nobody paid attention to chronos’s post?
curious… anyone here taken classical physics in college?
Well, to return to the OP,
It is very difficult to explain to someone that orbit is free-fall. You will have to send him for a physics course or two before he will believe it.
What keeps the moon in orbit is the fact that it’s orbiting. If you could put rockets on the trailing side of the moon powerful enough to stop it from orbiting, the moon would impact earth in a time period that I am not equipped at this moment to calculate. My earlier point about the tides should be enough to convince most people that gravity acts at that distance.
Let’s stay away from that line of reasoning, we’re not getting anywhere with that even in this thread.
:rolleyes: You probably have a hopeless case there. The passengers only experience weightlessness if the plane dives with an acceleration roughly equal to g.
I can’t believe the guy talking about Lagrange points is the same one saying that there’s no gravity in space. I had to look that one up, but it’s not terribly hard to understand. Show him the Wikipedia page for it , and point out the word *gravitational *in, "The Lagrange points mark positions where the combined gravitational pull of the two large masses provides precisely the centripetal force required to rotate with them. " These are not points having no gravity.
The space station is about 220 miles out, certainly well within the practical limits of the gravitational field of Earth. That’s why it has to orbit to stay up there. (See above thought experiment about stopping the Moon’s orbit.)
This is such nonsense you can’t refute it.
Two semesters of general physics, one semester quantum mechanics, one semester physics of music :). University of Michigan Class of '79. I was not a physics major but I have forgotten more about physics than Chessic Sense’s coworkers seem to know.
Perhaps you can find link to physics articles where physicists are referring to the weight of something in free fall.
i always thought “the physics of …” classes were the most interesting.
anyway… my attempt at analogizing:
tower of terror/drop zone/ whatever drop ride you get in… you initially feel the gut drop. then the loose ends like hair, necklaces, etc. start floating. that’s weightlessness. you only know you’re falling because of what you see, and the air wooshing by. lock yourself up in a tin can like the ISS and you’re just plain floating (and falling but not in the roller coaster sense).
to explain why the ISS isn’t crashing down to earth like the drop ride, you have to factor in “lateral movement”. this would require a second explanation. draw a circle. this is the earth. draw a triangle protruding from the circle. this is a really high mountain. pretend a cannon is shot off the mountain and draw the trajectory of it falling down to earth. now draw a trajectory if the cannonball is blasted off REALLY fast; so fast that it falls past where the earth ends. well… it can’t fall just straight down. gravity acts towards the earth’s center. the trajectory would turn into orbit.
also i suggest you fully understand the analogy before you go explaining it or it might end up confusing the both of you.
Reference frames are often a very useful tool in Newtonian mechanics, they’re not the sole province of Einsteinian relativity at all. Thinking about switching reference frames in Newtonian mechanics is where we get concepts like ‘centrifugal force’ from. Galileo used them when talking about dropping a ball while sailing on a ship.
But yeah, I would prefer to avoid discussion of reference frames at the office, and it’s perfectly possible to talk about ‘apparent weight’ without them.
For a start, that’s just what I’ve been doing!
But more seriously, the reason I was able to do it was because, in the way I would prefer to explain it, the difference between ‘apparent’ and ‘actual’ weight is NOT about reference frames, it’s about using an “incorrect” (okay, an “unhelpful”) definition of weight.
Another example: the wiki article on weight, uses the ‘everyday’ notion of weight to say that we weigh less when floating in water. I prefer (and I think physicists would, though IANAProfessional) to say that we weigh the same (force of gravity acting on us is the same) but the weight is counteracted by another force, which we call buoyancy.
Can’t be bothered to root around professional journals at the moment. BUT, google ‘weight free fall’. The first three hits (at the very least) explain it. The very first is from NASA.
Are we done here?
I think the most effective technique is to show him footage of the Vomit Comet, and show webpagesthat explain how astronauts use it. They use it because there’s no difference. They are the exact same thing, you use one to simulate the other.
If that doesn’t work, yell “Einstein said so!” at him over and over.
Chessic Sense said:
In general, you are correct, the experience is caused by exactly the same thing. In practice, the transitory nature of the events you mention may cloud the comparison value.
As we’ve demonstrated, “weightless” is a goofy term that can be confusing. Gravity is acting on the Space Station, and on the astronauts in the space station. It is slightly less strong than the amount pulling down at the surface of Earth, but it is there and not negligible. What differs, however, is that on the surface of Earth, you are standing on something. The ground, the floor, the chair, the couch all push up on you to keep you from moving down. Thus you feel the push against you as much as the pull down. It is the conflict of the forces that gives you the feeling of “weight”, the droopy, heavy arms and all that.
In the space station, the station is moving with the astronaut. Because they are moving at exactly the same speed, the amount of attraction by the Earth does not register because it is not being opposed by anything. There is no conflict of forces, so no feeling of heaviness. The station and the astronaut do have a mutual gravity between them, but it is [del]insignficant[/del] tiny (we talk of “microgravity”). They have no feelable constant reaction keeping them together.
How can you prove gravity is working on the Space Station? The Station is in orbit. What keeps it in orbit instead of floating away from Earth? Gravity. The ball on a string analogy is useful here. Without the string, the ball will fly away. It takes the string (gravity) to hold the ball near you.
Back to what is a plummeting elevator. When you get on an elevator, you feel a slight change in the interaction between you and Earth. Gravity is pulling down constantly, but the amount of push up changes. When the elevator starts lifting, the amount of push increases, and you feel heavier. When the elevator slows down, you feel less push, and inertia pulls you away from the floor, and you feel lighter. Same thing when the elevator goes down.
