Weightlessness (1 Viewer)

[Chinmoku03]

Could someone explain weightlessness to me? I've read thru some of the common study guides (Macquarie, Excel) and textbooks, (Phys Context, Jacaranda) but none of the explanations made sense to me. I get the formula, just need an explanation.

Thanks in advance.

 

[alcalder]

Well, there is "weightlessness" and there is weightlessness. If you were in space away from any large body, there would be no gravity acting you (to speak of) and hence you would have zero weight.

Now, the "weightlessness" astronauts/cosmonauts feel is due to something else entirely. They still have the Earth's gravitya cting upon them, effectively pulling them freefall back towards the Earth (in a vertical direction). However, their motion horizontal is such that while they are freefalling vertically they keep missing the Earth and continue on around.

Douglas Adams referred to such a state as flying because the person fell and missed the ground (however, the person missed from much closer proximity).

Skydivers also feel this weightlessness but because they have the immense wind resistance it doesn't really feel like it. BUT the training astronauts go up in the throw-up plane which flies up and then down in freefall. Effectively everyone inside the plane feels weightless because the environment, air and plane around them is also freefalling. So, they feel weightless and because there is nothing providing a reaction force to gravity (ie the ground) they effectively have no weight.

Does that help?

 

[Chinmoku03]

Yeah, the weightlessness I mean is the one where astronauts only feel weightless, not that they're actually weightless.

I'm looking at the second paragraph atm. So, if the astronauts are freefalling, I guess they are falling towards the centre of the Earth with the same speed as a skydiver would? (Minus wind resistance) And what is this horizontal motion that keeps astronauts from falling towards the centre? Is it the veolcity of the rocket that's travelling at around about the orbital velocity of the Earth?

Also, in the fourth paragraph, you mentioned that astronauts feel weightless because the environment around them is freefalling towards Earth with them. How come the environment around skydivers don't freefall towards Earth with them? What's keeping the atmosphere up there instead of falling down? I know it might sound a bit off topic, but I have this really annoying habit of knowing details -.-;

 

[alcalder]

Yeah, the weightlessness I mean is the one where astronauts only feel weightless, not that they're actually weightless.

I'm looking at the second paragraph atm. So, if the astronauts are freefalling, I guess they are falling towards the centre of the Earth with the same speed as a skydiver would? (Minus wind resistance) And what is this horizontal motion that keeps astronauts from falling towards the centre? Is it the veolcity of the rocket that's travelling at around about the orbital velocity of the Earth?

Yes, they are falling towards the centre of the Earth with the same speed as a skydiver - that is the vertical component of their motion - but there is also a horizontal component which is the velocity of the rocket or space station or whatever they are in, exactly.

Also, in the fourth paragraph, you mentioned that astronauts feel weightless because the environment around them is freefalling towards Earth with them. How come the environment around skydivers don't freefall towards Earth with them? What's keeping the atmosphere up there instead of falling down? I know it might sound a bit off topic, but I have this really annoying habit of knowing details -.-;

The air inside a plane/rocket/space station forms a closed environment, as does that in your car. So when you are travelling at a constant speed in your car you are now in an intertial frame of reference and it feels as though you are not moving at all (but you know you are moving by looking out the window of the car. Thus the astronaut in the plane/rocket/space station is in the other frame of reference and so compares his motion to what is around him. The skydiver is not in an enclosed environment and is actually falling through an external environment that interacts with him - air resistance. The air inside his lungs travels with him, but unless he traps the air around him in a bubble, he will move through it and it does not travel with him as much. Some may follow him (eddies of air and such) but the air all slips on by.

What keeps the atmosphere up there? Well, remember gas likes to fill all available space. It is acted upon by gravity, too, and thus held to the Earth but it also spreads out as much as it can.

 

[Chinmoku03]

Yes, they are falling towards the centre of the Earth with the same speed as a skydiver - that is the vertical component of their motion - but there is also a horizontal component which is the velocity of the rocket or space station or whatever they are in, exactly.

Alright, but what makes this horizontal component so special that it prevents the astronauts from freefalling towards Earth's centre? Are the astronauts and their environment undergoing the same orbital velocity that keeps them afloat up there or something?

 

[kony]

okay let me try to explain this one:

first let me explain something you'll need later. if an object is at a single height above the ground, and is suspended there (whether on top of a table, or just floating there), the entire system does not experience a net acceleration. this means that when you add all the forces which act upon the object, the net force is 0.

therefore, if an object is accelerating down, the net force is not 0 but is directed towards the centre of the earth.

ok onto weight.

when we feel weight, we don't actually feel the force of being accelerated towards the centre of the earth. the weight we feel is the force that the ground exerts on us. if we draw a picture, there would be an arrow pointing down that is our weight force, mg, and the reaction force, N, which points up.

the weight we feel is N.

now suppose we are in a lift, and the cable has been cut. we are falling towards the centre of the earth. from my first idea, this means the net force is mg towards the centre of the earth!

