Why Do Astronauts Feel Weightless in Orbit?

[Buckleymanor]

Nope. No special speed is needed for the astronauts to be "weightless". All that is needed is for them to be in free fall.

Well what is free fall if it's not a special speed. If you were falling at a rate faster or slower than free fall would you not feel some kind of weight.

 

[Doc Al]

Well what is free fall if it's not a special speed. If you were falling at a rate faster or slower than free fall would you not feel some kind of weight.

Free fall is not a special speed. Free fall means that the only force acting is gravity.

 

[Dewgale]

Well what is free fall if it's not a special speed. If you were falling at a rate faster or slower than free fall would you not feel some kind of weight.

Velocity in free-fall is determined by the force of gravity, as that is the only force on you.

Refer to the following formula.

$V=\sqrt{\frac{GM}{r^{2}}}$

An object in free fall's velocity is determined solely by its distance from the object it's orbiting, and the mass of the body being orbited unless it is acted upon by an outside force, in which case it's no longer in free fall, which makes the whole thing a moot point.

 

[A.T.]

Velocity in free-fall is determined by the force of gravity, as that is the only force on you.

Forces determine acceleration, not velocity.

An object in free fall's velocity is determined solely by its distance from the object it's orbiting

Free fall is more general than circular orbits.

 

[Buckleymanor]

Free fall is not a special speed. Free fall means that the only force acting is gravity.

If that is the case then is the second part of my reply true or false.
If it's true that a body falling at a faster or slower rate than the rate of acceleration due to gravity feels an effect caused by gravity acting upon it.Then we just disagree about what the meaning of special is with regards to free fall.
If it's false then I have to agree that free fall is not a special speed.

 

[jbriggs444]

If that is the case then is the second part of my reply true or false.
If it's true that a body falling at a faster or slower rate than the rate of acceleration due to gravity feels an effect caused by gravity acting upon it.

The second part in question being, I think:

If you were falling at a rate faster or slower than free fall would you not feel some kind of weight.

If you were in a uniform circular orbit and if your speed in this orbit were faster or slower than that which would make centripetal acceleration and gravitational acceleration match then there would have to be some other force, in addition to gravity, keeping you on this circular trajectory.

You would feel some kind of weight due to that other force. Not due to gravity.

If, on the other hand, you were moving horizontally near the earth, if the only force acting on you were gravity and if your speed were faster or slower than that which would put you in a circular orbit then you would still be in free fall. You would follow an elliptical, parabolic or hyperbolic trajectory.

But you would not feel any kind of weight.

Take this a step farther. If you are sitting at your desk, you are in a circular orbit around the Earth at a rate of roughly one revolution per 24 hours. The force of gravity is pulling you down. Compressive and rigid forces in the Earth's crust, mantle and core are holding you up. As others have pointed out, the weight you feel is due to the forces holding you up.

 

[Doc Al]

If that is the case then is the second part of my reply true or false.

You mean that if you are accelerating at at greater or lesser rate that the rate of acceleration due to gravity, do you feel some force acting upon you? Of course, since something other than gravity must be acting on you. (I see that jbriggs444 has addressed that.)

If it's true that a body falling at a faster or slower rate than the rate of acceleration due to gravity feels an effect caused by gravity acting upon it.Then we just disagree about what the meaning of special is with regards to free fall.
If it's false then I have to agree that free fall is not a special speed.

If you think there's something special about being in orbit that creates weightlessness, then you are wrong. Want to be weightless? Get shot out of a cannon! (Actually, that's not really weightless as there will be air resistance.)

As long as gravity is the only force acting, you will experience "weightlessness". Doesn't matter if you're in orbit or not. Consider the "vomit comet" used to train astronauts. That's never in orbit.

 

[Buckleymanor]

If you think there's something special about being in orbit that creates weightlessness, then you are wrong. Want to be weightless? Get shot out of a cannon! (Actually, that's not really weightless as there will be air resistance.)

If you were shot out of a cannon you would feel weight as you accelerated out of it.
The same as if you were in a lift going upwards.Coming down is a different matter.
If you were born in orbit and maintained it you never experience weight.
Being shot out of a cannon or going up and coming down in a lift you would.So there is something special about being in orbit you are unable to experience your own weight.

 

[p1l0t]

I'm pretty sure he meant after the initial acceleration. He also did note minus wind resistance.

 

[Dewgale]

Forces determine acceleration, not velocity.

That's true; however, the acceleration in the case of orbit is centripetal, and so velocity can be determined by the formula I posted above.

Free fall is more general than circular orbits.

Agreed. I thought he was just talking about orbital freefall. If he's talking about free-fall in general, then disregard what I said; it doesn't apply in those scenarios.

 

[Buckleymanor]

I'm pretty sure he meant after the initial acceleration. He also did note minus wind resistance.

