The problem I see with the whole “apparent weight” thing is that all weight is apparent. You keep acting like earth is some magical reference frame, assuming that the person’s weight on Earth is their only true weight. What about the Sun, which the Earth is orbiting and thus under its gravitational effects? I have a weight relative to the Sun. Why isn’t that my “real” weight, and my weight on Earth merely apparent? Why do you consider the Earth to be special?
And if the Earth isn’t special, why bother with this “apparent” crap when all things are apparent?
As for the OP: I generally like the explanation that the space station and the Moon are falling towards the earth. They are just moving so fast that they wind up missing it. Then you just go from that to, if I’m on the space station, then I must also be falling toward the Earth. And what’s that force that makes things fall? Gravity.
Bit meaningless to talk about weight relative to the sun or Earth, but if you want to talk about weight due to the sun, be my guest. “Relative” applies to motions, not forces.
And the answer to your q would be, because the weight due to the Earth is by far the largest. However if you like, you can include the contributions to the total force on the body from the weight due to the Sun as well, and the moon, the stars and everything else in the universe. I expect they will all be miniscule and cancel out to leave essentially just the Earth’s contribution.
ETA: “all weight is apparent” - no it most definitely isn’t! The force of gravity is quite real, whether you consider it to be an inverse square law force, or a distortion of spacetime.
If I understand tidal forces correctly, then your scattered objects will also tend to be attracted toward a line drawn from the ship’s center of mass to the center of the planet (assuming there is in fact a planet nearby). Moreover, the objects that are nearer to the planet will be attracted toward this line more strongly than the objects that are farther from the planet. If this is correct, then one could measure the time it takes for the objects to congregate at each of these two points; the ones that come together more quickly are the ones that are closer to the planet.
Is this correct?
In any event, the original assertion - that there is no known instrument that can detect the difference between freefall and a true absence of gravity - holds true if one is restricted to measurement at a single point in space.
If you’ve ever jumped out of an airplane (for fun) you do not experiance the drop in your stomach. That feeling from rollercoasters, elevators is not from the act of free-falling. Perhaps it comes with the transition of up to down … which I do not know if astronauts experiance.
True. I once asked Jim Lovell what it felt like when the spaceship attained “zero G.” “Is it like being in a roller coaster when you go over the top and begin to drop?” was my question. And he said no, it wasn’t. You just found yourself floating - even in your space suit.
While jumping out of a perfectly good airplane with only some fabric to prevent you from dying is, indeed, called free-falling, I’m not certain that the situations are equivalent. Air resistance eventually limits your speed to terminal velocity while those in orbit are constantly accelerating. On the other hand, you could make an argument that someone standing on the ground is constantly accelerating due to the Earth’s rotation…
… back to the OP and persuading coworkers, if the “swinging bucket in a circle with water in it” analogy doesn’t help them understand orbits, then try the “throwing rocks (or shooting cannons) on the moon” description. Most people get that if you throw a rock parallel to the ground, it goes sideways while falling towards the ground, and that the harder the throw, the farther the rock goes sideways as it falls. Now get them to imagine throwing a rock on the moon (so you don’t worry about air resistance) so hard that by the time it falls, the curved surface of the moon has curved down away from the rock. So the rock is falling towards the moon, but ends up missing the surface and falling completely around the moon.
To get into the terminology of “weightlessness”, I will say that there is no “true weightlessness”, unless you are the only body in the universe. Because if there is anything else out there, there is a gravitational attraction between the two of you, and that attraction is “weight”.
Of course, if you are in free fall[sup]1[/sup], then you don’t experience that weight, you think you are weightless. Thus the word. But that is an experience, not an actuality.
If you stood[sup]2[/sup] on a scale, it wouldn’t register, but you would still be affected by that other object (in this case, the scale). Unless that scale is very sensitive (i.e. not a household bathroom scale).
So what is weight? The feeling of being pulled (which is actually the feeling of the resistance to motion in some parts and the lack of that resistance in other parts)? Or is it the act of being pulled (i.e. gravity)? Physicists and engineers use the latter. Laypeople tend to think the former. Ergo, the heated fight in this thread about what “weightless” means.
Mosier said:
Incorrect. You are accelerating in the space station. However, the difference is that a roller coaster is not a constant acceleration, it is a jerky ride. (See my previous post.) In other words, it isn’t the acceleration that causes the problem, it is the change of acceleration.
chorpler said:
The cause of the knot in the stomach is typically because all your bits are getting sloshed around. That sloshing happens because the roller coaster (or boat, or car, or whatever gives you motion sickness) is not a constant pull, it is a sharp pull one way, then a pull a different way, then a different pull a different way. It’s the jerking around that causes the problems.
Well, the other cause of nausea and disorientation comes because your balance sense perceptors are the fluid canals in your ears, and they are getting funky signals from the fluid sloshing around rather than a constant feeling of settled fluid. Plus, your visual perception is not necessarily matching your balance perceptors - the disagreement between the sense organs triggers trouble.
Astronauts do have to worry about that during transition (typically 3 days, some better some worse). That is separate from the shaking/sloshing, which happens under thrust at launch, but then ceases once they get to orbit.
Stealth Potato said:
It’s the changing between the two that people sense. Plus, you would think it would be nice to free float all the time and not have your spine compressed under gravity, but some astronauts report back discomfort from not being under load. Some of them actually use the exercise equipment to load their backs just to hang out. jackdavinci said:
The problem with water tanks is the viscosity. Water pushes you around, unlike air. You feel the drag trying to move, and slosh from waves generated by others. But the floating sensation is close. Roller coasters change your motion a lot, which is not correct, and elevator rides are transitory, not constant. Zero-g plane rides give you the worst of both worlds - “weighlessness”, then a transition to 2 times weight, then a transition to weightlessness, then back, etc ad nauseaum.
BigT said:
Nancarrow said:
Still caught up on the term “weight” and whether it means the force or the feeling.
[sup]1[/sup] Technically, you are not in free fall, you are under the gravitational influence of that other object. It just happens to be miniscule.
[sup]2[/sup] How do you stand on a scale when the only force is the mutual gravity between you two? You figure it out. (For my job, we use bungees, or a mechanical contrivance with cables that hook to a harness and do the same job as the bungees.)
I think this would be a good enough example for the average person. Assume that the person brings a scale with him. In free fall, stick the scale under him. I don’t think there will be any problem with the him accepting that the scale won’t register anything. I think there will be even less of a problem with him accepting that gravity is > 0 in this situation. Yes, you eventually reach terminal velocity, but so does the scale.
Then the only problem would be convincing him that a satellite is falling around the earth. The best way to do this would be to use the analogy of a ball. If you throw a ball, it moves laterally while falling. (A vector diagram would be just confusing.) That much is clear. Once he accepts this, all he has to do is accept that if you throw the ball fast enough, the earth curves away at the same rate as the ball drops, and you fall forever. (Ignoring the effects of air resistance, etc.)
But what if I was in a little ship way out far away from the Sun? Like in the path of Pluto, but Pluto is no where near me at all. Would this be different compared to hanging out in the space station?
(ps I never had physics)
Not in any significant way. Sure, the amount of floating and clumping those drift pile accumulations will be smaller, but that’s such a unique experiment that it wouldn’t affect your day to day activities or general experience.
