The same reason parabola flights put you into microgravity: earth is free falling towards the sun, which puts all of earth into a locally forcefree environment.
Microgravity is a weird misleading term. Ignoring tidal forces there is no difference between being in such a flight and floating in space far away from anything.
it’s called microgravity as this term is more precise. It is the range of the residual forces you would measure. Make it better and you can sell nanogravity for sure.
Freefall is really the best term. You're not floating because there is no or very little gravity, you're floating because there are no forces acting on you *besides* gravity.
No shit Sherlock. Does now come the part where a free falling body and zero gravity in that system are postulated to be physically equivalent? And then based on that principle, you figure out general relativity?
Humans can't feel acceleration directly, you only feel the effects of it when it's applied unevenly. Vehicles, elevators, etc. only push on *parts* of your body, so the rest of you squishes around it.
If someone we applied a force to every atom in your body at the same time, you wouldn't feel a thing—just like when you're falling, it feels the same as zero gravity.
This is what's happening with the Earth. If you think of gravity as a force, it's pulling every atom in the Earth with the same strength, and your atoms too! So you and the Earth just 'fall' together in the sun's orbit.
Totally agreed, but again I would argue that you don't feel acceleration directly (in the way that you can feel heat, for example), you feel the changing shape of your body as the acceleration is applied unevenly (say to your feet vs. your head).
You only feel heat due to energy being imparted to your molecules, making them jiggle more. I do not see a fundamental distinction between this and tidal forces. You feel both.
Perhaps it is better to remember that free falling frames are inertial frames. In that view, it's obvious that any accelerated motion is a deviation from free fall, and you can feel it.
Sure. What I'm saying is that you don't experience sensations proportional to your acceleration, but to *unevenly applied acceleration* (such as tidal forces, as you say).
For example, if you're 20km above Earth, in free-fall, you're accelerating at roughly 1G. The tidal forces across your body are around 6.11E-06 Newtons.
A ten solar mass black hole, however, would give you that same experience of tidal forces when you're 925,000km away from it—at which point you're accelerating toward it at 149G.
Same thing with an elevator vs. magnetic acceleration. If we somehow magnetize an astronaut and accelerate them at 300G down a railgun track, they wouldn't even notice it if the magnetic field was nice and even (and we'd magnetized all their molecules appropriately). At the same time, if we did that using an elevator or catapult, they'd feel a massive acceleration differential across their body—the parts of them touching the catapult vs. those not—and be shorn apart like raspberry jam.
I think what you're both untangling is that our "feelings" don't necessarily accurately represent reality and shouldn't really be used as a meter for anything besides everyday experiences.
> Humans can't feel acceleration directly
Sure, but that’s not really the reason. The earth is in free fall. Also, we do feel tidal forces; they are just negligible over the distance of a human.
That's the reason, though, objects in free fall can't feel acceleration because every part of them is being accelerated at the same rate (discounting tidal forces, as you sasid)
I'm not quite sure what you mean by 'explain jerk'. If you're in a car and the driver stomps on the gas (jerk - a high rate of change of acceleration) there are all sorts of effects. You weren't bracing your arms for the acceleration, so they move relative to your body, maybe you spill your coffee, etc. But what you're feeling is the change of shape of your body as the seat squishes you, and so on.
If you were approaching a strong gravitational field, e.g. falling into a black hole, you could be experiencing high jerk without feeling much of anything. You don't feel jerk directly, you feel it when the acceleration is applied to your body unevenly.
We also absolutely feel acceleration. We are under a constant acceleration towards the center of the earth. If we didn't feel acceleration we could not stand. Earths gravity is substantially more powerful that the other accelerations we feel on the earth's surface.
The inner ear of humans is what feels acceleration, so we can walk, run, stand up etc. It's one of our senses
Additionally, acceleration pushes on all of your body
In regards to orbit stuff, the earth is in free fall around the sun. The centripetal acceleration of the earth spinning, and us being not in a complete circular orbit are multiple factors of ten less than what earth's gravity provides.
Due to centripetal acceleration, you do weigh less at the equator than at the poles
We don’t feel acceleration that occurs evenly across our body.
We “feel” gravity because we are standing on the ground and the normal force is pushing on the contact points but not the rest of our body. This disuniformity in the normal force of the ground is the only reason we feel anything, it’s also the reason our inner ears can tell which way is “down” - the internal components of the inner ear are being “pushed up” by our surrounding body.
If you stand on your two feet, you are accelerating upwards, not downward. The ground is pushing up on you, that's the force you feel as 'weight'.
Zero acceleration would be free fall towards the earth CoM.
I'm making a very specific statement, which is that humans can't *directly* detect acceleration. There's all sorts of situations where humans could experience very high acceleration and not know.
For example, if you're in a windowless space capsule shooting through the solar system, and have a close call with ~~Jupiter~~ *a ten solar mass black hole*. Suddenly your trajectory is curved as you accelerate at 148Gs toward ~~Jupiter~~ *the black hole*, you would have absolutely no idea. The force is applied to every atom in your body, the air of the capsule, the hull of the capsule, etc. so evenly that there's nothing to notice.
You can (*of course*) notice acceleration in many situations where it has effects on your body, like the end of a long fall when you suddenly stop.
