Why are astronauts weightless?

  • Thread starter Forge
  • Start date
In summary, if an object is orbiting Earth, it is experiencing a centripetal force and is constantly accelerating.
  • #71
Bandersnatch said:
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.
 
Physics news on Phys.org
  • #72
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.)
 
  • #73
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..
 
  • #74
p1l0t said:
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.
 
  • #75
MikeyW said:
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.
 
  • #76
p1l0t said:
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).
 
  • #77
p1l0t said:
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.
 
  • #78
MikeyW said:
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?
 
  • #79
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
 
Last edited:
  • #80
Mordred said:
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.
 
  • #81
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
 
  • #82
Mordred said:
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.
 
  • #83
The latter forget microgravity
 
  • #84
Mordred said:
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.
 
  • #85
So it feels like pushing 180lbs not <1lb if you pushed an astronaut that weighs 200lbs here in Earth but in LEO?
 
  • #86
p1l0t said:
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.
 
  • #87
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?
 
  • #88
Bandersnatch said:
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.

original.gif
 
  • #89
Bandersnatch said:
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.
 
  • #90
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.
 
Last edited:
  • #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
 
  • #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.
 

Similar threads

Acceleration in uniform circular motion is uniform or non-uniform?
    • Introductory Physics Homework Help
2
Replies
55
Views
615
B Confusion while trying to build intuition of centripetal force
    • Mechanics
Replies
15
Views
2K
What happens when a car turns?
    • Mechanics
Replies
16
Views
938
I Why can centripetal force balance out gravity?
    • Mechanics
Replies
23
Views
1K
B Gravitational acceleration in circular motion
    • Mechanics
Replies
2
Views
917
I Direction of friction of each wheel and total moment when a car turns
    • Mechanics
Replies
6
Views
889
B Why do we feel gravitational acceleration from the Earth and not from the Sun?
    • Mechanics
Replies
5
Views
1K
Circular motion of a particle around a track -- what provides the centripital acceleration?
    • Mechanics
2
Replies
37
Views
2K
I Remark on centrifugal force in Heino Falcke's black hole book
    • Mechanics
Replies
22
Views
1K
B What is the effect of the twin paradoxon?
    • Special and General Relativity
3
Replies
71
Views
3K

Hot Threads

Recent Insights

Back
Top