The Vomit Comet isn’t only, or indeed primarily, for astronauts. It’s for anyone doing zero-g experiments who need more duration than you can get from towers/pits but can’t afford spaceflight. If you’re on such a project, you can apply for time on it.
abashed, I’d like to let you know that I (and others here) appreciate the humility in admitting that you don’t know something, and asking for explanations.
And I do mean that, so I hope it doesn’t come across as negative, in saying that it would be really nice to take some of this humility and use it in the Quantum Mechanics/Non-Experts thread. Your attitude over there has been the opposite of this.
The phrase/concept you’re thinking of is “weightless”. And as chrisk already pointed out it’s caused by an absence of support (with or without gravity).
You can’t see, touch, feel, or otherwise sense gravity directly (that requires some very complicated machinery). Instead, you feel the resistance to gravity provided by the ground, a plane, or the thrust from a jetpack.
And since we all grow up being supported against gravity in one way or another, our brains are wired to treat this resistance is universal and omnipresent. Movement and personal direction are relative to that support.
Without an external reference (e.g. scenery, the horizon) or force (e.g. air resistance) you wouldn’t be able to tell the difference between falling and orbit, until you interacted with that support again (i.e. hit the ground). All you’d sense is an absence of that support we feel as weight. (in fact, even the most advanced scales just measure the compression between an object and it’s supporting surface).
It hasn’t been removed, it remains silently at the bottom, underneath the vacuum chamber. A few years after I took over as Facility Manager, there was talk of resurrecting it. I contacted my predecessor, the engineer who had overseen the original construction and trouble shooting. He was silent for a long time then said they would have to bring him out of retirement.
The accelerator didn’t use the long, rocket like drop vehicles. It used a hollow sphere about 40" across. Between the limited size and the internal bracing there was not a whole lot of room for the experiment. It also suffered from vibrations from the launch.
Here is a photo showing the accelerator piston and the socket that held the package.
This page has a schematic view of the facility that shows the accelerator:
The German drop tower at Bremen has had more success with their catapult, I guess it has a less abrupt acceleration profile.
Dennis
Right. When you’re free-falling, you’re in inertial motion. Every particle of you is moving in a straight line through four-dimensional spacetime (ignoring biology and such, obviously), at rest in its own frame of reference and, since we’re not on a neutron star, over the volume occupied by a human body space behaves as it would in deep space; to wit: We don’t get torn apart by tidal forces caused by our feet having a substantially different path than our head.
(All this neglects wind resistance.)
I like to look at it as the body in freefall being completely stationary, and the ground accelerating towards it. It’s no different from being way up in the middle of the air and being hit by a rocket.
Humans usually live on the surface of a planet, which is no different from living on an accelerating rocket, and, like all observers in a non-inertial reference frame, we experience forces which would disappear were we to shift our frame of reference. These are called fictitious forces for some stupid reason, and gravity is one of them. It’s exactly the same concept as being stuck to the inner side of a spinning barrel, and for the same reason: Inertial motion being stopped by something, such that it appears as though there’s a force drawing you to whatever is impeding your straight-line inertial progress.
A few bits of information on the difficulty of the accelerator. The piston itself, as can be seen, weighs many tons. It has a stroke of 20 feet. In that length, it must smoothly accelerate to 113 mph, back off to allow the experiment to leave, then come to a halt before crashing through the retaining cap of the cylinder.
It was, by all accounts, spectacular to watch through the viewing ports at the top. The experiment would shoot up from the bottom, gradually coast to the apogee only a few feet from the top of the vacuum tank, then drop back down.
Meanwhile, the massive decelerator cart filled with plastic beads had to be re-positioned. It was on wheels that ran up an incline. After the experiment passed, clamps were released to allow it to run back down the track. If it had not passed a certain point by an allowed time, explosive bolts cut the release mechanism off completely to get the cart moving.
The main package, by the way, is also released by snapping a very hard bolt that is reduced in diameter to just barely hold the weight of the package. The release bolt does not bear the weight of the package until moments before release. And it cannot be used if it holds the weight for more than a few minutes, as the high stress work hardens the bolt even more.
The bolts themselves are often shown during tours, but the manner in which they are broken is classified. The mechanism is covered with black cloth when it is visible.
Shortly after the facility was built, NASA had an open house. It was decided to drop the experiment in air (instead of a vacuum) to allow crowds to directly view a drop from the upper rim of the vacuum tank, and feel the earthquake from below as the 2500 lb vehicle slammed into the decelerator cart at 113 mph. It’s quite a feeling.
All went well for the first few drops. Then, despite the addition of bomb type fins, the vehicle veered a few inches off course and scraped the steel decelerator cart. The sparks ignited the polystyrene pellets which contain acetylene. A black mushroom cloud arose from the depths to the delight of the crowd, who thought it was normal.
