Why Do Astronauts Feel Weightless in Orbit?

[Forge]

If they're orbiting Earth, then they're in uniform circular motion. And if they're in uniform circular motion, then they experience a centripetal force (and thus centripetal acceleration) towards the center of Earth. Therefore, they are constantly accelerating (direction changes, magnitude does not change) and you can feel acceleration but it appears that they do not feel any force.

 

[HallsofIvy]

You don't have to go into orbit to be weightless. If you were to go to the "Burj Al Arab" in Dubai, the tallest building on earth, and jump of the top, you would be weightless- until you hit the ground! The force of gravity would still be acting on you but you would not feel any weight because there is nothing pusing up on you.

When astronauts are in orbit, the force of gravity is acting on them and their ship equally. Astronauts and ship are all falling toward the center of the earth. There is no "floor" or "ground" pushing up on them so they feel no weight.

 

[jtbell]

You don't have to go into orbit to be weightless. If you were to go to the "Burj Al Arab" in Dubai, the tallest building on earth, and jump of the top, you would be weightless- until you hit the ground!

Or for a safer experience, arrange a ride on the "Vomit Comet".

 

[A.T.]

you can feel acceleration but it appears that they do not feel any force.

What you usually feel as a force, is the deformation of your body caused by it. The special thing about the force of gravity is that is is proportional to mass, so it is accelerating small objects almost uniformly, without causing significant deformations. There fore you don't feel it, if it the only force acting on you.

 

[Forge]

When astronauts are in orbit, the force of gravity is acting on them and their ship equally.

So relative to the frame of reference of the ship, the astronauts appear to be weightless, but they are not actually weightless. However, do the astronauts not feel the pull of the force? If you're in an zero-acceleration frame of reference, you cannot tell what velocity you are traveling at. However if you're in an accelerating frame of reference, can't you feel that you are accelerating?

What you usually feel as a force, is the deformation of your body caused by it. The special thing about the force of gravity is that is is proportional to mass, so it is accelerating small objects almost uniformly, without causing significant deformations. There fore you don't feel it, if it the only force acting on you.

So because the force is proportional to mass, the acceleration is constant, right? Doesn't gravity acceleration both small and large objects almost uniformly?

 

[A.T.]

So because the force is proportional to mass, the acceleration is constant, right? Doesn't gravity acceleration both small and large objects almost uniformly?

If free falling objects are small enough, so gravity gradient is negligible, they are accelerated uniformly along their volume. You cannot "feel" such acceleration, because it doesn't create deformation.

 

[Bandersnatch]

If all parts of your body are accelerated uniformly, as it happens in the gravitational field(caveat: far away from the source), then no, you can't.

Human beings perceive linear acceleration mostly via the otholitic organ in the inner ear. In it, suspended in viscous fluid are small crystals. Since the crystals are not rigidly connected to the rest of the body, whenever your head experiences acceleration they lag behind the bulk of your tissues due to inertia. Their displacement bends the tiny hairs lining the walls of the organ. These hairs are connected to nerves that then transmit the signal to your brain.

Another, similar in principle, way we feel acceleration is via kinesthesia. I.e., the relative position of our limbs, that also changes due to inertia, as long as the accelerating force is applied non-uniformly.

If the force permeats the space in uniform fashion, all parts of your body change their velocities at the same rate, so neither the crystals in your inner ear, nor your limbs ever lag behind, and so, never produce a neural response in your brain.

So, in a way, one could say you never really feel gravity, even sitting in your chair. It's the ground pushing the chair pushing on your bottom parts that disrupts the uniform acceleration of the gravity, providing localised force that then is transferred to the rest of your body through the tissues, with inertial lag across the board.



Now, IF you are close to the source of gravity, the parts of your body farther from the source experience less acceleration(inverse square law) than the closer ones. These are tidal forces that you could feel as being stretched along one axis and compressed along the two other, providing your total length is significant as compared to the distance to the source and its strength(effect also known under the scientific term of spaghettification).

