View Full Version : Gravity and Freefall
08-23-2000, 10:29 PM
Why, in a weightless environment, does one feel like one's falling? Would the feeling be different in deep space, outside the gravity fields of Earth, the moon, the sun, etc?
08-24-2000, 12:26 AM
According to Relativity, acceleration cannot be distinguished from gravity. When you are in free-fall, your acceleration cancels the gravity. It feels like you are falling in a weightless environment because there is no difference. The feeling would be the same in deep space.
If you are in a (small enough) box, you there is no experiment you can perform to determine whether you are in a gravitational field or whether you are accelerating.
08-24-2000, 12:51 AM
To clarify DrMatrix's post: it's not that you feel like you're falling when you're in a weightless environment. Rather, it's that you feel like you're in a weightless environment when you're falling.
08-24-2000, 01:00 AM
When you are in technical freefall, you are not accelerating. You will fall and fall and gain speed, but eventually you will reach your limit. The air will push up against you with an equal force that you fall with. The more mass you have, the larger the limit. Now MASS is not WEIGHT. When you are in space, you have the same MASS but no WEIGHT.
08-24-2000, 01:24 AM
I was ignoring air resistance. You never actually reach your terminal velocity, you just approach it. The resistance to falling in a fluid depends upon not just the weight but also the size and shape of the falling object. When I said free-fall, I meant ignoring the air resistance.
08-24-2000, 01:59 AM
When astronauts are in the space shuttle in orbit they are technically freefalling. (With no need to account for wind resistance) A skydiver in an atmosphere is technically not in freefall. The wind resistance is technically a force that keeps said skydiver from falling freely.
As the good doctor said, "If you are in a (small enough) box, you there is no experiment you can perform to determine whether you are in a gravitational field or whether you are accelerating." The converse is also true. If you are in a small enough box there is no experiment you can perform to determine whether you are in a gravitational field too small to measure or in freefall. If I put you in a small box and pushed it out of the back of a plane, you would settle to the bottom of the box as the box approached terminal velocity.
08-24-2000, 08:09 AM
Like Lance Turbo said, the space shuttle astronauts are in a kind of free fall while they are in orbit. And I don't think they feel like they are falling while they are in orbit.
Falling in Earth's atmosphere is different because you are fighting wind resistence and retain a sense of motion.
Lots of people have "motion sickness" and various weird feelings in free-fall, because our bodies and orientation mechanisms have evolved to operate in a gravitational field.
KC-135 "Vomit Comet" (http://zeta.lerc.nasa.gov/kjenks/kc-135.htm)
Thrills - and spills - aboard the `Vomit Comet' (http://seattletimes.nwsource.com/news/health-science/html98/zero_19990330.html)
08-24-2000, 02:00 PM
At the Space Shuttle's orbital height, the gravitational field is about .8 g, or 80% what it is at the surface. So why doesn't the Shuttle fall? The answer is, it does: An orbit is a form of falling. However, due to the curvature of the Earth, the surface "falls" away just as fast, so it never hits ground. Meanwhile, the astronauts inside are falling at the same rate, so they appear to be weightless relative to the Shuttle.
You don't actually get motion sickness from being weightless or in free-fall... You actually get dropsickness. The mechanism is actually different, although the symptoms are very similar: Motion sickness arises because of conflicting information arriving from your eyes and the motion sensors in your ears. If you're in a car and accelerating (decelerating, turning) a lot, but staring at the dashboard, then you might get motion sick because your eyes tell you that you're at rest relative to the dashboard, but your ears tell you otherwise. You can also get it by watching a wide-screen movie where the camera is going through crazy antics (mounted on on the bumper of a snowmobile, say, or a stunt plane), because your eyes say you're accelerating, and your ears say you're not. In any event, you can't get motion sick with your eyes closed.
Dropsickness, on the other hand, relies soley on the signals from your inner ear. If you're feeling weightless, your body interprets it as a problem, regardless of what your eyes are telling you, with potentially technicolor consequences.
08-24-2000, 02:45 PM
The statement "If you are in a (small enough) box. . . ", leads me to wonder what experiments you could do in a big enough box.
How 'bout an experiment to detect a tidal force? There would be a tidal force in a gravity field, so closer objects (on the floor) would weigh more than farther away objects (on the ceiling).
What about detecting the direction of the gravitational pull? If objects in the box don't fall straight down, you're in a gravity field. The objects would fall toward the center of the gravity source.
Practically, it all depends on the precision of your measurement tools and the nature of the gravity source.
If you were closely orbitting a neutron star in a large box, I would guess that by performing these experiments you'd be able to tell that you weren't just accelerating.
08-24-2000, 03:03 PM
stolichnaya started a separate thread adressing why I said "small enough". See Gravity vs. Accel. in a Large Box. (http://boards.straightdope.com/sdmb/showthread.php?threadid=35836) You have the right idea though.
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