The first is correct, the second isn’t. Without acceleration, an object moves in a straight line at a constant speed. If the speed is constant but the path is curved (as when in orbit), you have acceleration.
True.
But there’s another usage of “free fall” that could be misleading here: a parachutist steps out of the jump plane and accelerates until he reaches terminal velocity (around 120 mph), at which point he’s unaccelerated, and has more or less normal sensations of gravity.
In a sense, the parachutist’s fall isn’t “free” because the air he falls through creates drag. Nonetheless, the common term for what he’s doing is freefall.
Well, it’s not that a straight line is the important part - it’s inertia that is important. When you are going around a curve, inertia is pushing you toward the outside edge of the curve. But if you were travelling forward, and then began a free fall you would continue to move forward without feeling that affect, thus your fall would be a curve - and it would not feel the same as going around a curve in a car.
Also, you DO accelerate when in a free fall, and you stop accelerating once you reach terminal velocity - I don’t understand your statements to the contrary.
Here’s an interesting article on space sickness . It seems that most of the problem of adjusting is that motion and gravity don’t work like your brain expects them to, so your brain has trouble interpreting what your eyes are seeing. It has very little to do with any feeling of free-fall.
I suspect that the “Stomach drop” feeling on a roller coaster is more an affect of your brain telling you that the floor has dropped out from beneath you. Also, inertia affects your internal organs, and the fluid in your inner ear - when the coaster begins it’s drop, you (and the stuff inside you) want to remain in rest, so there’s a physical sensation of everything being pulled downwards by the initial motion of the coaster. I imagine that if the fall was long enough so that your body could catch up to itself and so that the adrenaline shot subsided you would feel much more comforatable than the initial dropping sensation.
Depends how you’re measuring the ‘feeling’. Most of it is due to the villi in the inner ear being decompressed, as if you’re hanging upside down, or falling. This causes dizziness and disorientation. Most of the Skylab astronauts complained of never feeling that they were facing “up”, though after a while they got used to it.
Another problem is that the cardiovascular system is set up for pumping blood around a vertically-oriented body with 1g on it. When you remove all the gravity, the blood is pumped harder into the upper body and head than it normally would be. This led to a “stuffy” feeling in the head, complete with bulging veins. Unlike the disorientation, though, this does physiologically adjust after a week or so, but the problem then reverses back in gravity, which is why astronauts get faint very easily for a while on their return.
People who have done the “hammer” zero-G simulation on earth (i.e. a plane climbing to a great height, then dropping at 1g for up to a minute as used in NASA training and for Apollo 13) describe the “drop” sensation as being permanent. If you ever do a cliff jump, you’ll experience this until you reach the water. Further reading from Jerry Lingenger aboard Mir:
Sometimes, I think that people would understand physics a lot better if we taught relativity right from the get-go. It’s not only more technically correct, it’s also, in many cases, more intuitive. But we spend so much time learning non-relativistic physics that we lose track of that.
As I’m sitting here at my desk, not moving, if I look at an accelerometer, it’ll read 1g. If I jump out of a plane, and I have negligible air resistance, then it’ll read 0g. The basic idea of General Relativity is that the accelerometer is correct: When I’m sitting here, I’m accelerating at 1g, and when I jump out of the plane, I’m not accelerating. Likewise, the astronauts in orbit are not accelerating, either. Despite appearances, they’re travelling in a straight line, or the nearest thing to it you can have in the Universe. It just so happens that that straight line is a closed loop.
Or another example: In a well-executed loop in an airplane, when you’re at the top of the loop, you’ll feel perfectly normal. According to non-relativistic physics, you’re accelerating at 2g at that point, but it still feels just like when you’re sitting in your chair on the ground, and supposedly (according to that same non-relativistic physics) not accelerating at all. So why should accelerating at 19.6 m/s[sup]2[/sup] feel just like not accelerating at all? That’s a hard question to answer, in non-relativistic physics. With relativity, though, it’s easy: When you’re sitting on the ground, you’re accelerating, and when you’re at the top of a loop, you’re also accelerating, the same amount (though in a different direction). Since you have the same amount of acceleration in both cases, they feel the same.
That is beautiful.
Actually, you start accelerating again as soon as you jump out of the plane, with your acceleration increasing proportionally to the square of the velocity of the wind going past you. Terminal velocity is merely when you reach 9.8 m/s^2 and your acceleration levels off.
Really, though, I think I’m happy we didn’t learn aerodynamics from a relativistic perspective.