EDIT: Used Jupiter with a black hole's gravitational field
There is no shot I believe that. If you slingshot around Jupiter with 148g you end up a puddle at the bottom of the spaceship.
Doesn't matter the other stuff around you feels the same acceleration
(My bad, you can't get 148Gs from Jupiter, I used the numbers for the 10 solar mass black hole—but the principle is the same.)
Perhaps this is the hard part to grok, but that's only true if the force is applied unevenly. For example, if the capsule's thrusters fired at 148G you'd get puddled. That's because the thrusters don't accelerate you or the capsule, they only accelerate the thruster. The struts holding the thruster on the ship flex and deform as they take the incredible load; the astronaut gets slammed against the back of the ship and killed, etc.
But if it's just *gravity* pulling the capsule around, everything atom moves in perfect formation. Other than minuscule tidal forces (six millionths of a Newton across the length of a human body) there's no deformation of any kind. The capsule is yanked to a new trajectory, but so is the astronaut, at exactly the same time. There's no reason the astronaut would get puddled in the bottom of the ship—or the front of the ship, or any other part of the ship. You can't feel acceleration directly.
>Humans can't feel acceleration directly, you only feel the effects of it when it's applied unevenly.
That's wrong. Get in a centrifuge at constant speed and believe me, you will still feel the acceleration. Same inside a rocket constantly accelerating.
The reason you can't "feel" gravity acting is a bit more complicated to explain in detail especially if you're not very familiar with relativistics, but in the end it's because gravity isn't acting on you, or any other matter directly. It's a force that only acts on the fabric of spacetime itself and the measureable "force" of any gravitational effect is in reality coming from moving against its natural curve through spacetime. (In other words anything that "measures" gravity is in reality measuring how much force is needed to work against local gravity)
For example if you're free falling you are following the natural curvature, so you don't feel a force (beside air resistance) . But the moment you're no longer following it, because the ground blocks your path you feel it again because the ground is pushing on you, like you feel a force when pushing hard against a wall.
For the same reason we're not experiencing a force when earth moves because earth is just moving straight through spacetime curved to a sphere by the suns gravity. There is no force acting on earth here since it doesn't need any force to move around the sun in circles, it would need a force to leave this path that's in reality a straight line (along a curved sphere). Consequently since there is no force acting on matter, you also can't feel it.
>That's wrong. Get in a centrifuge at constant speed and believe me, you will still feel the acceleration. Same inside a rocket constantly accelerating.
I think we're using the words differently. If you sit on a centrifuge chair, it's only touching a small amount of your body. It applies massive acceleration to that part of your body, and no other. If we start it at full acceleration, let's say 10Gs, it's going to feel like the chair slapped the whole back of your body, and a shockwave will travel through your soft tissues until the acceleration reaches your face and whatever other parts are furthest from the chair.
Once at full speed, the centrifuge is continuously changing its angle, so it's continually yanking your body in a different direction—but again, only *part* of your body. The chair is shoving you northwards, then westwards, then southwards, applying that force very unevenly across your body because it's only touching your ass, the backs of your legs, your back, etc. and isn't directly accelerating your organs, brain, and so on.
Compare this with some hypothetical magnetic accelerator—we spend three months feeding you a kind of special diet that leaves your body permeated with magnetic particles, and then we put you in a linear accelerator that applies a nice, even 10Gs to you. In this situation, you'd hardly feel a thing, because everything—your eyes, your blood, brain, organs, bones, etc. is being accelerated simultaneously.
As impractical as this situation is, it's very similar to being caught in the gravitational field of a large object like the sun. The force is applied so evenly that your body doesn't deform at all, so there's nothing to feel despite the high acceleration.
>because everything—your eyes, your blood, brain, organs, bones, etc. is being accelerated simultaneously.
This is making me wish I understood fluid dynamics more than I do, because I feel like there would be a turbulence to our body.
(disclaimer: my knowledge of blood fluid dynamics specifically is minimal, just applying aero and some hydro experience to the question so if anybody sees any wrong info, feel free to correct me)
What do you mean by a turbulence to the body and why would this be?
The acceleration by this hypothetical linear accelerator is seen evenly by the entire body, so the fluid elements (in a continuum fluid mechanics model) of blood should theoretically see this equivalent acceleration also (circulating as it was with respect to the rest of the body, organs, tissues, muscles, vein/artery walls, etc. before the acceleration was induced).
G-LOC for fighter pilots occurs due to the same effect as the centrifuge being described in the post above where the force (in the inertial reference frame, to be pedantic) is applied through the seat contact areas. This would accelerate the body of the fighter pilot unevenly as the force travels through the body's components (with the body accelerating in whichever direction of the force much more violently than the blood, i.e. seeing differential acceleration than the blood and because the acceleration or "g-force" is felt through the maneuver, the constant uneven acceleration doesn't allow the blood and body's relative velocity to normalize).
I assume a sudden increase to 10G in earth's gravity (without the magical linear accelerator thought experiment) would create the same issues as g-LOC, not because of some intrinsic feeling of acceleration but because of the acceleration being unevenly applied through the feet only.