if we think about the forces that are actually acting: we can suppose we are accelerating down at g, causing weight force mg. the floor of the lift provides a reaction force, which is N, which is equal to mg. so far net force is 0. now we add another mg since the entire system is falling.

ok if we think about the force arrows, the 2 mgs are pointed down. N is pointed up. as said before, we dont actually feel "weight" but the force exerted on us by the ground. therefore the force here is N - mg = 0 => hence weightlessness.

in an orbit, we are in effect in free fall. don't think of it as being at one altitude, because we're not really, when compared to the sun. so it is like being in a lift that is continuously free falling. therefore weightlessness.

that was long! anyway this is how i think of it, i realise the N - mg = 0 part is a little weird. i have to ask my teacher ..

 

[alcalder]

Alright, but what makes this horizontal component so special that it prevents the astronauts from freefalling towards Earth's centre? Are the astronauts and their environment undergoing the same orbital velocity that keeps them afloat up there or something?

Let's imagine a flat Earth (as many still do today *snigger*). Then if a person were falling to Earth, towards the Earth's centre, and had horizontal and vertical components of motion then they would hit the Earth. Essentially this is like throwing a ball off a cliff and the vertical component of motion is caused by the Earth's gravity and the horizontal (and constant) motion is due to you throwing the ball.

Now, the Earth is actually round (I know, weird concept for some ), so when you throw that ball off the cliff with a horizontal speed, if you throw it fast enough it will not hit the Earth but miss it and keep on missing it even though the Earth's gravity keeps trying to suck it in.

So astronauts in a space ship are essentially continually falling back to Earth but their thrusters are giving them enough speed to keep missing the Earth.

Does that help?

 

[Chinmoku03]

Ok, so to sum up, weightlessness is when an object is freefalling towards Earth, but the horizontal velocity (Is this velocity any value greater than v = sqrt (GM/r)? No one has replied to this yet, I think) is great enough to resist the gravitational field strength of the Earth. And since the environment around the object is freefalling at the same rate with the object, the object appears to be not moving. Is that right?

Also, how does freefalling contribute to feeling weightless? I get that they feel like they're not moving, but people in a car travelling at constant velocity aren't moving either. Why do the astronauts "float" in their spaceship while people in cars don't?

 

[Kirjava]

EDIT: Subscripts were not working, changed algebraic symbols.

Ok, I had a little play with this. This was the result:


m = mass of the earth
o = mass of the object
a = acceleration towards the earth
v = velocity of the object (perpendicular to a)
G = gravitational constant
r = radius of the orbit
g = force of gravity acting on the object



g = Gmo/r^2 (Universal Law of Gravitation)


F = oa (Newtons Second Law)


Therefore Gmo/r^2 = oa


Gm/r^2 = a


Radial acceleration of an object in a stable orbit is given by v^2/r


Therefore a = v^2/r = Gme/r^2


v^2 = Gme/r


v = (Gme/r)^1/2


So that’s the object’s instantaneous velocity at a given orbit, just what you suggested. Please be swift to point out any errors .

So, how does free falling contribute to the “feeling” of weightlessness? Well, our bodies are quite used to being under the influence of gravity, or more specifically being “squashed” by the force of gravity into the ground (which pushes back up on us, we don’t fall through the ground thankfully =p). In free fall the force of gravity is still acting on our bodies, but there is nothing to oppose it- and consequently all your organs are left to float around inside you (hopefully not too much). A person moving horizontally in a car doesn’t feel weightless because the force of gravity is still opposed by the reaction force of the seat acting on your bum (you’re still being “squashed" into the seat). This changes if the motion of the car isn’t perfectly perpendicular to the force of gravity (i.e. horizontal), but hopefully that will have answered your question.

Actually, on second thoughts I'm beginning to wonder whether the feeling of weightlessness is actually caused by the acceleration downwards rather than what I stated above. Do you think skydivers at terminal velocity would still feel as though
their stomachs were setting up camp in their mouths? I'm not sure any more...

 

[bos1234]

Let's imagine a flat Earth (as many still do today *snigger*).

is it true?

 

[bos1234]

this is from http://science.howstuffworks.com/weightlessness1.htm

Weightlessness is more correctly termed microgravity. You are not actually weightless, because the Earth's gravity is holding you and everything in the shuttle in orbit. You are actually in a state of free-fall, much like jumping from an airplane except that you are moving so fast horizontally (5 miles per second or 8 kilometers per second) that, as you fall, you never touch the ground because the Earth curves away from you. It's like this: When you stand on a bathroom scale, it measures your weight because gravity pulls down on you and the scale. Because the scale is resting on the ground, it pushes up on you with an equal force -- this equal force is your weight. However, if you were to jump off a cliff while standing on a bathroom scale, both you and the scale would be pulled down equally by gravity. You would not push on the scale and it would not push back against you. Therefore, your weight would read zero.

