Almost exactly. What I need to show is that he has experienced the same feeling that astronauts feel on the space station. That falling is space and falling on earth is the same thing. And that that floating feeling he feels in his stomach on a rollercoaster, elevator, or space station is due not the lack of gravity, but to the act of free-falling.
Weight is the force I exert on the ground. If the situation is more complicated than that the the physicist talks about forces with vectors and they don’t talk about weight.
Answered in #2 in my first post.
In order to “fall” you need gravity to pull you. The roller coaster and elevator and space station are all falling because gravity is pulling them down. Just in the case of the space station it will fall forever and never meet the ground (well technically without an occasional push it would eventually crash into the Earth but that needlessly complicates things for this where we can say it will fall forever).
When you and the scale you want to weigh yourself with are both falling at the same rate (same reference frame) you are weightless. Gravity is still there though…it is the thing making you and the scale fall.
gazpacho, there are all sorts of situations in which the meaning of a word within a scientific discipline differs from its everyday usage. In some cases one can perhaps ‘agree to disagree’, and allow both meanings within ‘non-overlapping magisteria’ (and how I loathe that phrase in its more usual context… but I digress). But when one is discussing a phenomenon within that branch of science, it is essential to get the terms correct. If someone talks to me about a guy’s ‘force of personality’, I won’t bat an eyelid, I’ll accept the metaphor. But if they claim that the guy attracts hot babes to him because that’s what forces do, what about gravity huh, then there will be tears.
“Weight” as it might be used in everyday conversation could well be different from “weight” as used by physicists, but if you want to understand a physical problem, like, oh I dunno, somebody orbiting the Earth in a space shuttle (forgive me if I’m derailing the thread too much), you must replace whatever previous understanding you had of the technical word, with its technical definition.
If you don’t, you will NEVER grasp what’s actually going on.
ETA: yes, the physicists do indeed talk about forces with vectors. One of those vectors is… WEIGHT.
Slight hijack:
Do astronauts report that they get used to that free-falling feeling at some point? I think of the folks that go up in space stations for extended stays … that pit-of-the-stomach feeling would suck for months at a stretch.
Apparentweightless.
It’ll never catch on, but still…
The feeling in your tummy is due to the initial acceleration. You can feel it in an elevator. Once it all settles down into equilibrium there is no tickle in your tummy.
Not sure about vertigo and stuff like that though. I imagine your inner ear gets a bit turned around. Not sure if that settles down.
Well…from a purely physics POV, you are correct.
That said, the word “weight” – or at least it’s early English roots – go back to well before our Newtonian concept of gravity and weight. For instance, the OED gives these cites:
The modern English word “weight” carries with it multiple denotations. And in fact, the primary definition given by the OED is this:
And, for that matter, the word “gravity” was in use well before it came to have a specific scientific meaning, as well. The reality, from a historical point of view, is that the modern scientific meaning of “weight” is the interloper, so to speak. It is the consequence of a long and rocky process of trying to integrate conventional terminology into an entirely new framework. I again refer to a rather apt citation from the OED:
So you can see that nearly 120 years after Principia, science was still struggling to come up with a well justifiable meaning for “weight” in the scientific context. Even now, the term is not applied as consistently as I think you’re making it out. For instance, terms like “atomic mass” and “atomic weight” – there is a difference between these terms, but it has nothing to do with mass vs. weight.
In other words, they were here first. The modern scientific meaning is a bit of neologism. Science broke with tradition on the meaning of quite a few words. The scientific definition isn’t “wrong”, but neither is the traditional one – it’s just a difference of context. With respect to the traditional definition, there’s nothing incorrect about saying that an astronaut is weightless in space.
It’s always a good idea with these sorts of discussions to start by explaining that “weight” has a different and specific meaning in the context of physics that doesn’t exactly correspond to the meaning that people are familiar with. You’d be surprised how much confusion can be avoided that way.
What’s with the apparent weightless stuff?
Einstein gave us the Equivalence Principle. In an accelerating reference frame the force from an acceleration is indistinguishable from gravity.
So, suppose you are born on the space station and there are no windows. Your whole universe is the inside of the space station. Any experiment you do will to you seem as if there is no weight (the reverse is true too…if the station were under constant acceleration that it simulated 1g you would not be able to distinguish the acceleration from gravity).
So, how is it “apparent” weightlessness in that reference frame? Someone there who had no clue of the outside world would not describe it as “apparent weightlessness”.
Well now we’re getting into semantics. Perhaps it’s not so much “incorrect” to talk about weightlessness, as “unhelpful”, as evidenced by both the progression of this thread, and its very existence in the first place.
Completely agree with your last sentence. I rather thought that was the point I was making actually.
ETA: all that was for Stathol.
Re the equivalence principle: if you want to bring general relativistic concepts into the discussion, be my guest. I believe it is more fruitful to wear a Newtonian hat to discussions of the space station with lay office workers. OTOH if you ARE going to do GR, why not go the whole hog and say there’s no force of gravity at all, only the distortion of spacetime.