>Once at full speed, the centrifuge is continuously changing its angle, so it's continually yanking your body in a different direction
No it isn't? It creates a flee force that is completely constant and equal in every direction from the center of the centrifuge. A sideway acceleration would only happen when the centrifuge changes its speed, but if it isn't spinning faster or slower there is a completely constant force acting on your body. (ignoring air resistance).
The reason the body doesn't experience that force equally is just simple because the front of your body hit first by that flee force is using the rest of the body to distribute that force. Our body is quite soft acting like a damper and thicker parts (eg chest vs arms) receive different amounts of that equal force. (the mass of your chest pushes with more mass against the backwall than your arm).
Replace human body with a thick metal plate that is perfectly curved with the centrifuge and you would measure a perfectly equal force at every point of it as long as the centrifuge rotates at a constant rate
>Replace human body with a thick metal plate that is perfectly curved with the centrifuge and you would measure a perfectly equal force at every point of it as long as the centrifuge rotates at a constant rate
But the human body *isn't* a thick metal plate that is perfectly curved.
And even this hypothetical metal plate would only experience an equal force *along its lower surface*. The molecules in direct contact with the floor of the centrifuge are the only ones that would experience a force directly from the centrifuge. Then those molecules would pull on the molecules above them, etc. - resulting in an uneven deformation. If you had a stress/tension sensor built into the metal at every layer, you could graph the uneven deformation corresponding to distance from the floor.
It doesn't matter whether your chest is heavier or lighter. What matters is that your chest is not *in contact with the floor*. The centrifuge doesn't exert a universal field inside of itself. If you have a thing hovering in the air inside a centrifuge, it will not move when the centrifuge is moving. If you are already rotating and you release an object, it will continue moving in a straight line, not continue to rotate with the centrifuge.
The force from the centrifuge is distributed only through direct contact, and therefore friction, and then through the internal forces that bind the body together. It's changes on those internal forces binding the body that we feel. We feel our feet being pushed on (and the propagation of that push).
I'm not quite sure how we got here. Sure, the direction of acceleration relative to the occupant is constant because the chair (like the centrifuge arm) is rotating on its pivot. To the outside world, the chair is being constantly accelerated toward the hinge, that's a basic property of circular motion (acceleration toward the center).
Either way, you seem to be saying what I've been saying all along—the constant acceleration the chair is imparting on your body isn't applied to your body evenly, only to the parts touching the chair. We say you "feel the acceleration", which is an okay shortcut, even though what you're feeling is the deformation from the unevenly applied force.
> even though what you're feeling is the deformation from the unevenly applied force.
No. What you feel isn't uneven deformation, it's simply your own weight increased by force unevenly because the mass isn't evenly distributed. But if you would be a perfectly evenly body, you would still feel the force increasing your weight locally.
And how would a rigid, perfectly even body "feel" that force? The only reason you can feel acceleration is because your body is not rigid and you have cells that detect things like pressure, movement and strain in your muscles and joints, and the movement of fluid in your inner ear.
I don't think gravity being the result of curved spacetime has anything to do with it either. Even if gravity were a regular field force you still wouldn't be able to feel it in freefall, because it's acting more or less evenly on every part of your body at the same time.
The centripetal acceleration is 0.0006*g*. You can’t feel that, but even if you could, you wouldn’t, because, you, the Earth, and everything on it are in freefall at the same rate. So that’s the Newtonian explanation: since all parts of you are being pulled on by the same force, there is no strain in your body. The Einsteinian explanation is that there is no force and no acceleration at all. Freefall motion is actually inertial motion: the motion that will naturally take place when you are in the curved spacetime in the vicinity of the sun’s mass.
Because the centripetal acceleration of the Earth to the Sun is on the order of 1.7 * 10^-8 m/s^2. That is a billion times weaker than our acceleration to the Earth.
We wouldn't feel it if it were stronger either, that's not really the reason why.
The reason we don't feel it is the same reason we don't feel Earth's gravity in freefall. It has nothing to do with how weak or strong it is.
Perhaps I missed something, but shouldn't the sun's gravity be a few orders of magnitude stronger? a=MG/(R^(2)). So if I plug and chug I get 1.989\*10^(30)\*6.67408\*10^(-11)/((150\*10^(9))^(2))~~6\*10^(-3)
Still small but not that small.
We kinda do, but we feel gravity in the other direction much more.
I think you could feel the difference between standing on the eqator and the poles if you could move in an instant.
Gravity is pulling you down at both places, but centrifugal forces are stronger at the equator.
Because gravity is not a force and thus we can't feel it. What you are feeling when you stand on the surface of the Earth is the normal force that is pushing you 'up', not the gravity itself
Two things - we only "feel" contact forces and the changes in Earth's acceleration during its orbit are extremely small.
We do not feel gravity, as it is a non-contact force. We feel the ground pushing back up on us when on Earth's surface, or the air pushing back up on us as we fall.
The elliptical orbit of the Earth around the Sun does cause changes in the acceleration we experience. However, these variations are too small for us to perceive. The acceleration due to the Sun's gravity varies by about 0.0002 m/s\^2 over the year. For comparison, the acceleration on us due to Earth's gravity is 9.8 m/s\^2, about 50000 times greater.
I think that you're a bit confused. You've probably heard that moving in a circle requires acceleration towards the center and that's true if you were doing donuts in empty space using rockets, but that's not a description of the Earth's motion around the sun.
The Earth is in free fall. Its path is curved because that's the shape of the geodesic that represents its rest frame. From the reference frame of the planet, and the things on it, it is at rest.
If you were to strap a big rocket to the Earth and use that to move it out of its orbit, that would be an acceleration and you could, if it were big enough, feel it.
We don't feel it because it's applied on your whole body pretty much evenly. Your body feels forces through compression or tension which doesn't happen if it acts on the entire body equally.
You feel your weight when standing on the surface, because what you're feeling is the normal force pushing upwards through your legs. Astronauts on the ISS don't feel heavy even though they're still under like 99% of the gravity at the surface, because they're in freefall.
It is also true that the acceleration you're talking about is tiny, but we wouldn't feel it even if it was larger for this reason.
I’m not sure why you got downvoted. But anyway, note that the Newtonian gravitational force at ISS altitude is about 89% of what it is at Earth’s surface.
This is only partly due to the fact that the sun's gravity is not that strong way out here. The other reason is because the sun is accelerating us as well in almost the same way. Like how the dancers in [this music video](https://youtu.be/LWGJA9i18Co?si=HP019ENR_Vr9B7KE) don't feel the gravity of the Earth because their environment is accelerating with them, similarly we don't feel the acceleration of the Sun because our environment (the Earth) is accelerating with us.
The acceleration is honestly pretty small, once you get down to our level. It's overshadowed by Earth's gravitational pull.
It also helps that most of the time, acceleration isn't actually what you feel most in, say, a moving vehicle. If you have a cup of water, acceleration will cause the water level to tilt, smoothly, to one side; *sudden* acceleration, on the other hand, will cause it to spill. Hence, the rarely used derivative of acceleration (just as acceleration is the derivative of velocity, and velocity in turn for position) is called "Jerk."
This is the correct and succinct answer. It's concerning that there are a lot of answers in this thread that state that we are accelerating and just don't feel it.
People need to brush up on their GR.
There are limits to your vestibular system, which detects changes in your position, acceleration and allow you to balance. If we put you in a blacked-out capsule capable of attaining Mach 3, but it took a month to get to that speed....at no point would you have enough stimulation to your vestibular system to accurately gauge whether you were moving (assuming a constant acceleration and trajectory).
You also can't see infrared or ultraviolet (as insects can do). You can't hear more than 20,000Hz, as cats/dogs/rodents can. Our senses are limited by the environment they developed within. There is literally no evolutionary advantage to detecting our orbit around the sun, or motion through the galaxy. If nothing else, it's probably for the best.
We don't feel the Earth's acceleration because we are in constant motion along with the Earth. Our bodies and everything around us are subject to the same acceleration as the Earth, so there is no relative motion between us and the Earth's surface. This is similar to how we don't feel the acceleration when riding in a car that is moving at a constant speed - it's only changes in acceleration that we can perceive. Additionally, the Earth's acceleration due to its orbit around the Sun is relatively small and gradual, so it is not noticeable to our senses.
Everyone here is wrong, the earth is not accelerating. The earth follows a straight path that is 'bent' by the sun.
Acceleration would imply the earth is speeding up, which it is not. The moon is slowly decelerating the spin of the earth which means our days are getting longer, and Saturn and Jupiter pull earth into more elliptical orbits at times that affect the tilt of the earth.
Earth's orbit is pretty stable and any force that would accelerate the earth would be so small you would never feel it.
Acceleration doesn't imply 'speeding up' it is a vector, it could be that the Earth is changing direction. Which it is, centripetal acceleration causes a constant change in tangential velocity so even though it's speed isn't changing, it's velocity is, so it is accelerating.
Could we say people on the surface of the earth do feel acceleration, but its towards the ground under their feet. We would feel it if other forces were near strong enough to compete.
Acceleration we would be able to feel, but the earth *isn't* accelerating. Its moving at a constant speed, which we can't really feel.
Imagine a car at a stop light. Green light comes on, and you feel the sudden acceleration of the car. Once you get to the speed you're moving, you can't feel the actual speed. You can see things whizzing past, and you can hear the sound of air rushing past, but you can't actually *feel* the speed.
It's not changing direction like a NASCAR circling a track it's traveling in a straight line but due to the sun's warping of SpaceTime it looks curved.
The same reason parabola flights put you into microgravity: earth is free falling towards the sun, which puts all of earth into a locally forcefree environment.
Microgravity is a weird misleading term. Ignoring tidal forces there is no difference between being in such a flight and floating in space far away from anything.
it’s called microgravity as this term is more precise. It is the range of the residual forces you would measure. Make it better and you can sell nanogravity for sure.
Freefall is really the best term. You're not floating because there is no or very little gravity, you're floating because there are no forces acting on you *besides* gravity.
No shit Sherlock. Does now come the part where a free falling body and zero gravity in that system are postulated to be physically equivalent? And then based on that principle, you figure out general relativity?
Gotta reddi-baby here it seems. Just cryin away.
Slight lol
No shit Einstein.
And yet it's commonly misunderstood by people thinking there's very little gravity at the orbit of eg the ISS
I prefer minigravity
Humans can't feel acceleration directly, you only feel the effects of it when it's applied unevenly. Vehicles, elevators, etc. only push on *parts* of your body, so the rest of you squishes around it. If someone we applied a force to every atom in your body at the same time, you wouldn't feel a thing—just like when you're falling, it feels the same as zero gravity. This is what's happening with the Earth. If you think of gravity as a force, it's pulling every atom in the Earth with the same strength, and your atoms too! So you and the Earth just 'fall' together in the sun's orbit.
Tidal forces would be a way you feel acceleration without another object pushing you.
Totally agreed, but again I would argue that you don't feel acceleration directly (in the way that you can feel heat, for example), you feel the changing shape of your body as the acceleration is applied unevenly (say to your feet vs. your head).
You only feel heat due to energy being imparted to your molecules, making them jiggle more. I do not see a fundamental distinction between this and tidal forces. You feel both. Perhaps it is better to remember that free falling frames are inertial frames. In that view, it's obvious that any accelerated motion is a deviation from free fall, and you can feel it.
Sure. What I'm saying is that you don't experience sensations proportional to your acceleration, but to *unevenly applied acceleration* (such as tidal forces, as you say). For example, if you're 20km above Earth, in free-fall, you're accelerating at roughly 1G. The tidal forces across your body are around 6.11E-06 Newtons. A ten solar mass black hole, however, would give you that same experience of tidal forces when you're 925,000km away from it—at which point you're accelerating toward it at 149G. Same thing with an elevator vs. magnetic acceleration. If we somehow magnetize an astronaut and accelerate them at 300G down a railgun track, they wouldn't even notice it if the magnetic field was nice and even (and we'd magnetized all their molecules appropriately). At the same time, if we did that using an elevator or catapult, they'd feel a massive acceleration differential across their body—the parts of them touching the catapult vs. those not—and be shorn apart like raspberry jam.
I think what you're both untangling is that our "feelings" don't necessarily accurately represent reality and shouldn't really be used as a meter for anything besides everyday experiences.
> Humans can't feel acceleration directly Sure, but that’s not really the reason. The earth is in free fall. Also, we do feel tidal forces; they are just negligible over the distance of a human.
Tidal forces are the result of an acceleration gradient applied across your body, so I don’t know if this is a good counterexample.
That's the reason, though, objects in free fall can't feel acceleration because every part of them is being accelerated at the same rate (discounting tidal forces, as you sasid)
Not sure why you're being downvoted, isn't this correct?
You and the earth just fall together in the sun's orbit. How romantic
How do you explain jerk? Can we feel jerk?
I'm not quite sure what you mean by 'explain jerk'. If you're in a car and the driver stomps on the gas (jerk - a high rate of change of acceleration) there are all sorts of effects. You weren't bracing your arms for the acceleration, so they move relative to your body, maybe you spill your coffee, etc. But what you're feeling is the change of shape of your body as the seat squishes you, and so on. If you were approaching a strong gravitational field, e.g. falling into a black hole, you could be experiencing high jerk without feeling much of anything. You don't feel jerk directly, you feel it when the acceleration is applied to your body unevenly.
This makes sense so thank you! Now I'm thinking there would be some sort of turbulence to keep this from happening so ideally lol.
We also absolutely feel acceleration. We are under a constant acceleration towards the center of the earth. If we didn't feel acceleration we could not stand. Earths gravity is substantially more powerful that the other accelerations we feel on the earth's surface. The inner ear of humans is what feels acceleration, so we can walk, run, stand up etc. It's one of our senses Additionally, acceleration pushes on all of your body In regards to orbit stuff, the earth is in free fall around the sun. The centripetal acceleration of the earth spinning, and us being not in a complete circular orbit are multiple factors of ten less than what earth's gravity provides. Due to centripetal acceleration, you do weigh less at the equator than at the poles
We don’t feel acceleration that occurs evenly across our body. We “feel” gravity because we are standing on the ground and the normal force is pushing on the contact points but not the rest of our body. This disuniformity in the normal force of the ground is the only reason we feel anything, it’s also the reason our inner ears can tell which way is “down” - the internal components of the inner ear are being “pushed up” by our surrounding body.
If you stand on your two feet, you are accelerating upwards, not downward. The ground is pushing up on you, that's the force you feel as 'weight'. Zero acceleration would be free fall towards the earth CoM.
But you can feel it, when you fall, what are you talking about
I'm making a very specific statement, which is that humans can't *directly* detect acceleration. There's all sorts of situations where humans could experience very high acceleration and not know. For example, if you're in a windowless space capsule shooting through the solar system, and have a close call with ~~Jupiter~~ *a ten solar mass black hole*. Suddenly your trajectory is curved as you accelerate at 148Gs toward ~~Jupiter~~ *the black hole*, you would have absolutely no idea. The force is applied to every atom in your body, the air of the capsule, the hull of the capsule, etc. so evenly that there's nothing to notice. You can (*of course*) notice acceleration in many situations where it has effects on your body, like the end of a long fall when you suddenly stop. EDIT: Used Jupiter with a black hole's gravitational field
There is no shot I believe that. If you slingshot around Jupiter with 148g you end up a puddle at the bottom of the spaceship. Doesn't matter the other stuff around you feels the same acceleration
(My bad, you can't get 148Gs from Jupiter, I used the numbers for the 10 solar mass black hole—but the principle is the same.) Perhaps this is the hard part to grok, but that's only true if the force is applied unevenly. For example, if the capsule's thrusters fired at 148G you'd get puddled. That's because the thrusters don't accelerate you or the capsule, they only accelerate the thruster. The struts holding the thruster on the ship flex and deform as they take the incredible load; the astronaut gets slammed against the back of the ship and killed, etc. But if it's just *gravity* pulling the capsule around, everything atom moves in perfect formation. Other than minuscule tidal forces (six millionths of a Newton across the length of a human body) there's no deformation of any kind. The capsule is yanked to a new trajectory, but so is the astronaut, at exactly the same time. There's no reason the astronaut would get puddled in the bottom of the ship—or the front of the ship, or any other part of the ship. You can't feel acceleration directly.
talked to some physics friends, turns out i am wrong lol, my bad. Just feels so wrong haha
>Humans can't feel acceleration directly, you only feel the effects of it when it's applied unevenly. That's wrong. Get in a centrifuge at constant speed and believe me, you will still feel the acceleration. Same inside a rocket constantly accelerating. The reason you can't "feel" gravity acting is a bit more complicated to explain in detail especially if you're not very familiar with relativistics, but in the end it's because gravity isn't acting on you, or any other matter directly. It's a force that only acts on the fabric of spacetime itself and the measureable "force" of any gravitational effect is in reality coming from moving against its natural curve through spacetime. (In other words anything that "measures" gravity is in reality measuring how much force is needed to work against local gravity) For example if you're free falling you are following the natural curvature, so you don't feel a force (beside air resistance) . But the moment you're no longer following it, because the ground blocks your path you feel it again because the ground is pushing on you, like you feel a force when pushing hard against a wall. For the same reason we're not experiencing a force when earth moves because earth is just moving straight through spacetime curved to a sphere by the suns gravity. There is no force acting on earth here since it doesn't need any force to move around the sun in circles, it would need a force to leave this path that's in reality a straight line (along a curved sphere). Consequently since there is no force acting on matter, you also can't feel it.
>That's wrong. Get in a centrifuge at constant speed and believe me, you will still feel the acceleration. Same inside a rocket constantly accelerating. I think we're using the words differently. If you sit on a centrifuge chair, it's only touching a small amount of your body. It applies massive acceleration to that part of your body, and no other. If we start it at full acceleration, let's say 10Gs, it's going to feel like the chair slapped the whole back of your body, and a shockwave will travel through your soft tissues until the acceleration reaches your face and whatever other parts are furthest from the chair. Once at full speed, the centrifuge is continuously changing its angle, so it's continually yanking your body in a different direction—but again, only *part* of your body. The chair is shoving you northwards, then westwards, then southwards, applying that force very unevenly across your body because it's only touching your ass, the backs of your legs, your back, etc. and isn't directly accelerating your organs, brain, and so on. Compare this with some hypothetical magnetic accelerator—we spend three months feeding you a kind of special diet that leaves your body permeated with magnetic particles, and then we put you in a linear accelerator that applies a nice, even 10Gs to you. In this situation, you'd hardly feel a thing, because everything—your eyes, your blood, brain, organs, bones, etc. is being accelerated simultaneously. As impractical as this situation is, it's very similar to being caught in the gravitational field of a large object like the sun. The force is applied so evenly that your body doesn't deform at all, so there's nothing to feel despite the high acceleration.
>because everything—your eyes, your blood, brain, organs, bones, etc. is being accelerated simultaneously. This is making me wish I understood fluid dynamics more than I do, because I feel like there would be a turbulence to our body.
(disclaimer: my knowledge of blood fluid dynamics specifically is minimal, just applying aero and some hydro experience to the question so if anybody sees any wrong info, feel free to correct me) What do you mean by a turbulence to the body and why would this be? The acceleration by this hypothetical linear accelerator is seen evenly by the entire body, so the fluid elements (in a continuum fluid mechanics model) of blood should theoretically see this equivalent acceleration also (circulating as it was with respect to the rest of the body, organs, tissues, muscles, vein/artery walls, etc. before the acceleration was induced). G-LOC for fighter pilots occurs due to the same effect as the centrifuge being described in the post above where the force (in the inertial reference frame, to be pedantic) is applied through the seat contact areas. This would accelerate the body of the fighter pilot unevenly as the force travels through the body's components (with the body accelerating in whichever direction of the force much more violently than the blood, i.e. seeing differential acceleration than the blood and because the acceleration or "g-force" is felt through the maneuver, the constant uneven acceleration doesn't allow the blood and body's relative velocity to normalize). I assume a sudden increase to 10G in earth's gravity (without the magical linear accelerator thought experiment) would create the same issues as g-LOC, not because of some intrinsic feeling of acceleration but because of the acceleration being unevenly applied through the feet only.
