B For Lack of a Better Term...Moon Motion

Atari_Me

It has often been posed that one reason humans cannot feel the motion of the Earth as it spins and moves around the Sun is that our bodies have acclimated to this motion due the fact that it is all we have ever known. If true, does this mean that astronauts have experienced feeling the motion of the Moon as they walk around its surface?

I imagine the above is a bit of pulp fiction as you would not feel any effects of the the Moon moving around the Earth due to the constant speed, however, as the Moon is a spinning object as well, logically would there not be a certain measure of centripetal force that the astronaut would be able to feel, much like a car moving around a curve? Or is this counteracted by the Moon's gravity?

I apologize for the third-grade level of this question, but as I am presently unable to afford a trip to the Moon, my brain is having trouble working out how the physics of this situation ends up.

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BvU

Homework Helper
Do you notice the attraction of the moon or any centrifugal force from the motion of the earth and moon around a common center of mass ? The sea does -- so we have tides. Humans are insensitive to small very low-frequency changes in gravity, whether on earth or on the moon.

For fun you could calculate the reduction in gravitational acceleration on the equator due to the rotation of the earth...

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russ_watters

Mentor
It has often been posed that one reason humans cannot feel the motion of the Earth as it spins and moves around the Sun is that our bodies have acclimated to this motion due the fact that it is all we have ever known. If true, does this mean that astronauts have experienced feeling the motion of the Moon as they walk around its surface?
No. You can't feel motion, only acceleration. And there's nothing to "acclimate to" unless the acceleration is changing. And as soon as it stops changing, everything's fine again. Think about what it feels like when you ride in an elevator car, roller coaster, etc.

Atari_Me

All the above makes sense. A side question though. Centrifugal force, much like you experience on a merry-go-round is not acceleration dependent. It is constantly attempting to push you away from the center. You do feel this force in constant motion. I assume you cannot feel this motion on the Moon due to how slight it is and when combined with the Moon's gravity, it is essentially nullified and overcome by the Moon's gravity.

Logically, if the above is true, one would never feel centrifugal force on a planetary body because a) it would not be strong enough to feel, and b) is counter-acted and overcome by the stronger centripetal force generated by gravity, correct? Otherwise, if the centrifugal force were strong enough that you could feel it pulling you away from the planetary body, it would also be strong enough to pull rocks, dirt, etc. away from the planetary body and essentially rip the planetary body apart due to the spin. As a result, you should only ever feel the stronger positive gravitational forces pushing down on you on a planetary body.

Would the above be a fair assessment?

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russ_watters

Mentor
Centrifugal force, much like you experience on a merry-go-round is not acceleration dependent. It is constantly attempting to push you away from the center. You do feel this force in constant motion.
No, I'm not sure what you're thinking there - centrifugal force is acceleration (or rather, what you feel because you are being accelerated).
I assume you cannot feel this motion on the Moon due to how slight it is and when combined with the Moon's gravity, it is essentially nullified and overcome by the Moon's gravity.
Right - just like you can't feel it on Earth. It's measurable in terms of its effect of countering gravitational force, but so small we couldn't feel the difference if it wasn't there.
Logically, if the above is true, one would never feel centrifugal force on a planetary body because a) it would not be strong enough to feel, and b) is counter-acted and overcome by the stronger centripetal force generated by gravity, correct?
Gravity isn't a centripetal force. But no, it's possible for a planet to rotate fast enough that the difference would be noticable.
Otherwise, if the centrifugal force were strong enough that you could feel it pulling you away from the planetary body, it would also be strong enough to pull rocks, dirt, etc. away from the planetary body and essentially rip the planetary body apart due to the spin. As a result, you should only ever feel the stronger positive gravitational forces pushing down on you on a planetary body.

Would the above be a fair assessment?
No, the net force can still be down, just smaller than if the rotation weren't there. You're at least right that the net force can't be positive (up).

Atari_Me

No offense Russ, would prefer if you didn’t respond if you aren’t going to provide data.

Gravity behaves as a centipetal force regardless of its classification. It is a force pushing objects inward toward a planetary body.

I’m also aware that centifugal force is classified as an acceleration but in the sense that the body is moving around an object at a consistent speed it also behaves as a constant.

You know exactly the points I’m trying to make.

davenn

Gold Member
No offense Russ, would prefer if you didn’t respond if you aren’t going to provide data.
you are crusin' for a brusin'

Gravity behaves as a centipetal force regardless of its classification.
it isn't !

It is a force pushing objects inward toward a planetary body.
it doesn't

You know exactly the points I’m trying to make.
well it would be wonderful if those points were actually based in reality as known by modern physics

from people trying to teach you .

And yet again in this thread you continue to repeat incorrect statements
I really suspect that this thread is on the same path because you refuse to listen to those much wiser than you

Dave

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PeterDonis

Mentor
It has often been posed that one reason humans cannot feel the motion of the Earth as it spins and moves around the Sun is that our bodies have acclimated to this motion due the fact that it is all we have ever known.
Can you give a specific reference? "Acclimated to this motion" could mean a number of different things.