 

[Naty1]

forget about what people may 'feel'...that's too subjective. [I assume you are interested in the physics, not 'feelings']

So relative to the frame of reference of the ship, the astronauts appear to be weightless, but they are not actually weightless. However, do the astronauts not feel the pull of the force? If you're in an zero-acceleration frame of reference, you cannot tell what velocity you are traveling at. However if you're in an accelerating frame of reference, can't you feel that you are accelerating?

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.]


Some interesting Newtonian [not GR] comments here:

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

 

[technician]

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.

It depends on what you mean by 'weightless'
If 'weight' is the force due to gravity then they are NOT weightless. There is a resultant force on the astronaut and the spacecraft ...centripetal force.

How do you 'experience'/'feel' (this is not subjective) weight?...It is the upwards force on your feet/backside as indicated on a weighing machine. The astronaut experiences no force between himself/herself and any weighing machine.
They EXPERIENCE weightlessness

A fish 'floats' in water...is it weightless...does a fish experience the force due to gravity?...have you seen pictures of fish floating...there ARE NO NET forces on a fish !
What is the difference between a fish and an astronaut??

Draw a freebody diagram of forces on an astronaut and forces on a fish...use this diagram to explain 'weightlessness' (do not refer to GR)

 

[A.T.]

Everything nearby floats because there are no net forces.

The astronaut is undergoing centripetal acceleration, so there obviously is a net force on him. Bringing GR into it will probably just confuse the OP.

 

[technician]

forget about what people may 'feel'...that's too subjective. [I assume you are interested in the physics, not 'feelings']



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.]


Some interesting Newtonian [not GR] comments here:


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

These are ridiculous statements, completely contrary to conventional teaching and textbook explanations.
This is not physics !
Someone with some authority should check posts like this.

 

[Drakkith]

Someone with some authority should check posts like this.

This is a forum, not a scientific journal. We don't really appeal to an authority. If you feel someones post is incorrect, then correct them. If you are right, then you're right.

 

[p1l0t]

There is a difference between mass and weight I think that's what he is missing.

 

[technician]

The rules of these forums dictate that posts should be in line with current/text book explanations.
What is stated here is plane wrong... in my opinion.
As far as I understand it these are not 'opinion' forums
Anything else should be backed up by some sort of references that can be checked.
The 'report' option is only for spam, advertising, etc.

 

[p1l0t]

The rules of these forums dictate that posts should be in line with current/text book explanations.
What is stated here is plane wrong... in my opinion.
As far as I understand it these are not 'opinion' forums
Anything else should be backed up by some sort of references that can be checked.
The 'report' option is only for spam, advertising, etc.

It's not really wrong, they are [just about] weightless in microgravity. Their mass doesn't change but their weight changes with gravity.

 

[The Juzzagon]

Don't astronauts feel weightless due to the equivalence principle? Technically, above earth, in space, the gravity is around 9/10 as strong as on the ground. The reason that causes weightlessness is because the station astronauts are in, is traveling so fast that the Earth curves away under the ship. So they are not weightless, they are constantly falling, but due to the speed of their orbit, the Earth curves away below them just as fast that they never seem to fall relative to the ship: thus, causing what they feel as weightlessness.

 

[technician]

What is 'microgravity'...cant find it in the index of my textbook !
How does weight change with gravity (I think I have the answer to this but I am interested in your view)

 

[technician]

juzzagon...you are not wrong

 

[A.T.]

Don't astronauts feel weightless due to the equivalence principle?

That's just a another way to say the same thing:

force proportional to mass -> force can be seen as an inertial force -> equivalence principle

 

[A.T.]

It's not really wrong, .

It's not really wrong, in the context of GR. That should be made clear, otherwise it will confuse.

The general explanation (Newton or GR): Gravity is a force (real or inertial) that accelerates everything by the same amount. So it doesn't cause deformation of the body and cannot be felt.

 

[technician]

What's the difference between
1) a fish floating
2) A skydiver with parachute not open
3) A skydiver with parachute open
4) A passenger in a free falling lift
5) an astronaut in the space station

Which of the above could be described as 'weightless'?

