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.A.T. said: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.
Can you explain that in terms of F = GMm/r^2 ?p1l0t said:The total gravitational force is near zero in orbit.
Ok. But it is also dependent on there being a force applied to the body to keep it from accelerating due to gravity.Weight is dependent on gravity.
So there is no centripetal acceleration in this circular motion?Gravity is acceleration. If you are going at 17k relative to someone on Earth in LEO you are not accelerating.
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.Chestermiller said: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.
Can you provide a formula to calculate that "total gravitational force" which is near zero for an object in orbit?p1l0t said:The total gravitational force is near zero in orbit.
A.T. said: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. said:Can you provide a formula to calculate that "total gravitational force" which is near zero for an object in orbit?
p1l0t said: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.
Bandersnatch said:Seriously, this is secondary school-level.
For uniform circular motion over body M at radius R:
[tex]F_g=F_c=G \frac{Mm}{R^2}=mRω^2≠0[/tex]
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.technician said: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.
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.p1l0t said:The original post claimed they are accelerating. They are not. They are at a constant speed of probably around 17, 000ish mph
Which is an odd choice of reference frame, for reasons explained by A.T. just above your posts.p1l0t said:(Relative to an observer on the surface)
Bandersnatch said: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).
Towards the centre of the Earth, of course. Vectors, remember?p1l0t said: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...
p1l0t said: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.
A.T. said: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.
Bandersnatch said:Towards the centre of the Earth, of course. Vectors, remember?
It's pure Newton, too.
Well, they have such a high tangential velocity that they "fall" towards the Earth at the same rate as they "fly away".p1l0t said:Lol then how do they maintain altitude?
Bandersnatch said:Well, they have such a high tangential velocity that they "fall" towards the Earth at the same rate as they "fly away".
I don't know what else to tell you, these are not high-level concepts. Should I direct you to some physics book maybe?
Resnick&Halliday Physics part I, chapter 4-4 ("uniform circular motion").
And your answer is... Wait for it...p1l0t said:Yeah so the net force is what? Wait for it...
ZERO!
Doc Al said:And your answer is... Wait for it...
Incorrect!
The term "weightless" is something of a misnomer. Something is "weightless" because there is no supporting force, not because there is no weight. An astronaut in the space shuttle is still being pulled by Earth's gravity. It is Earth's gravity which keeps them in orbit. They feel "weightless" because both astronaut and shuttle are in free fall.p1l0t said:If you are falling away just as fast you are falling in then are you not weightless?
Doc Al said:The term "weightless" is something of a misnomer. Something is "weightless" because there is no supporting force, not because there is no weight. An astronaut in the space shuttle is still being pulled by Earth's gravity. It is Earth's gravity which keeps them in orbit. They feel "weightless" because both astronaut and shuttle are in free fall.
Since they are accelerating (moving in a circle) about the earth, there must be a net force on them. There is--gravity!
You can experience "weightlessness", albeit for a short period, by jumping off a cliff. Your weight--the pull of gravity--doesn't disappear.
There is more to acceleration than changing speed. As has already been mentioned, changing direction of motion is also acceleration. You need to learn a little physics before being so adamant with your opinions.p1l0t said:You still have mass, yes. Weight, no. They are not accelerating. They are at a constant speed. If they are accelerating they would have weight. How much force does it take to move an astronaut? Practically none because he is weightless. F = M * 0
Doc Al said:There is more to acceleration than changing speed. As has already been mentioned, changing direction of motion is also acceleration. You need to learn a little physics before being so adamant with your opinions.
Look up centripetal acceleration. Something moving in a circle is accelerating and that requires a force. (Try driving your car in a circle on a patch of ice. No friction to provide the centripetal acceleration, so you won't be able to turn.)
Mordred said:I highly suggest you google Einsteins equivelence principle. Bandersnatch an Doc have been giving you the correct answers. So maybe after reading up on the equivelence principle will aid your understanding
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.]
..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. ]
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...
... Accelerometers, can only detect g-force i.e. weight2 (= mass x proper acceleration) They cannot detect free fall.
An object in uniform circular motion undergoes centripetal acceleration towards the center.p1l0t said:If they are at a constant speed and not changing altitude which way do you propose they are accelerating?
That's great, but first you should learn Newtonian physics.p1l0t said:This is why I chose to explain this with Newtonian physics...
A.T. said: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.
Did you check Newton's 1st Law? Does the astronaut move in a straight line? No? Then it is accelerating.p1l0t said:How does it accelerate towards the center while not actually losing any altitude?
It makes perfect sense:p1l0t said:That doesn't make any sense at all.
Microgravity has nothing to do with it. It refers to negligible tidal effects.p1l0t said:I can see if the forces are in equilibrium but that's microgravity.
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.p1l0t said:How does it accelerate towards the center while not actually losing any altitude?...They are weightless because of the lack of acceleration.
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 said: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 said: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.
Read the 1st post again. He's talking about the reference frame where the astronaut is in uniform circular motion and undergoes centripetal acceleration.p1l0t said:Considering I don't know what reference frame he is talking about, your probably right.
That's a frame independent fact.p1l0t said:But part of his question was why do they "feel" no force.