My impression is that our bodies can feel when we accelerate in a plane, car, roller coaster, etc but the velocity itself doesn’t really cause any physical effects.
Why is that
Generally speaking, we feel motion when one part of our body is pushing or pulling against another part.
So we don’t feel acceleration in all situations. If you are being accelerated by gravity in a vacuum, all atoms of your body are being affected in the same way and you don’t feel anything. (Except perhaps existential fear)
During acceleration in a car, you feel the back of your seat pushing against you because it is trying to move faster than you due to the acceleration of the car. During constant velocity, you are moving at the same speed of the car, and there isn’t that extra push from the seat.
With velocity there is nothing to feel. One of the principles of relativity (Galilean relativity) is that there is no experiment that you could perform to determine your velocity; only your velocity relative to some other thing.
…which is why Newton’s 1st law of motion starts with the idea of a body at rest or at a constant velocity. They are physically pretty much the same thing. Note that a speed of zero is also a constant velocity.
https://www.physicsclassroom.com/class/newtlaws/Lesson-1/Newton-s-First-Law
Then, to change that velocity, you need to have a net force acting on the body. That causes acceleration (rate of change of velocity). And when you accelerate, you feel that force acting on your body!
What would you expect to feel differently sitting in a car driving at 60 MPH or sitting in a plane going 600 MPH?
…both relative to the surface of the Earth, FWIW.
Or on a planet doing 66,000 mph?
This is either expressed poorly or just wrong. Objects in the vacuum of outer space are certainly affected by gravity. You don’t experience a gravitational force if you are in free fall, but that has nothing to do with being in a vacuum or not.
Who said they aren’t? I was answering the question of what a person would “feel.” And you don’t “feel” acceleration due to a gravitational force acting on every atom of your body.
That must be news to astronauts in orbit.
Perhaps we should list our scholastic transcripts in our user information.
I guess you could say our bodies sense change more that they do constants. A steady velocity is a constant so there is nothing to feel. An acceleration is a change in velocity that we can sense.
Same with temperature. When we touch something that is the same temp as our hand we don’t feel anything because the temps remain constant. If we touch something hot or cold we are sensing the change in the heat transfer either into or out of our hands.
Well, … If you’re not in a vacuum, you won’t be in a true free fall, because the atmosphere will be slowing you down.
To the same degree that the vacuum is total or partial, that’s the degree to which you won’t experience the gravitational force.
This is why they don’t call it “zero gravity” anymore, because even in orbit, it’s not a perfect vacuum, hence not zero gravity… So to keep the nitpickers happy, they call it “microgravity”.
Maybe you’re again not being clear, but it certainly would be news to the astronauts in the ISS that they’re experiencing gravity, since they and everything in there that isn’t nailed down is all floating around in apparent zero-G.
“Microgravity” is certainly a more accurate term, but it has fairly little to do with air resistance due to not being in a perfect vacuum. If there was any measurable acceleration from that, the ISS wouldn’t be up there very long. There are larger effects from any rotation of the ISS, from the gravity of the ISS itself, and from the motions and activities of the astronauts. IIRC experiments that are super sensitive to microgravity need to be carried out near the center of mass of the ISS.
I guess I am being not clear. Maybe I’m being too terse. There is a difference between experiencing gravity and “feeling” gravity, which again the question was about “feeling.”
If the astronauts weren’t “experiencing” gravity, they wouldn’t be in orbit. They aren’t “feeling” gravity because they are floating around. They aren’t “feeling” the acceleration on their bodies because they are in free fall. They aren’t “feeling” the acceleration of free fall because they are enclosed in a space station or whatever protected from what little atmosphere is pushing back. A sky diver will “feel” their acceleration after jumping out of an airplane because the atmosphere pushing back on their bodies will increase until they reach terminal velocity.
An astronaut finding themselves in a random spot of the solar system will not “feel” the acceleration they are suddenly experiencing towards the sun (or less likely a nearby planet).
Well, to be really precise about it, you can’t be accelerated by gravity “in a vacuum” if you mean “while not touching any other objects”. When we say, generally, that gravity is acting on something, we mean that thing is in a gravitational field. A gravitational field makes the space itself accelerate. If you want to stay stationary with respect to the planet that is creating the gravity field, you have to have some other force to accelerate you equally but in the opposite direction. That’s why standing all day is hard on the feet.
Guys, what level of nitpicking do y’all think is answering the question of the OP? I never thought the statement “if the only force accelerating you is gravity you won’t feel it” would be so controversial.
OK. I’ll head off a nitpick. If the gravitation field is so strong that you experience tidal forces between one part of your body and another, you’ll feel that.
But get just a little too close to the event horizon of a black hole, and you probably won’t feel a thing.
Take a ride in the Vomit Comet (or one of the similar commercial operations), and you get about 30 seconds of true freefall in air. The trick of course is to make sure the air is in freefall as well.
There is measurable acceleration from air on the ISS, and in fact it wouldn’t be up there very long if it didn’t get boosted by rockets on a regular basis.
I did the math on the acceleration from air vs. that of tidal forces a while back, and while I forget which was more important, they were of the same order of magnitude, and in fact both on the order of a millionth of a gee (so “microgravity” really is accurate).
Fair enough. I haven’t seen authoritative numbers on the magnitude of air drag, but I’ll take your word for it. The fact remains, however, that those other factors I mentioned – and others – are also important. In particular, both tidal effects and also centrifugal force differentials between the nearest and farthest parts of the ISS together amount to a microgravity of 0.384 μg/m (according to Wikipedia).
I think the reason some of us are talking past each other is lack of clarity about the distinction between acceleration, defined as a rate of change of velocity, and the force associated with that acceleration. Einstein’s observation that, without independent knowledge about the motion of a reference frame, it’s impossible to tell if that RF is in a gravitational field or if it is accelerating is equally valid in classical physics.
The earth’s gravitational field always produces an acceleration (near the surface) of about 9.8 m/s2 and hence can be defined as the constant g that is independent of mass. Thus if you were in a space ship accelerating at that rate, you would feel a force of 1 g against the back of any chair or wall or whatever surface was pushing you against your own inertia to accelerate along with the ship. For the same reason, on the surface of the earth you always feel a downward force of 1 g. But if you’re in free fall in a vacuum, the gravitational force is no longer discernible; one can regard it as the gravitational force being exactly balanced by your acceleration of 9.8 m/s2, which is producing a force of 1 g in the opposite direction.