Most certainly. I was just nitpicking that one statement. Some people here go through the math and come up with a figure of 0.656 μm/s2 from air drag, which is about 0.067 μg. So, less than typical tidal forces, but not negligible. You can see the altitude loss between boosts in the past 6 months here:
(Interestingly, that range also includes a de-boost, which sometimes happens–sometimes to allow a craft to dock which otherwise wouldn’t have enough propellant, other times to avoid space debris)
Not at all. Gravity is still pulling on them just like it does on earth- slightly less because they’re a little higher, but still something like 90% of sea level gravity.
But they’re in orbit, which means that in effect, they’re traveling fast enough to fall around the Earth as they orbit, instead of re-entering. So the astronauts themselves experience a constant sensation of free fall, because that’s exactly what they’re doing- constantly falling around the Earth.
Not coincidentally, that’s why getting to orbit is all about going fast enough, not getting high enough. You have to go fast enough to fall around the earth, and the faster you go, the wider ellipse you describe as you fall around.
As far as what we feel, our bodies feel force, whether it’s as pressure, weight, etc… Going at a constant velocity doesn’t exert force on us, but acceleration (positive OR negative) does.
That seems like an extremely misleading thing to say. Objects in orbit or otherwise falling in a vacuum experience a gravitational force that is not balanced by anything, causing a net acceleration towards the centre of the earth. (If the force were balanced, the space ship would fly in a straight line.) The relation between force and acceleration is simply Newton’s second law, so objects near each other will experience about the same acceleration.
Actually it does not take that long for infalling objects to be destroyed by a black hole, and furthermore unless the hole is ridiculously huge they will be ripped apart by tidal forces before even making it past the event horizon. All that just from gravity, without even counting being fried by radiation.
Perhaps a simple answer for the OP is that velocity is constant while acceleration is not. You don’t feel the earth turning because you’re turning along with it at constant velocity. If it was suddenly to accelerate (or decelerate), you would certainly feel it.
Depends on your coordinate system. In a rotating frame, the object is stationary and the gravitational force is balanced by the centrifugal force. Gravity is just as much a pseudoforce as centrifugal force is; neither is more valid than the other.
Of course. Both of those sentences – the first, and the parenthesized second – are obviously true. I’m simply trying to provide an intuitive insight into why free-fall is not subject to acceleration forces. In a sense, the orbiting spacecraft really is traveling in a straight line – according to GR, it’s describing a geodesic in curved spacetime.
Another way of describing this situation is that the only thing we feel is deviation from free-fall. On Earth, that deviation is caused by electromagnetic forces that prevent us from falling directly towards the Earth’s surface. In a parachute that deviation is caused by the parachute’s motion being resisted by the air (and the deviation is transmitted to the parachutist by the lines connecting him or her to the chute). When there’s no deviation, you don’t feel anything - even if you are accelerating towards the center of the Earth (but are currently in a vacuum, or inside a space vehicle traveling through a vacuum). Gravity always affects your motion - but only causes a “feeling” when resisted.
I typed in too much of a hurry and now too late to edit. I obviously disagree with your first sentence that I was “misleading”, and meant to refer to the second and the parenthesized third sentence in what I quoted. Also, I should have said that I was “trying to provide an intuitive insight into why free-fall is not subject to perceived acceleration forces in its own reference frame”.
This is not a physics question; this is a biology question. (Note, I am a physicist, but not a biologist.) To answer it, we need to consider our various senses.
Our vision will sense speed by detecting the motion of nearby objects.
Our inner ear can detect rotational motion.
Our skin can detect the pressure of something acceleration towards us.
There’s probably more senses that we use to feel acceleration and velocity.
PS I don’t know about you, but when it comes to astronomical velocities, I prefer km/sec rather than mph or km/hr. This is because I read a lot of astronomical literature and that’s what they usually use there. Also the numbers are a bit more user-friendly; there won’t be any numbers greater than 300,000.
Pleonasm touched on the actual answer to the OP, which basically asked why we feel acceleration but not velocity. First we need an understanding of acceleration, which is a change in velocity either in speed or direction. Velocity is simply a constant speed.
Our inner ear has special interconnected ring shaped chambers and fluids that are arranged in 3 dimensions. When your head is moved in any direction (accelerated), the fluid in the rings stays in place as the rings turn. To picture this imagine a cup of soda and ice sitting on a table; as you rotate the cup the fluid will stay in place rather than rotating with the motion of the cup. So there is relative motion between the cup and the fluid. So how does that relate to your senses in your inner ear?
Inside the chambers attached to the rings are tiny hairlike structures that are bent as the fluid passes over them. These are specialized nerve structures that can send messages to your brain to indicate motion - both direction of movement and velocity, which is acceleration.
However, if your velocity is constant, the fluid does not rotate in the rings and no signal is sent to the brain.
(as an additional bit of anatomy, your inner ear also has sac-like structures that have a fluid in them, and some hairs in a gel containing small mineral crystals. The crystals are dense, and bend the hairs down in the direction that gravity pulls them. These hairs provide your brain with “down” information…in addition there are visual clues and sensors all over your body that tells your brain about your body position.)
This is a bit simplified. I would suggest that you google “human inner ear balance”
If you are in a box and can’t see the outside world, there is no way of determining what reference frame you are in. They are all equivalent.
You can start in an inertial frame of reference, accelerate in various ways, and integrate acceleration to determine velocity, but that velocity is only relative to the frame you started in. Humans lack the equipment to integrate, so they can’t even do this. (Moving in an elevator is a good example.)
This isn’t a human perception problem. It is intrinsic to the universe we live in.