 

[p1l0t]

It doesn't matter if the reason is they are falling constantly the net gravity is near zero and therefore they are just about weightless. I am not sure if LEO is 9/10ths when your not at the right speed (zero instead of 17,000 mph or so depending on alt). Either way it is irrelevant because the net force is basically null.

 

[A.T.]

It doesn't matter if the reason is they are falling constantly the net gravity is near zero

What is "net gravity" and how do you compute it exactly for an orbiting astronaut?

 

[technician]

When is 'net gravity' near to zero.?
When is the net force basically null?

You need to make statements like this absolutely clear...

 

[technician]

From post #8 this has degenerated into ...I do not know...it is time for someone to close this thread.
I am not coming back.

 

[p1l0t]

Read about Microgravity

 

[A.T.]

What's the difference between
1) a fish floating
5) an astronaut in the space station
Which of the above could be described as 'weightless'?

Floating in water and in space is not quite the same, because of the non-uniform density of the body.

 

[p1l0t]

lmgtfy.com/?q=micro gravity

 

[p1l0t]

The first link is the best answer http://www.nasa.gov/centers/glenn/shuttlestation/station/microgex.html

 

[Naty1]

These are ridiculous statements, completely contrary to conventional teaching and textbook explanations.

Why not just rewrite the Wikipedia article as well??

 

[cjl]

What's the difference between
1) a fish floating
2) A skydiver with parachute not open
3) A skydiver with parachute open
4) A passenger in a free falling lift
5) an astronaut in the space station

Which of the above could be described as 'weightless'?

I would make the case that only #4 and #5 could be considered "weightless", since those are the only cases of free fall listed above (since a skydiver, even with no parachute, has substantial air resistance). Floating in water is subjectively similar, but it is not the same, since the buoyant force acts on the surface of the object, while gravity acts on the whole volume.

I would also say that based on the equivalence principle, free fall and weightlessness are the same thing - in the absence of tidal forces, there is no difference of any significance.

 

[technician]

Why not just rewrite the Wikipedia article as well??

I have textbooks that have stood the test of time.
Don't need wiki
It is the worst thing to have happened to education...full of irrelevant information and incorrect facts...check their explanation of faradays laws. Who checks wiki...how much does it cost??
You get what you pay for in this world
Goodbye

 

[p1l0t]

Obviously he is trolling

 

[A.T.]

What is "net gravity" and how do you compute it exactly for an orbiting astronaut?

The first link is the best answer http://www.nasa.gov/centers/glenn/shuttlestation/station/microgex.html

The term "net gravity" is not used there. What is your definition of it?

 

[p1l0t]

The term "net gravity" is not used there. What is your definition of it?

The total gravitational force is near zero in orbit. Weight is dependent on gravity. Gravity is acceleration. If you are going at 17k relative to someone on Earth in LEO you are not accelerating. No acceleration and your weight is zero. It doesn't matter if your speed relative to an observer on the Earth is really fast. The force it takes to move an object is its mass times it's acceleration. If it is in a microgravity environment wear the acceleration is near zero than the mass is almost meaningless and therefore it weighs just about nothing. The reason it seems conter-intuitive is that we have just become used to constantly accelerating through spacetime at 9.8mps2 or whatever is here in this planet. That's why we have weight on the surface even when standing still.

 

[Chestermiller]

The astronaut is undergoing centripetal acceleration, so there obviously is a net force on him. Bringing GR into it will probably just confuse the OP.

I don't really think you should have dismissed what Naty1 said like that. We know from GR that there really is no net force acting on him when he is in free fall (which I'm sure you are aware of). For a question like this, the OP certainly should at least be made aware that there is a pre-relativistic model of what is happening, and a (now believed by most to be correct) GR version of what is happening, and that the two descriptions are very different.

 

[Andrew Mason]

The total gravitational force is near zero in orbit.

Can you explain that in terms of F = GMm/r^2 ?

