3 ms·
Something weird about this is that humans very often ascribe the sensation of d^ns/dt^n, to feeling d^n-1s/dt^n-1. So people will say that something which acce
by jameshart 3y ago
Something weird about this is that humans very often ascribe the sensation of d^ns/dt^n, to feeling d^n-1s/dt^n-1.
So people will say that something which accelerates quickly is 'fast'. Or they will say, when they feel themselves initially being pressed back into their seat as a plane starts its takeoff roll, that they are experiencing 'acceleration' when what they are experiencing is actually jerk.
The thing is, you can't actually feel motion at a constant speed - so the only thing that tells you you are acquiring speed is your body's experience of acceleration - so when you feel yourself accelerating, you associate that with speed. Likewise, your body also can't really tell the difference between constant acceleration and just... being at a different angle, and maybe a bit heavier than normal. So it's when you experience changes in the apparent direction of 'down' and the overall 'weight' you're feeling that you think 'oh, we're accelerating'.
My favorite way to get a sense of what acceleration, jerk and snap feel like is to focus on what happens when you're in a car that's braking hard. You're decelerating at a relatively constant rate while the brakes are applied - it feels as if 'down' is pointing slightly forward, meaning you'd be sliding off the seat if it weren't for your seatbelt holding you back. When the car finally stops though, there's a very abrupt change in acceleration - a 'jerk'. 'Down' switches to pointing straight down again, very quickly. You're pulled back into your seat. That's jerk. And specifically the sudden onset of that swing in what direction 'down' is pointing, and then its rapid disappearance is snap. Your body feels like it's being 'jerked' around when the car stops precisely because that motion has high snap - you experience a sudden high amount of jerk, then the jerk ends.
- pixl97 3y ago>The thing is, you can't actually feel motion at a constant speed In a vacuum. If you roll the window down, or if the atmosphere starts ablating you into a hot plasma then you may realize you're going very fast.
- jameshart 3y agoOr the air's going very fast past you. Who can tell?
- brokencode 3y agoUnless you’re in a wind tunnel or a hurricane, you can be pretty confident in the reason for the fast air.
- pixl97 3y agoWell, if you're on a 2D plain, it's pretty easy to tell. If the solid plain is stationary to your motion, then the air is fast. If the solid plain is moving, then it's a little tricker to figure out.
- lambdasquirrel 3y agoOh cmon, who rolls down the windows in an airplane? ;)
- uoaei 3y agoThen you're experiencing acceleration, just in the opposite direction.
- jameshart 3y agoNot if you’re moving at constant speed. The air molecules are experiencing acceleration.
- uoaei 3y agoPlease define "drag" in a way that somehow contradicts what I said...
- jameshart 3y agoThe setup for our thought experiment here is someone traveling at constant speed. By definition they are not experiencing any acceleration. If they are experiencing a drag force they are also experiencing a thrust force that is equal to it because they are traveling at a constant speed. ‘Feeling the effect of a force’ is not the same thing as ‘experiencing an acceleration’.
- codethief 3y agoHmmm, from a Newtonian perspective I would have argued (or at least my personal impression is) the only thing we actually perceive is force (i.e. acceleration). All the situations you described are just (higher-order) changes of acceleration and even if you give them a name: At the end of the day, the only thing that matters to our bodies is the force.
- jameshart 3y agoSure, but my argument is that you only really take note of changes in the acceleration you experience. Yes, your internal sense of ‘which way am I accelerating’ is the sensor you’re using (also ‘which way and how hard are my limbs being pulled’ and ‘how much force am I feeling in my joints and on the parts of my body that are touching the objects around me’) - but when those all indicate a generally constant vector with a magnitude close to 10m/s^2 your body just intuits that that direction is ‘up’. If that vector is changing your body figures you must be moving because ‘up’ normally doesn’t move. So it’s changes in acceleration that create movement sensations, not acceleration itself.
- codethief 3y agoInteresting thought but I'm not entirely convinced yet: 10m/s² down (along your body axis) is not the same as 10m/s² in any other direction. I would assume the body is perfectly able to distinguish between those.
- jameshart 3y agoYes, you know if you are standing up or lying down. But your body doesn’t interpret either of those sensations as movement. If you flip from one to the other suddenly, your brain figures out ‘we’re rotating’ pretty quickly though.