# Why are astronauts weightless?

#### BobG

Homework Helper
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).

#### mikeph

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.

#### p1l0t

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?

#### Mordred

If you were to dangle a man on ropes with his feet barely touching a scale would you say hes 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:

#### p1l0t

If you were to dangle a man on ropes with his feet barely touching a scale would you say hes 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.

#### Mordred

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

#### p1l0t

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.

#### Mordred

The latter forget microgravity

#### ZVdP

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.

#### p1l0t

So it feels like pushing 180lbs not <1lb if you pushed an astronaut that weighs 200lbs here in Earth but in LEO?

#### Bandersnatch

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.

#### technician

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?

#### p1l0t

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.

#### Dewgale

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.

#### technician

Ps. I think this thread will get to 100 posts before someone stops it.

Last edited:

#### technician

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

Gold Member
HUH??

Dave

#### Dewgale

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).

#### p1l0t

This is what I was saying earlier.. without the math to back it up.

#### technician

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

#### technician

This is what I was saying earlier.. without the math to back it up.
And it is wrong

#### Dewgale

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.

#### Dewgale

I'm not totally sure how that resolves the issue.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving