Comment by paulannesley | Hacker News Reader

Comment by paulannesley | Hacker News Reader

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The answer to the title question seems to be:

> Inside the building, on those top floors, the oscillation is what unnerves us. A forty-story building may sway a foot to the left, a foot to the right. The span of that period might last around four seconds. A hundred-story building, by comparison, may move on the order of two-and-a-half to three feet to each side, cycling through a ten-second period. Typically, the taller the building, the longer the period of its cyclical motion.

And the implicit question of how much movement we feel:

> Acceleration is what causes the body forces that might tip us off our firmly planted feet, or nudge us back into the passenger seat of a car pulling away from a stoplight. Fighter pilots experience acceleration at many times the magnitude of gravity—“4 Gs” or more. The top of our hundred-story skyscraper accelerates through its period, as it sways from one side to the other, at a mere fraction of what a fighter pilot feels: maybe ten milli-g’s, or one hundredth of the force of gravity.

Surely it's not actually acceleration that you feel most of the time. A constant acceleration is, after all, indistinguishable from constant gravity, and so small acceleration vectors which, when added together with the local gravity vector, are still just a little more or less than 1 g, aren't really detectable - they just feel like gravity is pointing in a different direction. Obviously larger accelerations are detectable as being 'not quite like normal gravity'. When it takes different-to-normal effort to move your limbs, your body gives you feedback about it.

What you can feel is change in acceleration - a sudden change in the direction your body perceives as 'down'. Braking in a car doesn't feel much different to driving down a hill, but when the car's speed reaches zero and suddenly stops decelerating, the jerk as gravity snaps back to vertical is definitely noticeable. It's the jerk, not the acceleration, that throws you off your feet when you're standing on the subway and it pulls into or out of a station.

One can certainly feel constant acceleration "in their bones" and more importantly, the inner ear [1]. I don't think that jerk or higher-order derivatives of position contribute significantly to these sensations. As you wrote, in cases where the overall deviation from the day-to-day experience of the Earth holding on to you via gravity is small, you probably don't feel anything even when there are significant "swings" in acceleration, just not in terms of magnitude.

  > Braking in a car doesn't feel much different to driving down a hill

This is why simulators can be bolted down and still give a close approximation of a moving vehicle by tilting the pilot instead.

[1] http://en.wikipedia.org/wiki/Vestibular_system

My initial reading of this comment caused me a bit of confusion. We really do feel acceleration in a literal sense, but I think the point is that as human beings, we're accustomed to the constant force of gravity, so any constant acceleration due to other forces feels like gravity. Just a nitpick: gravity doesn't change in any way when a car stops decelerating. The net force does change (causing the jerk, i.e., second derivative of velocity), but we feel it most because we become accustomed to the gravity-like forces (and their non-vertical direction) acting prior to stopping.

And the technical term for it is actually the jerk:

http://en.wikipedia.org/wiki/Jerk_(physics)

Yes - da/dt being called jerk is a great example of a technical term providing precision to something people have an intuitive understanding of. When we describe motion as 'jerky' it really is generally the case that it has high peak absolute 'jerk'.

What I wasn't aware of until I checked out that Wikipedia link, though, was that one school of thought for naming the next three derivatives of motion with respect to time is to call them 'snap', 'crackle' and 'pop', which I just love (much better than the more formal name for the next derivative, which is 'jounce').

It's unbalanced forces you feel, not unbalanced acceleration per se. If you have a constant acceleration, it's because there's a consistent force on you.

It's why this part of the article is wrong: >Humans are also terrible at perceiving velocity at a constant speed. [not perceiving sway] is why, when you’re traveling on a train at a steady fifty miles an hour, your body believes you might as well be sitting perfectly still.

Humans in a train moving steadily are enclosed in an environment where everything is moving at the same speed as them - seats, air, everything. There are no unbalanced forces to feel in the first place. Just the same as we don't perceive the earth's movement around the sun (well, in a moving-body sense) or the sun's movement around the galaxy (which is blisteringly fast on a human scale); because our frame of reference moves with us.

