4 ms·

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

• mapt 11y ago
  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.
• jameshart 11y ago
  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.