A plummeting elevator (who has been in one of those? Show of hands) would be similar, just like falling off a building would be similar. Ever jumped off a high dive platform into a pool? That falling period before the [del]splat[/del] splash is free fall.
Use the ball on a string analogy, the fact that the Moon orbits instead of flying away, and the cannon examples to explain orbits and why gravity is required to keep things from flying away.
Then ask them if the building or car prevents gravity on Earth from pulling on them. If it doesn’t, then how does the Space Station (metal can) shield gravity? And more importantly, if it can shield gravity, why haven’t we figured a way to use that to carry heavy loads on Earth?
Too late. 
Also, regarding planes, the way they achieve “weightlessness” (note the quotes to skip any arguments over the word) is not by the altitude at which they fly, but rather the path that they fly. They fly parabolas, like roller coaster humps. That is exactly how the planes (Vomit Comet) simulate the effect for astronaut training and hardware testing. At the top of the curve, they fly exactly the path that you would travel if lauched out of a cannon and flying through the air. That is the same curve a ball travels if you throw it. That makes the people in the plane float as if weightless. Then 30 seconds later, they transition from going down to going up. That transition shifts them from floating in the air to lying on the floor with 2 gravities of force pulling down - just like a roller coaster.
Francis Vaughan said:
We don’t really feel acceleration, we feel the conflict between two accelerations. One force is pushing against a limited portion of you (your feet), the other is pulling on all parts evenly. Thus your arms hang limp and you feel heavy. In free fall (i.e. nothing holding you up), there is only one force on you acting on all parts evenly, so you do not feel your parts getting pulled/pushed against each other. Ergo, you feel light.
**bordelond ** said:
The main cause of motion sickness is neurovestibular. The sense organs in your body report what they sense, and your brain feeds those signals together. When in free fall, the fluid in your ears floats instead of hangs, thus triggering all the sense hairs all over, not in connection with the visual signals you are receiving. Those various mixed signals are what make a person dizzy and nausead.
People have different responses, everyone is an individual, blah blah blah, but typically most astronauts adapt within 3 days. After that, the nausea and dizziness go away. Then they get a bout when they return to Earth. Often their second trip to space will be easier - the body remembers the last adaptation, and adjusts back quicker.
Chessic Sense said:
That’s a fair understanding.
Whack-a-Mole said:
The term you are looking for is “jerk”. That is the technical term for rate of change of acceleration with respect to time.
I read those links and they are talking about how your are weightless in free fall.
You might point out that one of the constants of space travel is space sickness. As has been stated, it probably results from the lack of, dare I say it, gravity on the liquid in the semi-circular canals. But it’s a problem that hasn’t been solved yet. We don’t hear a lot about it, but almost all astronauts suffer from it. If your friend were falling in a long enough elevator shaft - or in the vomit comet, for that matter - the liquid would distribute itself randomly, presenting very mixed signals to the area of the brain responsible for orienting, resulting in extreme nausea.
So it’s fair to say that it’s commonplace for mechanical engineers to work with jerks?
That’s a good post Irishman.
What would the effects of essentially true weightlessness be (eg. if you were floating around outside the solar system), and would the body be able to differentiate it from apparently weightlessness? Would there be any different physiological affects from, say, orbiting Jupiter vs. Earth?
It took forever, as he kept finding links showing how spinning cylinders can create artificial gravity. He kept saying that having to create gravity means that it’s not there to begin with. I don’t know what link he saw that finally made it click for him, but he eventually stood up and announced that there was gravity in the space station after all.
Then just for kicks, we calculated the gravitational effect of Jupiter aligning with the earth as a baby is born vs the gravitational effect of the doctor holding the baby. At half a meter away, the 100kg doctor ‘gravitates’ at a tenth of a very, very close Jupiter. Against a distant Jupiter, it only takes 4.5 doctors to cancel it out. Once the baby is in the doctor’s arms, he pulls on the baby harder than even the close Jupiter…in case anyone was curious.
There is no experiment you could do inside a spacecraft that would tell you the difference between being in a windowless spacecraft orbiting Jupiter, orbiting Earth, in deep space not orbiting anything in particular, or falling into an atmosphere-less planet about to hit the ground. In all cases, whatever gravity there is acts equally on the spacecraft and everything in it.
Weight isn’t the force of gravity on an object. That’s simply Newton’s law of gravitation, F=Gm[sub]1[/sub]m[sub]2[/sub]/r[sup]2[/sup]. Weight is the force required to keep you from falling through the ground. If you’re not standing on the ground, you have no weight.
nitpick police:
1a - not “generating” gravity/gravitational acceleration. it’s centripetal acceleration, no different than swinging a bucket filled with water.
1b - gravity is generated by mass, not motion.
2 - a baby being born has no bearing on adding mass to the planet.
Are/were you under the impression that anyone thought differently? I don’t understand where that came from
Who said it did? It proves that Jupiter pulls on you comparably to a fat person in the same room. Just in case anyone thinks that, say, being born with the planets in a certain orientation would affect the baby’s future.
Gravity does not act equally on everything. Objects closer to a planet are pulled more strongly to it, objects further away are pulled less strongly, and these differentials are called tidal forces. There are microgravity experiments that can tell you whether or not you’re near a significant mass. By repeating the experiments over time, you’ll be able to tell whether you’re inbound, outbound, or in a stable orbit. If the tidal forces keep getting steadily stronger… Say your prayers. 
what then were you talking about when you said that the artificial gravity generated convinced your friend of the existence of gravity in the space station? it has absolutely nothing to do with gravity. he is no closer in understanding the effects of gravity on a space station than he was a day ago - possibly more confused.
also, what calculations were you doing involving doctors and babies? a doctor holding a baby out vs holding a baby close to the vest?