Weight is dependent on gravity.

Ok. But it is also dependent on there being a force applied to the body to keep it from accelerating due to gravity.

Gravity is acceleration. If you are going at 17k relative to someone on Earth in LEO you are not accelerating.

So there is no centripetal acceleration in this circular motion?

AM

 

[A.T.]

For a question like this, the OP certainly should at least be made aware that there is a pre-relativistic model of what is happening, and a (now believed by most to be correct) GR version of what is happening, and that the two descriptions are very different.

I have no problem with explaining it in different contexts. But the explanation should clearly state which statements apply to which model, and which reference frame. The OP makes it quite clear that he wants an explanation for the frame, where the orbiting astronaut is in uniform circular motion. In that frame the net force and thus the acceleration are obviously not zero or near zero.

 

[A.T.]

The total gravitational force is near zero in orbit.

Can you provide a formula to calculate that "total gravitational force" which is near zero for an object in orbit?

 

[p1l0t]

I have no problem with explaining it in different contexts. But the explanation should clearly state which statements apply to which model, and which reference frame. The OP makes it quite clear that he wants an explanation for the frame, where the orbiting astronaut is in uniform circular motion. In that frame the net force and thus the acceleration are obviously not zero or near zero.

GR is more accurate but often Newtonian physics are enough to explain it to someone. And yes the acceleration (g-force) IS practically nil. The only way F = 0 when the mass hasn't changed is if you multiply by (almost) zero.

Can you provide a formula to calculate that "total gravitational force" which is near zero for an object in orbit?

I'm not ignoring this, but my orbital mechanics book is at home and I am at work so let me get back to this later. BTW you can attack my definitions and ask more and more questions until you can find something to trip me up on but I know that an object in orbit is not only in unaccelerated flight, but are weightless because of the microgravity environment, and that g-force/acceleration are two sides of the same coin. Whether or not it has to do with lack of force or opposing forces it doesn't matter they are weightless because they are in a zero-g environment. If you have a better explanation I would like to hear it.

 

[Bandersnatch]

GR is more accurate but often Newtonian physics are enough to explain it to someone. And yes the acceleration (g-force) IS practically nil. The only way F = 0 when the mass hasn't changed is if you multiply by (almost) zero.

I'm not ignoring this, but my orbital mechanics book is at home and I am at work so let me get back to this later.

BTW you can attack my definitions and ask more and more questions until you can find something to trip me up on but I know that an object in orbit is not only in unaccelerated flight, but are weightless because of the microgravity environment, and that g-force/acceleration are two sides of the same coin. Whether or not it has to do with lack of force or opposing forces it doesn't matter they are weightless because they are in a zero-g environment. If you have a better explanation I would like to hear it.

Seriously, this is secondary school-level.

For uniform circular motion over body M at radius R:
F_g=F_c=G \frac{Mm}{R^2}=mRω^2≠0

 

[p1l0t]

Seriously, this is secondary school-level.

For uniform circular motion over body M at radius R:
F_g=F_c=G \frac{Mm}{R^2}=mRω^2≠0

Thanks for that, I don't remember those off the top of my head. Just the simple ones like Newtons Second Law where the net force equals mass times acceleration. Fnet = M * A

 

[technician]

but I know that an object in orbit is not only in unaccelerated flight
This statement is completely wrong. In orbit is the result of a centripetal force/acceleration.

but are weightless because of the microgravity environment,

The only way this statement can make any sense to me is if it relates to astronauts in the orbiting station. If you are born in an orbiting station and spend your life in an orbiting station there is no way that you can experience 'weight' in the conventional way that we mean...something that can be measured on bathroom scales. If astronauts read their textbooks they will find the explanation has something to do with 'free fall' which has something to do with centripetal force.

In basic physics lessons it is required that students can explain the readings on bathroom scales in an accelerating and decellerating lift.
We should be pleased that the physics principles are the same for all.

 

[A.T.]

but are weightless because of the microgravity environment,

The only way this statement can make any sense to me is if it relates to astronauts in the orbiting station.