The choking cloud quickly panicked the viewers as it spread throughout the building. The fire department had no way to fight a fire 500 feet underground so it just burned out. The facility was shut down for 6 months before it even had the first data drop. After that, all pellets must be aged for several months before using to allow the acetylene to dissipate. We buy them from a single approved supplier in cube bags 10 feet on each side made of canvas. They only last a dozen drops or so before they must be replaced as they get flattened and the deceleration rate goes up.
Those pellets and the shape of the decelerator were developed through scale model experiments conducted in the elevator shafts of the Terminal Tower in downtown Cleveland after hours.
The ZGF is not approved for test of live subjects, but the JAMAC facility at Hokkaido did some animal tests.
Early on, some technicians decided that they would see who could climb up the fastest using the emergency ladders. A 500 foot vertical climb. Each ladder flight is 20 feet between platforms. Whew!
Dennis
Netwons laws dont explain what happens inside a body. They describe what happens to a body.
So while a person falling is accelerating due to an external force, that acceleration is a bit different to a rocket accelerating in space - there is a difference in what is happening INSIDE the body is different.
This too is the same difference between orbital mechanics lack of “centrifugal force” and a ball on a strings experience of centrifugal force. Its the same difference.
Gravity is affects every little part of the body… so there is no pressure felt inside the body.
rocket thrust applies to the tail end of the body, and then that end pushes on the middle, and the middle pushes on the front… and break that down … Thats causing pressure in the body, with a pressure gradient throughout the body… higher where the external force is applied, lesser at the other end.
So that is why freefall feels so gravity-less - there’s no way to detect the acceleration. Its when you are held up by the ground that you feel a pressure at the surface supporting you, and you can get sore knees, hips, back, etc… due to pressure inside your body !.
For the same reason, engineers consider the most important measurement of a materials strength is its ability to avoid breaking apart due to pressure ! eg concrete measured in its megapascals.
Why is this classified? :dubious:
ITAR regulations, probably. The government can get pretty absurd about what counts as a “munition”. I’m guessing that a similar mechanism is used somewhere in something that’s tangentially related to some weapon system.
mixdenny, were you still working at the facility last summer? That’s when I toured it (with a group of kids from the Science Center), and I might have met you.
The key to a low gravity experiment is eliminating any “G jitter” since vibrations affect the delicate fluid surface or flame propagation. It is very hard to do and once our system was perfected, we couldn’t let those darn Russkies copy it. Remember, the facility began testing in 1966.
Our 2.2 second Drop Tower uses a taut wire that is pinched between blades to release, but that has jitter. The G levels there are no where near the ZGF so it isn’t as critical.
The JAMAC was a very high tech facility at Hokkaido. It used a magnetic release that worked well. JAMAC could provide 10 seconds of free fall. It was in an abandoned mine shaft. It did not drop in a vacuum, rather a sensitive instrument measured the G level and a variable thruster steadily increased thrust to maintain everything at zero G.
Their drop vehicle was guided on rails by non contacting magnetic bearings. At the bottom of the drop it decelerated using the magnets as a linear motor. Their braking distance was longer then the entire depth of the ZGF, we stopped in 15 feet.
Chronos, I retired in 2005, I was manager during the 1990s.
Dennis
Only to the extent that inter-atomic forces are many orders of magnitude stronger than gravity, so the tidal forces experienced by even a large molecule can be ignored when predicting bond lengths and similar. As far as the mathematical foundation of the theory, I’m sure even Newton himself would have said that every fragment of matter would be attracted to every other fragment of matter, down to atoms, and that is, indeed, what his theory predicts.
Moving to modern physics, all subatomic reactions take place in a curved spacetime near a massive object, but at the subatomic scale, you can model that spacetime as flat and lose almost no accuracy until you’re well into the extremes.
The centrifugal (center-fleeing) force of orbital mechanics is what keeps planets bound in orbits, and it’s the same thing as what keeps someone inside a spinning barrel pinned to the side: Their linear momentum is being redirected by something, but not destroyed. In one case, it’s the curvature of spacetime, in the other, it’s the wall of the barrel.
Think everyone answered this, if they were “Free” from gravity, they would not hit the ground, they would just float off which ever way they jump until friction stops them.
But is it true to call it weightless? Feels like weightless kind of maybe
You still have weight, if you stopped in mid air and stepped on a scale (that is not also falling, pretend it is on a long pole) you would show what ever you weigh.
Someone said well if you stand on a scale that is also falling you will weigh nothing
but that does not seem exactly correct.
The scale may say 0, but that is simply because the scale is also falling, it isn’t in a state to compare you against anything?
