Why Do Astronauts Feel Weightless in Orbit?

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Discussion Overview

The discussion revolves around the phenomenon of weightlessness experienced by astronauts in orbit. Participants explore the underlying physics, including concepts of gravity, acceleration, and the perception of force, while addressing both theoretical and practical implications of weightlessness.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants argue that astronauts in orbit experience centripetal acceleration but do not feel any force due to the lack of a supporting surface pushing against them.
  • Others propose that weightlessness can also be experienced outside of orbit, such as during a free fall from a height, where gravity still acts but is not felt due to the absence of a counteracting force.
  • A few participants emphasize that while astronauts appear weightless, they are not truly weightless since gravitational forces are still acting on them, leading to a discussion on the nature of weight and force perception.
  • Some contributions highlight the role of the inner ear and body mechanics in perceiving acceleration, suggesting that uniform acceleration does not create a sensation of force.
  • There are differing views on the implications of General Relativity in understanding weightlessness, with some participants cautioning against introducing it into the discussion as it may complicate the explanation.
  • Participants also draw analogies, such as comparing astronauts to fish floating in water, to illustrate the concept of weightlessness and the absence of net forces.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of weightlessness and the role of gravitational forces, indicating that the discussion remains unresolved with no clear consensus.

Contextual Notes

Some claims depend on specific definitions of weight and force, and there are unresolved questions regarding the perception of acceleration and the effects of gravitational gradients on the human body.

  • #91
Dewgale said:
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.
You do not need to 'overcome' some force to cause acceleration. You need a (resultant) force if you want to cause a MASS to accelerate.

Basic physics F = ma...F is the resultant force
 
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  • #92
technician said:
Fake video of students messing about instead of reading their physics textbooks!

HUH??

Dave
 
  • #93
technician said:
You do not need to 'overcome' some force to cause acceleration. You need a (resultant) force if you want to cause a MASS to accelerate.

Basic physics F = ma...F is the resultant force

You misunderstand me.

If, on earth, you want to accelerate a 200 Kg (I'm using Kg, since it's more convenient) object 1 m/s2, you obviously need a net resultant force of 200 N. However, that's not the amount of force your body would actually have to exert on the object. Since the force of gravity is Fg=mg, the weight of the object is 1960 N. In order to accelerate the object 1 m/s2, you would need to exert 200 N + Fg, or 2160 N.

However, in space, due to the effects of gravity being minimal, we can discount it, and say that it requires only 200 N of force to accelerate a 200 Kg object by 1 m/s2.

So, yes, it would be a lot easier to move a 200 Kg object in space than it would be on earth, at least with respect to pushing it in the upwards direction (or lack thereof, since there's no "up" in space).
 
  • #94
This is what I was saying earlier.. without the math to back it up.
 
  • #95
To accelerate a mass of 200kg at 1 m/s2 needs a resultant force of 200N.
No more, no less
It needs exactly the same in the space station
 
  • #96
p1l0t said:
This is what I was saying earlier.. without the math to back it up.

And it is wrong
 
  • #97
technician said:
To accelerate a mass of 200kg at 1 m/s2 needs a resultant force of 200N.
No more, no less
It needs exactly the same in the space station

If you exert 200 N upwards on a 200 Kg object on Earth, you'll find it being pulled down with a force of 1760 N.

You're thinking of Fnet, whereas I'm talking about Fa. In space in this case, Fa=Fnet, since there is no Fg. So, in the space station, Fa would be 200, while on Earth Fa would be 2160 N.

In both cases Fnet is 200 N, but they would feel a hell of a lot different.
 
  • #99
I'm not totally sure how that resolves the issue.
 
  • #101
Disregard that last post my thinking was off. Was using my phone on top of a mountain doing tower work. Needless to say long day lol
 
  • #102
Congratulations everyone...100 posts.
Isaac Newton would be impressed
 
  • #103
103.. :)
 
  • #104
Why are astronauts weightless.
Because the speed at which they are moving around the planet is balanced by the centerpetel effect.
For example there is a point where the astronauts speed is just right to maintain orbit without falling to Earth if too slow or speeding off into space if too fast.
When either of these undesired speeds are maintained then weight will be felt by the astronauts.
When the speed is balanced and just right they maintain orbit and no weight is felt.
If a ring of unobtanium was placed stationary around the Earth a mile above it a person standing on that ring would feel and weigh much the same as someone stood on Earth. If you were to start rotateing the ring with the person on it his weight would get less and less as the ring rotated faster and faster until there was no weight registered on the scale.
 
  • #105
Buckleymanor said:
Why are astronauts weightless.
Because the speed at which they are moving around the planet is balanced by the centerpetel effect.
For example there is a point where the astronauts speed is just right to maintain orbit without falling to Earth if too slow or speeding off into space if too fast.
When either of these undesired speeds are maintained then weight will be felt by the astronauts.
When the speed is balanced and just right they maintain orbit and no weight is felt.
Nope. No special speed is needed for the astronauts to be "weightless". All that is needed is for them to be in free fall.
 
  • #106
Doc Al said:
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.
 
  • #107
Buckleymanor said:
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.
 
  • #108
Buckleymanor said:
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.
 
  • #109
Dewgale said:
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.
 
  • #110
Doc Al said:
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.
 
  • #111
Buckleymanor said:
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.
 
  • #112
Buckleymanor said:
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.
 
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  • #113
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.
 
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  • #114
I'm pretty sure he meant after the initial acceleration. He also did note minus wind resistance.
 
  • #115
A.T. said:
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.
 
  • #116
p1l0t said:
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.
 
  • #117
Buckleymanor said:
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!
 
  • #118
117 posts ! everything has been said over and over again. Nothing new is being discussed
 
  • #119
technician said:
everything has been said
but not yet by everyone
 
  • #120
ok, I'll continue to watch from the sidelines and keep count.
 

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