears in inertial reference frames.

Considering I don't know what reference frame he is talking about, your probably right. But part of his question was why do they "feel" no force.

 

[A.T.]

Considering I don't know what reference frame he is talking about, your probably right.

Read the 1st post again. He's talking about the reference frame where the astronaut is in uniform circular motion and undergoes centripetal acceleration.

But part of his question was why do they "feel" no force.

That's a frame independent fact.

 

[A.T.]

Since this is a non-inertial reference frame, the force of gravity is exactly canceled by the emergent centrifugal force. His acceleration is 0 and his velocity is 0.

The problems with this choice of ref.frame are as follow:
It is not the ref.frame chosen by the OP.
In this frame, the astronaut remains at all times stationary, so we cannot talk about circular motion.
The force cancelling gravity is fictitious. I.e., it dissapears in inertial reference frames.

And as I pointed out in post #44: being at rest in some non-inertial frame doesn't imply weightlessness, so this fact cannot be used as a reason for the weightlessness in this case.

 

[Bandersnatch]

I was aiming at sorting out the "why no change in altitude if acceleration" misunderstanding.
The weightlessnes, as you said, is frame independent.
(Now, wouldn't that be handy if we could change our weight by reassigning reference frames.)

 

[p1l0t]

Well here is my issue with weight though. In order to measure weight you have to be pushing against something. Your mass may not change but how can measure weight without being accelerated by something like the surface of the planet or a rocket engine pushing you? I supposed you could see how much it takes to move them..

 

[mikeph]

In order to measure weight you have to be pushing against something.

Not true.

When you stand on a scale, the spring deforms by a measurable distance, from which you deduce the restoring force. This is a measurement of the reaction force acting on you from the Earth, not your weight.

Weight is the force due to gravity. There is no requirement for it to be measurable by scales.

From reading the last few posts, I think your perception is that there is no weight/acceleration if you can feel no weight or acceleration, but this isn't true. Weight is a force acting on every kilogram of mass on your body equally, so your leg will accelerate at the same rate as your head. This is what makes you feel "weightless", whereas if you're being pushed from a certain direction, your brain detects the compression as that force is transmitted across the mass of your body.

 

[p1l0t]

Not true.

When you stand on a scale, the spring deforms by a measurable distance, from which you deduce the restoring force. This is a measurement of the reaction force acting on you from the Earth, not your weight.

Weight is the force due to gravity. There is no requirement for it to be measurable by scales.

From reading the last few posts, I think your perception is that there is no weight/acceleration if you can feel no weight or acceleration, but this isn't true. Weight is a force acting on every kilogram of mass on your body equally, so your leg will accelerate at the same rate as your head. This is what makes you feel "weightless", whereas if you're being pushed from a certain direction, your brain detects the compression as that force is transmitted across the mass of your body.

Yeah on Earth but on the ISS, for example, you cannot just stand in a scale.

 

[BobG]

Yeah on Earth but on the ISS, for example, you cannot just stand in a scale.

If you're in a free falling elevator that happens to have a scale, how much do you weigh in the free falling elevator?

You're right on the concept. "Weightlessness" just means you're weightless relative to your surroundings.

As long as your surroundings are reacting exactly the same as you, free falling due to gravity, whether the free fall happens to be straight down or have so much tangential motion that you constantly miss the Earth, then you're weightless.

However, it doesn't have anything to do with circular motion around the Earth. You'd still be weightless if you were in a spaceship that was in an elliptical orbit, as well. In fact, the ISS is virtually always at least slightly elliptical, since no orbits can be perfectly circular around an oblate Earth (the Earth bulges around the equator).

 

[mikeph]

Yeah on Earth but on the ISS, for example, you cannot just stand in a scale.

I'm guessing you didn't read beyond the first paragraph.

 

[p1l0t]

I'm guessing you didn't read beyond the first paragraph.

How about the fact that astronauts go through massive bone density loss in the weight bearing bones?

 

[Mordred]

If you were to dangle a man on ropes with his feet barely touching a scale would you say he's weightless?

The force of gravity acting upon him is still the same so his weight is still the same.
Now replace the ropes with centrifical force. Will his weight change?.

On the ISS the force of gravity is roughly 90% of Earth norm.
Weight is a relation of mass and force of gravity not other forced

 

[p1l0t]

If you were to dangle a man on ropes with his feet barely touching a scale would you say he's weightless?