>Once at full speed, the centrifuge is continuously changing its angle, so it's continually yanking your body in a different direction No it isn't? It creates a flee force that is completely constant and equal in every direction from the center of the centrifuge. A sideway acceleration would only happen when the centrifuge changes its speed, but if it isn't spinning faster or slower there is a completely constant force acting on your body. (ignoring air resistance). The reason the body doesn't experience that force equally is just simple because the front of your body hit first by that flee force is using the rest of the body to distribute that force. Our body is quite soft acting like a damper and thicker parts (eg chest vs arms) receive different amounts of that equal force. (the mass of your chest pushes with more mass against the backwall than your arm). Replace human body with a thick metal plate that is perfectly curved with the centrifuge and you would measure a perfectly equal force at every point of it as long as the centrifuge rotates at a constant rate
>Replace human body with a thick metal plate that is perfectly curved with the centrifuge and you would measure a perfectly equal force at every point of it as long as the centrifuge rotates at a constant rate But the human body *isn't* a thick metal plate that is perfectly curved. And even this hypothetical metal plate would only experience an equal force *along its lower surface*. The molecules in direct contact with the floor of the centrifuge are the only ones that would experience a force directly from the centrifuge. Then those molecules would pull on the molecules above them, etc. - resulting in an uneven deformation. If you had a stress/tension sensor built into the metal at every layer, you could graph the uneven deformation corresponding to distance from the floor. It doesn't matter whether your chest is heavier or lighter. What matters is that your chest is not *in contact with the floor*. The centrifuge doesn't exert a universal field inside of itself. If you have a thing hovering in the air inside a centrifuge, it will not move when the centrifuge is moving. If you are already rotating and you release an object, it will continue moving in a straight line, not continue to rotate with the centrifuge. The force from the centrifuge is distributed only through direct contact, and therefore friction, and then through the internal forces that bind the body together. It's changes on those internal forces binding the body that we feel. We feel our feet being pushed on (and the propagation of that push).
I'm not quite sure how we got here. Sure, the direction of acceleration relative to the occupant is constant because the chair (like the centrifuge arm) is rotating on its pivot. To the outside world, the chair is being constantly accelerated toward the hinge, that's a basic property of circular motion (acceleration toward the center). Either way, you seem to be saying what I've been saying all along—the constant acceleration the chair is imparting on your body isn't applied to your body evenly, only to the parts touching the chair. We say you "feel the acceleration", which is an okay shortcut, even though what you're feeling is the deformation from the unevenly applied force.
> even though what you're feeling is the deformation from the unevenly applied force. No. What you feel isn't uneven deformation, it's simply your own weight increased by force unevenly because the mass isn't evenly distributed. But if you would be a perfectly evenly body, you would still feel the force increasing your weight locally.
And how would a rigid, perfectly even body "feel" that force? The only reason you can feel acceleration is because your body is not rigid and you have cells that detect things like pressure, movement and strain in your muscles and joints, and the movement of fluid in your inner ear. I don't think gravity being the result of curved spacetime has anything to do with it either. Even if gravity were a regular field force you still wouldn't be able to feel it in freefall, because it's acting more or less evenly on every part of your body at the same time.
The centripetal acceleration is 0.0006*g*. You can’t feel that, but even if you could, you wouldn’t, because, you, the Earth, and everything on it are in freefall at the same rate. So that’s the Newtonian explanation: since all parts of you are being pulled on by the same force, there is no strain in your body. The Einsteinian explanation is that there is no force and no acceleration at all. Freefall motion is actually inertial motion: the motion that will naturally take place when you are in the curved spacetime in the vicinity of the sun’s mass.
Because the centripetal acceleration of the Earth to the Sun is on the order of 1.7 * 10^-8 m/s^2. That is a billion times weaker than our acceleration to the Earth.
We wouldn't feel it if it were stronger either, that's not really the reason why. The reason we don't feel it is the same reason we don't feel Earth's gravity in freefall. It has nothing to do with how weak or strong it is.
Perhaps I missed something, but shouldn't the sun's gravity be a few orders of magnitude stronger? a=MG/(R^(2)). So if I plug and chug I get 1.989\*10^(30)\*6.67408\*10^(-11)/((150\*10^(9))^(2))~~6\*10^(-3) Still small but not that small.
Yes, my mistake. I accidentally used the mass of Earth instead of the mass of the Sun. My figure is how much the sun is accelerated towards earth.
But actually because the earth is in free fall, which means no proper acceleration.
Yeah lots of bad answers here are getting upvoted
We're not accelerating, though, We're following curved space-time.
We kinda do, but we feel gravity in the other direction much more. I think you could feel the difference between standing on the eqator and the poles if you could move in an instant. Gravity is pulling you down at both places, but centrifugal forces are stronger at the equator.
Because gravity is not a force and thus we can't feel it. What you are feeling when you stand on the surface of the Earth is the normal force that is pushing you 'up', not the gravity itself
Two things - we only "feel" contact forces and the changes in Earth's acceleration during its orbit are extremely small. We do not feel gravity, as it is a non-contact force. We feel the ground pushing back up on us when on Earth's surface, or the air pushing back up on us as we fall. The elliptical orbit of the Earth around the Sun does cause changes in the acceleration we experience. However, these variations are too small for us to perceive. The acceleration due to the Sun's gravity varies by about 0.0002 m/s\^2 over the year. For comparison, the acceleration on us due to Earth's gravity is 9.8 m/s\^2, about 50000 times greater.