Yeah! Solve the geodesic equation for those space station dwellers. Hope your tensor algebra’s up to scratch. That’ll explain them.
Yeah, I figured that the stomach feeling went away but I’m assuming that the “lofty limbs” feeling still feels the same, even months later. I argued that it feels similar to a swimming pool, minus the water resistance.
Uh, yeah, this is all what the word “apparent” means. Someone who has no clue of the outside world would not describe it as “apparent”. Someone who has a clue of the outside world, for instance, by being there, like say in an office on planet Earth, would describe it as “apparent”. (at least once they had studied it carefully!)
Yet again, we’re bringing in concepts of different reference frames, which MIGHT be handy in describing these things in lay terms, but I think it’s best to START out by sticking to the Earth reference frame. Bring in fictitious forces once they’ve understood what a real force is, first.
Well, seems to me “apparent weightlessness” confuses the issue for laymen with flimsy grasp on Newtonian gravity. You need to discuss reference frames to get into “apparent” weightlessness as in:
“Compared to the scale on the space station the astronaut appears to be weightless but compared to a scale on Earth the astronaut weighs 99.99% (or whatever it is because he is further from the earth) on the space station.”
What most people want is what they weigh if the step on a scale on the space station. That answer is easy and not confusing to them. Zero.
Now if we want to get pedantic over it then we can wrestle with the equivalence principle and all that jazz.
And I think the best way to remedy this is to strengthen their grasp of Newtonian gravity, not persist in allowing fuzzy definitions to lead to confusion. It’s almost certainly their fuzzy definition that got them into thinking ‘there was no gravity in space’ in the first place.
Not confusing until they start thinking there is no gravity in space, because, hey, the scale says zero, right?
YOU brought the equivalence principle up, I said forget about that!
I thought the whole point of this thread was that you don’t settle into equilibrium. A person in orbit (and the vehicle around him) are being acted on by a force and are constantly accelerating.
Now, maybe that pit-of-the-stomach feeling goes away for reasons of biology or perception, but I don’t think your explanation settles it.
I think you can say you are weightless on the space station and still convey there is gravity there simply by noting they would fly off into deep space without it.
I guess I am saying I do not see how you get into apparent weightlessness without invoking reference frames (a Relativity concept) while you wanted to stick with Newton to avoid confusion.
And that is the funny thing about reference frames. People in two different frames can disagree and yet still both be right. To the guy on earth the astronaut saying he is weightless really means apparent weightlessness to him. To the astronaut it is not apparent weightlessness. It IS weightlessness.
Both are correct if they define the reference frame they are appealing to.
A fun thought experiment:
Imagine that the Earth is a perfect sphere and has no atmosphere. With enough angular momentum (about 7.9 km/s by my calculation), you could orbit the Earth 10 feet off the ground. The interesting thing is that you would feel just as “weightless” as someone orbiting at 2,000km above the surface.
Now consider this:
http://www.asdnews.com/news/15027/General_Atomics_Team_Powers_Navy_Rail_Gun_to_New_World_Record.htm
General Atomics (GA) just developed a new rail gun with a muzzle velocity of 2520 m/s (2.52 km/s). It used 41MJ (megajoules) of energy to deliver 10.64 MJ of kinetic energy (at the muzzle).
Suppose we wanted to fire this thing at ~7900 m/s. That’s roughly 3 times the velocity, and therefore roughly 9 times the kinetic energy. Let’s say 100 MJ to make it nice and even. The rail gun is apparently about 25% efficient, so it would take 400MJ of power to deliver 100MJ at the muzzle. 400MJ is roughly 110 kilowatt hours. At industrial power rates (average for the U.S. in 2009), that’s about $7.70 worth of power.
So for less than $8 (plus $237M in R&D costs :p), you could theoretically fire a bullet into a stable orbit at ground elevation. :eek:
I think the difference is it is a constant acceleration (not changing). You notice the tickle on the rollercoaster most as you go over the top of the hill and acceleration is noticeable. Like a plane taking off pushes you back into your seat…you notice that. However, despite the plane continuing to accelerate for several minutes you kinda forget that you are being pushed into your seat. All your organs settle into an equilibrium with the acceleration. Only when the acceleration changes and things start moving around inside again do you take note.
No, what’s causing the feeling in the pit of your stomach isn’t acceleration, it’s the lack of acceleration. You’re used to being under a continual 1g of acceleration, so when you’re not, it feels funny, and it continues feeling funny for as long as you stay unaccelerated.
But we’re getting a bit far afield of the OP, which is about how to explain this to his co-worker. I think the best course might be to show him videos from inside the Vomit Comet, like this one. These are folks who are in an airplane, and are by no measure in “space”, and yet they’re moving around just like the astronauts on the space station. In fact, what they’re doing on that airplane is precisely the same thing that the folks on the space station are doing; the folks on the station are just doing it for longer stretches of time.