The force of gravity acting upon him is still the same so his weight is still the same.
Now replace the ropes with centrifical force. Will his weight change?.

On the ISS the force of gravity is roughly 90% of Earth norm.

I am not arguing that they are not within the Earth's gravitational field. What I am saying is the tangential velocity opposes that force. It gives them essentially a microgravity environment because the net acceleration is almost zero. If they were accelerating they would be pinned against one of the walls. I can get in my airplane and feel weightless If I climb aggressively and then push the nose forward with just the right amount of force to make stuff appear to levitate. Maybe that's not exactly the same but the stuff still has mass and sure appears to be weightless floating around the cockpit. I went skydiving one time in Las Vegas, at first I had the feeling of rapid acceleration but after awhile it stabilized, maybe due to wind resistance but whatever it was comfortable after that besides the wind noise and the airport below getting "larger." Then of course decelerating when the chute opened. So if you were in an elevator falling towards the Earth would you be pinned to the ceiling as it fell, probably not I THINK since it is not itself pushing you. And even though it appears to be accelerating towards the Earth, really it is the Earth that is accelerating through spacetime. If your worldline is geodesic with spacetime then you're in freefall? Now I'm sure you guys can cut this up but I'm just trying to explain why *I think* weight requires acceleration. Mass stays the same, weight requires acceleration. OR maybe it just requires something to be pushing against regardless? I can also weigh 400lbs (2Gs) if I put the airplane in a 60 degree back and maintain level flight.

 

[Mordred]

No the physics definition of weight only apllies gravity and mass. Thats the key your missing

edit as a sum of other forces think of it as apparent weight or relative weight

 

[p1l0t]

No the physics definition of weight only apllies gravity and mass. Thats the key your missing

edit as a sum of other forces think of it as apparent weight or relative weight

Ok so if your tangential velocity is enough to oppose the inward acceleration are you in microgravity? Or it is simply that the astronaut and the ship are falling at the same rate and never hit the planet? All my college courses are always using "microgravity environment" perhaps incorrectly. I guess I just never realized that there was a difference between gravity and acceleration.

 

[Mordred]

The latter forget microgravity

 

[ZVdP]

No the physics definition of weight only apllies gravity and mass. Thats the key your missing

edit as a sum of other forces think of it as apparent weight or relative weight

There are simply two definitions of weight. Wikipedia calls them the 'gravitational definition' and 'operational definition'. I wouldn't say that physics only uses the gravitational definition.
I think it may be bound to the geographical location. On a Dutch science forum there was confusion too when most of the Belgian people assumed the operational definition, while most of the Dutch people used the gravitational one.
A bit like whether log(x) has base 10 or base e.

 

[p1l0t]

So it feels like pushing 180lbs not <1lb if you pushed an astronaut that weighs 200lbs here in Earth but in LEO?

 

[Bandersnatch]

So it feels like pushing 180lbs not <1lb if you pushed an astronaut that weighs 200lbs here in Earth but in LEO?

No, it feels like pushing 200lbs. The amount of force to produce the same acceleration on astronaut's body is the same as on Earth's surface. The mass doesn't change, after all.

 

[technician]

Ok so if your tangential velocity is enough to oppose the inward acceleration are you in microgravity? Or it is simply that the astronaut and the ship are falling at the same rate and never hit the planet? All my college courses are always using "microgravity environment" perhaps incorrectly. I guess I just never realized that there was a difference between gravity and acceleration.

OK...how much would 200lb weigh at the centre of the Earth, where there is no tangent?

 

[p1l0t]

No, it feels like pushing 200lbs. The amount of force to produce the same acceleration on astronaut's body is the same as on Earth's surface. The mass doesn't change, after all.

 

[Dewgale]

No, it feels like pushing 200lbs. The amount of force to produce the same acceleration on astronaut's body is the same as on Earth's surface. The mass doesn't change, after all.

Correct me if I'm wrong, but some of the force we feel when pushing 200lbs on Earth either comes from gravity pulling it down if you're throwing it, or from friction if you're pushing it along the ground. Either way, you wouldn't have to put forward as much force in space to achieve the same acceleration, because you don't need to overcome either of these forces.

 

[technician]

Fake video of students messing about instead of reading their physics textbooks!

Ps. I think this thread will get to 100 posts before someone stops it.