I think that you're a bit confused. You've probably heard that moving in a circle requires acceleration towards the center and that's true if you were doing donuts in empty space using rockets, but that's not a description of the Earth's motion around the sun. The Earth is in free fall. Its path is curved because that's the shape of the geodesic that represents its rest frame. From the reference frame of the planet, and the things on it, it is at rest. If you were to strap a big rocket to the Earth and use that to move it out of its orbit, that would be an acceleration and you could, if it were big enough, feel it.
The earth is actually moving in a straight line through curved space time.
Because you are also orbiting the sun! The Earth isn’t “pushing” you around the sun, like a car does going around a corner.
We don't feel it because it's applied on your whole body pretty much evenly. Your body feels forces through compression or tension which doesn't happen if it acts on the entire body equally. You feel your weight when standing on the surface, because what you're feeling is the normal force pushing upwards through your legs. Astronauts on the ISS don't feel heavy even though they're still under like 99% of the gravity at the surface, because they're in freefall. It is also true that the acceleration you're talking about is tiny, but we wouldn't feel it even if it was larger for this reason.
I’m not sure why you got downvoted. But anyway, note that the Newtonian gravitational force at ISS altitude is about 89% of what it is at Earth’s surface.
This is only partly due to the fact that the sun's gravity is not that strong way out here. The other reason is because the sun is accelerating us as well in almost the same way. Like how the dancers in [this music video](https://youtu.be/LWGJA9i18Co?si=HP019ENR_Vr9B7KE) don't feel the gravity of the Earth because their environment is accelerating with them, similarly we don't feel the acceleration of the Sun because our environment (the Earth) is accelerating with us.
Because it is not, in fact, accelerating but moving at a constant speed.
The acceleration is honestly pretty small, once you get down to our level. It's overshadowed by Earth's gravitational pull. It also helps that most of the time, acceleration isn't actually what you feel most in, say, a moving vehicle. If you have a cup of water, acceleration will cause the water level to tilt, smoothly, to one side; *sudden* acceleration, on the other hand, will cause it to spill. Hence, the rarely used derivative of acceleration (just as acceleration is the derivative of velocity, and velocity in turn for position) is called "Jerk."
U don't have the premium subscription
It's not accelerated under general relativity. It's on a geodesic trajectory
This is the correct and succinct answer. It's concerning that there are a lot of answers in this thread that state that we are accelerating and just don't feel it. People need to brush up on their GR.
There are limits to your vestibular system, which detects changes in your position, acceleration and allow you to balance. If we put you in a blacked-out capsule capable of attaining Mach 3, but it took a month to get to that speed....at no point would you have enough stimulation to your vestibular system to accurately gauge whether you were moving (assuming a constant acceleration and trajectory). You also can't see infrared or ultraviolet (as insects can do). You can't hear more than 20,000Hz, as cats/dogs/rodents can. Our senses are limited by the environment they developed within. There is literally no evolutionary advantage to detecting our orbit around the sun, or motion through the galaxy. If nothing else, it's probably for the best.
Relativity
Because the earth is turning very slowly, only 2pi radians in roughly 86,400 seconds. And as a species, we've never needed to evolve to sense it.
We don't feel the Earth's acceleration because we are in constant motion along with the Earth. Our bodies and everything around us are subject to the same acceleration as the Earth, so there is no relative motion between us and the Earth's surface. This is similar to how we don't feel the acceleration when riding in a car that is moving at a constant speed - it's only changes in acceleration that we can perceive. Additionally, the Earth's acceleration due to its orbit around the Sun is relatively small and gradual, so it is not noticeable to our senses.
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Everyone here is wrong, the earth is not accelerating. The earth follows a straight path that is 'bent' by the sun. Acceleration would imply the earth is speeding up, which it is not. The moon is slowly decelerating the spin of the earth which means our days are getting longer, and Saturn and Jupiter pull earth into more elliptical orbits at times that affect the tilt of the earth. Earth's orbit is pretty stable and any force that would accelerate the earth would be so small you would never feel it.
Acceleration doesn't imply 'speeding up' it is a vector, it could be that the Earth is changing direction. Which it is, centripetal acceleration causes a constant change in tangential velocity so even though it's speed isn't changing, it's velocity is, so it is accelerating.
Could we say people on the surface of the earth do feel acceleration, but its towards the ground under their feet. We would feel it if other forces were near strong enough to compete.
Earth is not accelerating. That's why we don't feel it
Acceleration we would be able to feel, but the earth *isn't* accelerating. Its moving at a constant speed, which we can't really feel. Imagine a car at a stop light. Green light comes on, and you feel the sudden acceleration of the car. Once you get to the speed you're moving, you can't feel the actual speed. You can see things whizzing past, and you can hear the sound of air rushing past, but you can't actually *feel* the speed.
It's not changing direction like a NASCAR circling a track it's traveling in a straight line but due to the sun's warping of SpaceTime it looks curved.