 

[kurt.physics]

I'll just have a jab at this one (mathematically)

say your in an elevator on some scales, S, and you have a mass, m, mass being the amount of matter inside you

There will be a force pulling down on you, mass multiplyed by gravity, mg, there would also be a force equil to mg but in the opposite direction, Fs (because if it wasnt equil the elevator would be accelerating!)

This is, mathematically defined as weight, a non mathematical definition would be

Weight is the pressence of a body in a gravitational field i.e. Earth

Now, lets say that you accelerate the elevator up in a negative direction

Fs must be greater than mg or it wouldnt accelerate, Fs>mg. Think of it as a super fat guy, the elevator has to push, or exert a force, greater than the fat man or he wont budge.

using newtons law with Fs being > than mg

Fs - mg = ma

Fs = m(a + g).............................equation 1

Now lets say that your mass, m, is 50kgs and you are accelerating upwards at approx 9.8, but because acceleration is in a negative direction then a = -9.8

Fs = 50(-9.8 + 9.8)
= 50(0)
= 0 Newtons i.e. Weightless (or shall i say "percieved" weightlessness)

Now lets do this but in the opposite direction
mg>Fs or we couldn't go down. Again use the fat guy senario. Say this time that the fat guy is so heavy that he breakes the cable, because his mg is greater that the force of the elevator, and he goes into free-fall.

so,
mg>Fs

useing newtons law

mg - Fs = ma
Fs = m(g - a)............................equation 2

so if the fat mans mass, m, is 1500kgs and is in free fall then,

Fs = m(g - a)
= 1500(9.8 - 9.8)
= 1500(0)
Fs = 0

Again percieved weightlessness
We could have just used equation 1 and made the acceleration in a negative direction

m(g + a)

= 1500(9.8 + -9.8)
= 1500(9.8 - 9.8)
= 1500(0)
= 0

That is percieved weightlessness but "true" weightlessness is when you are not under the influence of the gravitational force.

Thankyou,

Kurt Yr. 8 2007,
F*@k yehh

appoligies if i made a mistake, but im (100 - dx)% sure.

*dx being an infitesimal bit of x and x being 100 (Calculus reference)
​

 

[Looking Glass]

There's a way of phrasing it that helped me, which I didn't see anybody use here (might've missed it). That is that your horizontal velocity is such that the Earth is continually falling away from you as you fall towards it. Consider how the trajectorial parabola keeps extending as you throw a ball harder and harder. Eventually there is a velocity that locks it into orbit.

 

[hkdrmark]

you guys have clearly explained what is freefalling, but not weightlessness

Alcalder have explained that, but not simple enough

In fact, when the astronaut was in a capsule travelling around the Earth in a circular orbit with speed V, there must be a force preventing it from escaping from the earth. The force doesn't change the magnitude of V, but the direction.

that force is just F=mv^2/r

ok, then where does that force come from??

it comes from the gravitational force of the Earth.

In a circular motion, the gravitational force is used up to change the direction of the V.

OK, that means the gravitational force is no longer used to drag the capsule towards the Earth, but to change the direction of the V only.

Then , there will be no force acting on the astronaut to pull him towards the floor inside the capsule, hence no reaction force is acting on him , then he feels that he is weightlessness.

 

[helper]

you guys have clearly explained what is freefalling, but not weightlessness

In terms of physics weightlessness only exists if you leave the gravitational field, which doesn't ever occur.

Alcalder have explained that, but not simple enough

Maybe for you but is correct unlike what you wrote

In fact, when the astronaut was in a capsule travelling around the Earth in a circular orbit with speed V, there must be a force preventing it from escaping from the earth. The force doesn't change the magnitude of V, but the direction.
that force is just F=mv^2/r


ok, then where does that force come from??

it comes from the gravitational force of the Earth.

True

In a circular motion, the gravitational force is used up to change the direction of the V.

OK, that means the gravitational force is no longer used to drag the capsule towards the Earth, but to change the direction of the V only.

Wrong. The gravitational force always is pulling (dragging) you towards the center of the Earth by definition. If it wasn't you wouldn't be in circular motion.

Then , there will be no force acting on the astronaut to pull him towards the floor inside the capsule, hence no reaction force is acting on him , then he feels that he is weightlessness.

The force of gravity is stilling pulling him into the centre. Because the spacecraft is falling at the same rate as the astronaut there is no force of the floor acting on his feet, so no force is felt from this and the person appears weightless but isn't really.

 

[hkdrmark]

In terms of physics weightlessness only exists if you leave the gravitational field, which doesn't ever occur.


Maybe for you but is correct unlike what you wrote


True



Wrong. The gravitational force always is pulling (dragging) you towards the center of the Earth by definition. If it wasn't you wouldn't be in circular motion.



The force of gravity is stilling pulling him into the centre. Because the spacecraft is falling at the same rate as the astronaut there is no force of the floor acting on his feet, so no force is felt from this and the person appears weightless but isn't really.

thx for telling me the truth