Taking this just as you state it, I would say that it's an incorrect statement of the reason we don't feel the motion of the Earth. The correct reason is the one @russ_watters gave: that you can't feel motion, you can only feel acceleration. More precisely, you feel proper acceleration; we normally call this feeling "weight".

You appear to realize this, since you go on to say:

you would not feel any effects of the the Moon moving around the Earth due to the constant speed
I agree.

as the Moon is a spinning object as well, logically would there not be a certain measure of centripetal force that the astronaut would be able to feel, much like a car moving around a curve? Or is this counteracted by the Moon's gravity?
I think you mean centrifugal force here, not centripetal force. The spin of the Moon, or of the Earth, creates a force away from the center of rotation, not towards it.

However, there is another issue here as well. Centrifugal force is a "fictitious" force; what that means is that it's not a force you actually feel. Neither, for that matter, is gravity. If you are moving solely under the influence of gravity, you don't feel any weight; you are in free fall, weightless. The reason you feel weight standing on Earth, or on the Moon, is that the thing you're standing on is pushing up on you, preventing you from free-falling. In other words, the weight you feel is the proper acceleration due to being pushed away from the free-falling paths in your vicinity.

It is true that the rotation of the Earth, or the Moon, makes the weight you feel standing on the surface slightly different from what you would feel if it were not rotating. (This effect is much smaller for the Moon because it's rotating so slowly as compared with the Earth.) But you don't feel two separate forces, one from gravity and one from centrifugal force. You just feel whatever the total resultant is of the surface pushing on you.

Centrifugal force, much like you experience on a merry-go-round is not acceleration dependent. It is constantly attempting to push you away from the center.
But, as above, you don't feel the centrifugal force. The force you feel is the merry-go-round pushing on you, preventing you from flying off on the free-fall path that is tangent to the circular motion about the center. (If you analyze what's going on in a non-rotating frame, this is obvious.) And this force is acceleration dependent: it depends on the proper acceleration required to push you inward just enough to keep you in the circular path. Change the radius of the merry go round or its rotation frequency and you will change the force you feel, because the required proper acceleration changes.

PeterDonis

Mentor
Gravity isn't a centripetal force.
If we are taking the viewpoint I was taking in my previous post, I would say that gravity isn't a force at all. Or it's a "fictitious" force, which means it's not a force you actually feel, which means you might as well say it's not a force at all.

If, on the other hand, we take the Newtonian viewpoint that gravity is a force (it's just one that happens to not be directly felt), then it does point towards the center, so "centripetal" would not be an incorrect adjective to apply to it. I don't applying that adjective to gravity is very useful, but it's not logically inconsistent.

PeterDonis

Mentor
would prefer if you didn’t respond if you aren’t going to provide data.
I'm not sure what data you would want him to provide.

Gravity behaves as a centipetal force regardless of its classification. It is a force pushing objects inward toward a planetary body.
See my response to @russ_watters just now. This viewpoint is not logically inconsistent, but I don't think it's very useful either, certainly not for the topic we're discussing. I explained the alternate viewpoint that I recommend two posts ago; I think the problem looks a lot simpler with that viewpoint.

PeterDonis

Mentor
centrifugal force is acceleration (or rather, what you feel because you are being accelerated).
Careful. You don't feel centrifugal force. You only feel any force (weight) if there is some other force (like a merry go round pushing on you) that is preventing you from following the (free fall) trajectory you would follow if centrifugal force were the only force acting.

anorlunda

Mentor
Gold Member
Have you seen flight simulators like the one in the picture? They have a limited freedom of movement. However, they are able to "fool" the pilots into thinking that it is a much larger range of motions. That succeeds because the human body's perception of accelerations is poor, and it has a deadband. We cannot perceive accelerations below some threshold value.

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Atari_Me

If we are taking the viewpoint I was taking in my previous post, I would say that gravity isn't a force at all. Or it's a "fictitious" force, which means it's not a force you actually feel, which means you might as well say it's not a force at all.

If, on the other hand, we take the Newtonian viewpoint that gravity is a force (it's just one that happens to not be directly felt), then it does point towards the center, so "centripetal" would not be an incorrect adjective to apply to it. I don't applying that adjective to gravity is very useful, but it's not logically inconsistent.
So...my follow-up question would be...how would you describe gravity from a philosophical point of view? If it is not a force, then what is it? Would you agree that it behaves in a field, much like magnetism?

Just to make sure I state some aspects of Relativity correctly:

Would you agree with this statement? Gravity is inherent to all matter and cannot be separated from that matter. I suppose to be more precise, any object with mass.

If the above is true, how do we arrive at light bending as it passes a star? If light has no mass, how can gravity change the behavior of light? Is it as simple as gravity is not changing the behavior of light but instead changing the behavior of spacetime which light is traveling through?