Of course, in the real world, trains do sway side-to-side and also up and down a little where the rails meet up (click-click click-click...), and we all feel that. But we're talking about a theory train here, and only looking at forward velocity :)

Edit: Wikipedia says the sun orbits the galaxy at 220km/s. Monty Python's "Galaxy Song" says 40,000mph, which works out to 18km/s. Either way, it's pretty zippy for us humans.

Galaxy Song: https://www.youtube.com/watch?v=buqtdpuZxvk

Right: We're only terrible at perceiving constant velocity without visual reference because it's physically impossible to do so in a controlled environment, according to Newton.

What we're terrible at is perceiving constant acceleration in very fine increments, like 10 milli-G while standing, or on up past 100 milli-G while sitting or prone. This is directly equivalent to sensing a certain slope/grade in the terrain, if one is robbed of accurate horizontal (horizon) and vertical (trees/buildings) reference. The mind is capable of tolerating several degrees of tilt while being perfectly convinced everything is flat, so long as the visual references point in that direction... and even at greater extremes we really only notice topography when it's highly variable, cliffs and abrupt hills and sharp changes in slope. I have an unconfirmed notion that our ability to, for example, carry things on our back, or walk while pregnant, would be sharply curtailed if our body didn't automatically adjust to the different center of mass, and different perceived gravity vector from the standpoint of our skeletal centerline.

Humans directly perceive jerks, and they perceive them with alarm, because if the ground is jerking in a natural context it means you're about to fall off a slope and die.

So I think most people with a basic grasp of Newtonian dynamics get that if you're in a sealed box moving at a constant velocity, you can't actually tell if you're moving, or how fast. That's not too tricky (though you need Special Relativity to deal with why that's true even if you are moving very very fast, to explain why light inside your box still seems to go the same speed in all directions).

My issue with the article is that it agrees with that but then claims people are disturbed by small accelerations - and my contention is that you also can't detect small constant accelerations. As another poster wrote, a small constant linear acceleration is indistinguishable from the floor being slightly angled.

If you get back in your sealed train carriage and I accelerate it very gently forwards, you can't actually tell whether you are accelerating or simply at a slight angle. A small enough acceleration might not be noticed at all, just as a small angle of the floor would also go unnoticed. But if I change the acceleration - stop accelerating you or increase the acceleration - you will notice. It might be perceived as motion or the carriage rotating but it will be perceived.

Note that I'm not making the strong claim that you can't tell there is a net force acting on you. I'm only making the claim that you can't tell the difference between an accelerating force and being stationary in a gravitational field. And I'm claiming that when the net acceleration is close to 1g, your body can't tell at all that there is anything other than gravity acting on you. Higher g's are of course readily attributed to movement.

Obviously when a building sways, though, the acceleration is changing, switching between + and - a fraction of a g, not constant. And it's my contention that it is the changing acceleration - the presence of 'jerk' - that people detect and react to, precisely because the accelerations involved are too small to be perceived directly. You feel like you must be accelerating (or the floor must be tipping) because you perceive a change in the net acceleration you feel - a change in the direction you sense as down. You feel the jerk not the acceleration itself.

There seems to be a colloquial usage of the word "acceleration" that only means "change in acceleration". It's not strictly correct, but it's pretty par for the accuracy on a Kinja property.

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I'm afraid the OP had it right. Constant acceleration is what you feel when you are standing on the ground, and feeling the earth pressing against your feet. Just as you would feel standing in a rocket accelerating constantly. If the rocket were moving at a constant motion, you would be floating weightlessly inside it.

> Constant acceleration is what you feel when you are standing on the ground

Acceleration only occurs when there's a change in velocity. What you feel while standing on the ground is a constant force, but due to the normal force (of equal magnitude but opposite direction) of your feet pushing back on the ground, no acceleration occurs. The rocket on the other hand is accelerating, and that acceleration causes the normal force between your body and your chair to accelerate your body with the rocket.

You're going to have to take that one up with Einstein: http://en.wikipedia.org/wiki/Equivalence_principle

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