His statement probably relates to the rest frame of the astronauts center of mass, which in Newtonian context is an non-inertial frame. Therefore there are inertial forces which cancel gravity.

Why is it a bad answer to the OPs question?
1) The OP asked about the frame where the astronaut is in circular motion, if you chose to explain it in a different frame you should explicitly state and justify this.
2) Non-inertial frames and inertial forces up which are not needed to answer the question about a frame independent effect.
3) Being at rest in some frame says nothing about feeling acceleration. In the non-inertial rest frame of an accelerating car the passengers are at rest too, but they do fell the acceleration. The reason is that the force from the seats is applied to their backs only, while gravity is a applied uniformly to the astronaut, as stated on page 1 several times.

 

[p1l0t]

The original post claimed they are accelerating. They are not. They are at a constant speed of probably around 17, 000ish mph

 

[p1l0t]

(Relative to an observer on the surface)

 

[Bandersnatch]

The original post claimed they are accelerating. They are not. They are at a constant speed of probably around 17, 000ish mph

Force, acceleration and velocity are vectors. As such, they can change direction without changing the magnitude. This is exactly the case with uniform circular motion as per OP's question. Constant acceleration at right angle to the velocity vector causes it to change direction but not the magnitude.

This is acceleration.

Take the equation I've provided earlier, and divide everything by m. Now you've got gravitational acceleration equal to centripetal acceleration which is not equal 0.

Additionally, the speed(i.e., the magnitude of the velocity vector), is not necessarily 17k mph. This value is a function of the radius of the orbit, which has not been specified in the OP.

(Relative to an observer on the surface)

Which is an odd choice of reference frame, for reasons explained by A.T. just above your posts.
It is also not true, as the relative speed changes due to the varying angles between the orbiting body's velocity vector and the one of the observer "riding" on the rotating surface.
The only case when it's constant w/r to the surface observer, would be the geostationary orbit(and it'd be equal to 0 then).

 

[p1l0t]

Force, acceleration and velocity are vectors. As such, they can change direction without changing the magnitude. This is exactly the case with uniform circular motion as per OP's question. Constant acceleration at right angle to the velocity vector causes it to change direction but not the magnitude.

This is acceleration.

Take the equation I've provided earlier, and divide everything by m. Now you've got gravitational acceleration equal to centripetal acceleration which is not equal 0.

Additionally, the speed(i.e., the magnitude of the velocity vector), is not necessarily 17k mph. This value is a function of the radius of the orbit, which has not been specified in the OP.Which is an odd choice of reference frame, for reasons explained by A.T. just above your posts.
It is also not true, as the relative speed changes due to the varying angles between the orbiting body's velocity vector and the one of the observer "riding" on the rotating surface.
The only case when it's constant w/r to the surface observer, would be the geostationary orbit(and it'd be equal to 0 then).

It's not EXACTLY zero but the net force IS almost negligible. That is why they are weightless. If they were accelerating they would feel g-force. If they are at a constant speed and not changing altitude which way do you propose they are accelerating? This is why I chose to explain this with Newtonian physics rather than GR because I don't think he cares about time dilation...

 

[Bandersnatch]

If they are at a constant speed and not changing altitude which way do you propose they are accelerating? This is why I chose to explain this with Newtonian physics rather than GR because I don't think he cares about time dilation...

Towards the centre of the Earth, of course. Vectors, remember?
It's pure Newton, too.

It's not EXACTLY zero but the net force IS almost negligible. That is why they are weightless. If they were accelerating they would feel g-force.

3) Being at rest in some frame says nothing about feeling acceleration. In the non-inertial rest frame of an accelerating car the passengers are at rest too, but they do fell the acceleration. The reason is that the force from the seats is applied to their backs only, while gravity is a applied uniformly to the astronaut, as stated on page 1 several times.

 

[p1l0t]

Towards the centre of the Earth, of course. Vectors, remember?
It's pure Newton, too.

Lol then how do they maintain altitude?