And if we put a parachute on the scale and deployed it, while you are still falling just slower, the scale now shows you having weight
If you were truly weightless like in space you simply would not go anyplace.
If i pushed you, you would just float away at a constant rate, never accelerating.
On earth you really quickly change directions to down, and reach a lovely velocity
that will result in your mass making a much amplified spat when you weigh in on the ground
It is true to call it weightless. For the technical physics definition of “weight”.
What you’ve done throughout your post is ignore 500 years of scientific progress about the difference between weight and mass and acceleration and such.
Said another way, you’re mixing the ideas of mass and weight and acceleration and force all together into a goopy mess. In the standard “commonsense” (but wrong) way that humans have always done. Because they grow up at the bottom of a gravity well on a planet and think of that gooped-up confusion as “normal”. But it’s far from normal and far from technically correct.
Physics 101 teaches people how this stuff really works. But step 1 in this particular chapter is unlearning the idea that weight is some consistent property of matter. It simply isn’t. Weight, to have any meaning at all, is simply the manifestation of force applied to mass. Any mass, absent an imposed force, has no weight.
And as long as you (any you) persist in thinking of weight as an intrinsic something you’re stuck in that confused and goopy state. And won’t make progress towards enlightenment.
That’s post of mine is not very good. Too late to fix. The big picture is OK but the details are a mess. Oh well. Maybe tomorrow.
You end up needing to explicitly point out the equivalence principle. It might be about time.
There are two ways of looking at mass. Mass as it determines gravitational attraction, and mass as it determines inertia. Galileo realised that these two were the same. Prior to that people didn’t understand this. The implications is critical.
To a good approximation (one body vastly larger than the other) the gravitational attraction (the force) between a pair of bodies of masses m[sub]1[/sub]m[sub]2[/sub] is given by:
F = m[sub]1[/sub]m[sub]2[/sub]G/d[sup]2[/sup]
Now what about your inertia? The acceleration body 1 has when a force is applied is of course:
a = F/m[sub]1[/sub]
which means, if we substitute the result for the gravitational force back in:
a = m[sub]2[/sub]G/d[sup]2[/sup]
If you are mass 1, your mass cancelled out.
Which is why, even though we are talking about the force due to gravity we just talk about the acceleration; it is because we don’t need to know your mass. This happens because gravitational mass is the same as the inertial mass.
So what about weight? Weight is not mass. Your weight is dependant upon the gravitational field you are subject to. Usually however you are actually interested in your mass, not your weight. But since we are almost always only determining our mass whilst on the Earth, we get away with measuring the force we are attracted to the Earth by, and using that as a proxy for our mass. It usually is fine to do this. Most people are used to using a spring balance, which only measures force, but is calibrated to be used on the surface of the Earth to read in units of mass. Use in free fall and it measures nothing, use it on the Moon, and will read 1/6[sup]th[/sup] of your mass. But what it does measure is fhe force you feel, and thus is does measure your perceived “weight”. But your inertial mass does not change.
Use a beam balance, and you are always measuring mass, as the balance is balancing out the force due to two masses. It will read your correct mass on the moon. In freefall it won’t work - rather than reading zero, it just won’t be able to work and will just clatter about and never get a stable answer.
Using your inertia directly and you can work out your mass anywhere, including free fall. You could make a balance that uses a reference mass and shakes it against you with a known force and measures the amplitude of the shake. (OK, this won’t work so well with a bit sloppy meat bag, as you won’t get a reliable consistent shake, but assume it does work.) If you had such a mass measuring device, if you used it standing atop the building, it would read your mass. Step off into free fall, and it will still read exactly the same mass, even though a spring balance now reads zero weight.
The problem with weight versus mass is compounded in those countries that still do engineering with imperial units. Here you have to be very careful to distinguish between pounds of force and pounds of mass. Which is why in the SI system we use kilograms for mass, and Newtons for force. And rather critically, they are not numerically interchangeable. To get the force in Newtons for a body you multiply by the acceleration. F = ma. For a body sitting on the surface of the Earth that acceleration is 9.8ms[sup]-2[/sup]. You might note that this allows us to use any acceleration in addition to, or instead of, the Earth’s gravity, and still get the right answer.
Bravo Good Sir.
To your very last paragraph, it’s a slightly unfortunate coincidence that in SI units the Earth’s gravity is real close to 10. That invites the mistaken notion that a Newton is just a ~10x kilogram or is ~1E5 grams. Rather than a unit measuring a fundamentally different kind of thing.
To be sure, that confusion only arises is people who start with the weight/mass confusion as a bedrock “truth”. Which, sadly, is anyone raised in a non-SI country and many residents of SI countries too young or undereducated to have had much physics in school.