Well, yes, but at the moment you jump, your acceleration is zero, and a short time after that moment, your acceleration is still very small. I’m aware that terminal velocity is not just some abrupt step function that doesn’t kick in until you reach that velocity, I just didn’t want to go into that much detail.
I just read up on what an accelerometer is and what the definitions of “acceleration” and “velocity” are in order to understand this (I thought “velocity” and “speed” were the same thing; chalk it up to not being a native English speaker). I still don’t get it.
Why? What is the thing relative to which the accelerometer is moving so it can measure this acceleration?
Again, why? You’re moving relative to the Earth, relative to the plane, relative to the air, probably relative to the contents of your stomach if you’re anything like me.
Put me back in the free-fall theme park ride and give me an accelerometer. Are you saying it will measure 0g while I’m falling?
The distinction, as I once heard it, is between “falling freely” (skydiver) and “being free of falling” (orbiting astronaut)
(Yes, technically the astronaut is falling continually, but this is from the viewpoint of the person involved, and the astronaut feels he/she is “floating” and not falling)
Who says that accelerations need to be relative to something? Or, if you insist, it’s accelerating with respect to any inertial frame.
Or, if you want a more practical, empirical answer, all an accelerometer is is a mass attached to a spring. How much the spring stretches tells you how much acceleration you have. When you’re just sitting at your desk, the spring is stretched by an amount corresponding to 9.8 m/s[sup]2[/sup]. When you’re jumping out of a plane, without significant air resistance, the spring is not stretched.
But then it isn’t measuring acceleration, but rather the force exerted on the mass. Right?
Acceleration means change of velocity, correct? How is my velocity changing as I’m sitting here and typing this?
Weird. Personally, I love the feeling of freefall. I adore rollercoasters and rides like that, which make me giggle uncontrollably. And I’m not trained in the least.
Now, I’m sure that if I actually had to spend significant amounts of time in zero-G I’d get the same physical motion-sickness reaction that the astronauts do, and would have to train to get over it. But the immediate psychological impact of going into freefall is all positive, as far as I’m concerned. (Of course, if I were actually plummeting to my death in an uncontrolled fall I’m sure it would produce some anxiety, but as long as I rationally know that there’s nothing to be scared of, then it’s just one big WHEEEEEEEE!!!)
Hmmm, I never really considered the possibility that other folks on the roller-coaster were screaming because the freefall feeling made them anxious. I thought they were just doing it for fun, or maybe because the visual appearance of swooping around so fast is alarming, but I didn’t realize the physical feeling of falling could be scary just on its own.
I get motion sick in a hammock, on swings, or even looking at spinning stuff. I’ve had good luck with Bonine (meclizine). I once got a virus with the single symptom of making me dizzy when I turned too fast, and the doc gave me a prescription for meclizine.
Force is mass times acceleration. So you can’t really have one without the other.
I admire what Chronos is trying to do here, but I think there’s something important that’s being left out: as physics is currently understood, there is no difference between the “force” that throws you towards the outside of a carousel, and the “force” of gravity that pulls you towards the Earth. Both “forces” are what are usually called “fictitious forces”, and are the result of following a path in space & time that isn’t a straight line.
In the case of a carousel, it’s pretty obvious that you’re not travelling in a “straight line”. In the case of me, sitting in my chair and feeling the “force” of gravity, it’s not quite so obvious why I’m not travelling in a straight line; the answer is that space & time themselves are “curved”, and while you can define a notion of a “straight line” on a curved surface, the path I’m following through space & time is not one of them. If I were freely falling, though, I would be following one of these “straight lines” (at least until I went splat on the pavement.)
This whole idea that freely falling observers are the “best” ones (i.e. the ones that don’t experience any fictitious forces) is called the “equivalence principle”, and it was Einstein’s big insight that led him to the formulation of General Relativity. It can take some time to wrap your head around, though.
That’s just too cool. I wrote a grad school paper around 1993 or so on microgravity envirnoments (biological anthropology perspective), and there was almost no hard data I could find, although lots of nontechnical stuff. And most of what I found was in Russian (for obvious reasons, I guess).
Maybe I was looking in the wrong places.
I hate to say it, Kimstu, but if the rides make you giggle* and not scream, you’re the one that’s a bit weird – if the many people screaming on rollercoasters are to be considered “normal,” that is. 
- I’ve done that too, if hysterical, nervous laughter counts.