Wind resistance noted.If he meant after the initial acceleration it would have been easier to say, throw oneself of a building which might sound a bit impolite.
Reagardless out of a cannon of a building or on board the vomit comet there is no difference than being in orbit apart from the amount of time you are weightless.
So it's not so so special.

 

[Doc Al]

Wind resistance noted.If he meant after the initial acceleration it would have been easier to say, throw oneself of a building which might sound a bit impolite.

I had mentioned jumping off a cliff way back in post #55.

Reagardless out of a cannon of a building or on board the vomit comet there is no difference than being in orbit apart from the amount of time you are weightless.
So it's not so so special.

Good!

 

[technician]

117 posts ! everything has been said over and over again. Nothing new is being discussed

 

[A.T.]

everything has been said

but not yet by everyone

 

[technician]

ok, I'll continue to watch from the sidelines and keep count.

 

[p1l0t]

117 posts ! everything has been said over and over again. Nothing new is being discussed

Actually I kind of like where this is going. We seem to be finding common ground and working from there to further our understanding.

 

[technician]

Actually I kind of like where this is going. We seem to be finding common ground and working from there to further our understanding.

I agree, I learn something every time I drop in here, long may it continue
Got to go now

 

[Buckleymanor]

If, on the other hand, you were moving horizontally near the earth, if the only force acting on you were gravity and if your speed were faster or slower than that which would put you in a circular orbit then you would still be in free fall. You would follow an elliptical, parabolic or hyperbolic trajectory.

But you would not feel any kind of weight.

As a matter of interest your speed might be faster or slower than that which put you in a circular orbit but would it remain constant if it followed an elliptical, parabolic or hyperbolic path.

 

[jbriggs444]

As a matter of interest your speed might be faster or slower than that which put you in a circular orbit but would it remain constant if it followed an elliptical, parabolic or hyperbolic path.

No.

Assuming that the only force that is present is gravity, then an object whose path takes it farther away from the Earth would slow down and an object whose path takes it closer to the Earth would speed up.

 

[Doc Al]

As a matter of interest your speed might be faster or slower than that which put you in a circular orbit but would it remain constant if it followed an elliptical, parabolic or hyperbolic path.

No, the speed would not remain constant. It varies with the distance from the earth. As that distance increases, gravitational potential energy goes up and kinetic energy, and thus speed, goes down.

 

[Buckleymanor]

No, the speed would not remain constant. It varies with the distance from the earth. As that distance increases, gravitational potential energy goes up and kinetic energy, and thus speed, goes down.

I don't understand how this can be true if speed changes.

If, on the other hand, you were moving horizontally near the earth, if the only force acting on you were gravity and if your speed were faster or slower than that which would put you in a circular orbit then you would still be in free fall. You would follow an elliptical, parabolic or hyperbolic trajectory.

But you would not feel any kind of weight.

If your speed changes then you accelerate and decelerate or vise versa how come you would not feel any kind of weight.

 

[A.T.]

If your speed changes then you accelerate and decelerate or vise versa how come you would not feel any kind of weight.

Read page 1 (post #4, #7). This thread is already repetitive enough.

 

[p1l0t]

I don't understand how this can be true if speed changes.

If your speed changes then you accelerate and decelerate or vise versa how come you would not feel any kind of weight.

I know where your coming from. W = kg is basically F = ma. I think it depends on the reference frame though. The astronaut may not feel any acceleration but relative to Earth he is.

 

[Doc Al]

I don't understand how this can be true if speed changes.

If your speed changes then you accelerate and decelerate or vise versa how come you would not feel any kind of weight.

Please take A.T.'s advice and read this thread from the beginning. Acceleration does not matter as long as gravity is the only force acting.

 

[Buckleymanor]

Please take A.T.'s advice and read this thread from the beginning. Acceleration does not matter as long as gravity is the only force acting.

Sorry for the late reply but I could not get access to the beginning of the thread for some time.
From what I can gather acceleration does not matter as long as the object is small.
For example if you were to have a very large accelerometer traveling in an eliptical path around the Earth it would register gravitational changes.
So is the argument only about size.

 

[A.T.]

For example if you were to have a very large accelerometer traveling in an eliptical path around the Earth it would register gravitational changes.

I would't call it an accelerometer, but a tidometer.

 

[sophiecentaur]

Microgravity keeps popping up here and it has some significance for objects in orbit.
You could detect whether you were in orbit (significantly close to a large object) or just floating out in deep deep space because of the presence of microgravity whilst in orbit, which would reveal itself as detectable 'weight forces' by an accelerometer, for instance, placed on the innermost wall and outermost walls - a gradient of force across the width of the craft. The inner and outer parts of the craft will be going at the 'wrong' speeds to maintain a circular orbit (with a period which is only correct for the CM orbital radius) so there would be detectable 'outward' forces against the walls (and the equal and opposite reaction forces, of course).