PeterDonis

Mentor
how would you describe gravity from a philosophical point of view?
This is a forum to discuss science, not philosophy. I don't think this question is answerable within the scope of this forum.

If it is not a force, then what is it?
According to general relativity, it's spacetime geometry. But I don't think that's a philosophical answer; it's just a description of how GR models gravity mathematically.

Would you agree that it behaves in a field, much like magnetism?
There is a sense in which the spacetime metric, or more precisely its deviation from the flat Minkowski metric, can be thought of as a spin-2 field. This viewpoint is useful, for example, in studying gravitational waves. But it has limitations.

The spin-2 field model of gravity was also the basis for attempts to quantize gravity in the 1960s and early 1970s. It is quite possible that when we finally have a good theory of quantum gravity, there will be some level at which such an interpretation is feasible.

Gravity is inherent to all matter and cannot be separated from that matter.
This is too vague to know what you mean. If you mean there can only be gravity inside a region containing matter, this is obviously false. If you mean that in our real universe, any time we see gravity, we know there is matter somewhere, that is a reasonable statement.

If light has no mass, how can gravity change the behavior of light?
Light moves in the same spacetime geometry as everything else. That means the spacetime geometry affects the paths that light takes. Bending of light by the Sun is an example of such an effect: the Sun causes the spacetime geometry around it to be curved, and light passing close to the Sun has its path affected accordingly.

The issue of light not having mass (more precisely, rest mass) only comes into play if we ask whether light can act as a source of gravity, the way the Sun or other such bodies do. The answer to that is that, while light does not have rest mass, it does have energy, and energy can act as a source of gravity. In GR, the source of gravity is the stress-energy tensor, which includes rest mass, energy, momentum, pressure, and other stresses. So all of these things can act as a source of gravity.

Is it as simple as gravity is not changing the behavior of light but instead changing the behavior of spacetime which light is traveling through?
See above.

Atari_Me

This is a forum to discuss science, not philosophy. I don't think this question is answerable within the scope of this forum.
Science, in any field, is applied philosophy. Therefore the postulation of the theory of gravity is the philosophical aspect, and the mathematics that demonstrate how it works is the applied aspect. In this context the original question postulates if you remove the math, what is gravity. And you went into that a bit when you stated "It's spacetime geometry." It is a bit more complicated than that I think we can all agree, and what it is could fill a book, if not volumes.

The philosophical side of things can be quite baffling. Take colors and heat as an example. Certain colors absorb heat better than others, we've known that since the beginnings of humanity, but when you dive into the philosophical side of what that means it gets quite crazy. A color does not have any physical properties and absent light has little to no meaning at all, but color can change how an object transfers energy. It's easy to explain how it works from a scientific perspective, but from a why perspective it is a bit more difficult.

PeterDonis

Mentor
Science, in any field, is applied philosophy
This is your opinion, and is off topic for this forum. Further statements along these lines will receive a warning and will be deleted.

In this context the original question postulates if you remove the math, what is gravity. And you went into that a bit when you stated "It's spacetime geometry."
Geometry is math. And GR makes quantitative predictions about the results of measurements based on modeling gravity as spacetime geometry; it uses math to make those predictions.

russ_watters

Mentor
If we are taking the viewpoint I was taking in my previous post, I would say that gravity isn't a force at all. Or it's a "fictitious" force, which means it's not a force you actually feel, which means you might as well say it's not a force at all.

If, on the other hand, we take the Newtonian viewpoint that gravity is a force (it's just one that happens to not be directly felt), then it does point towards the center, so "centripetal" would not be an incorrect adjective to apply to it. I don't applying that adjective to gravity is very useful, but it's not logically inconsistent.
I think tripped over that - I think the OP used the wrong word and switching between them not necessarily correctly and and I followed it without correcting it.
Careful. You don't feel centrifugal force. You only feel any force (weight) if there is some other force (like a merry go round pushing on you) that is preventing you from following the (free fall) trajectory you would follow if centrifugal force were the only force acting.
For that, I was following the colloquial (agreed; incorrect) wording of the OP:
"Centrifugal force, much like you experience on a merry-go-round is not acceleration dependent. It is constantly attempting to push you away from the center."
Again, I followed it instead of correcting it. In a car turning left, people will say they are being "pushed" to the right (because they slide to the right in their seat), when in fact they are being pushed to the left.

Mentor

sophiecentaur

Gold Member
does this mean that astronauts have experienced feeling the motion of the Moon as they walk around its surface
the astronaut would be able to feel
I think the OP started off at a much more basic level. The thread may have veered of in the direction of 'expressing things correctly', rather than dealing with the basic content.
Those questions in the Original Post were not picked up on directly. Is the OP aware that the Moon actually spins on its axis at 1/28 the rate that the Earth spins (the rate we are used to). Also, the Moon is about 1/3 the diameter of Earth. So the rotational forces acting on an astronaut due to rotation would be 1/(3X (28)2) or about 0.0004 of the force on Earth. Why would we ever have evolved to detect that level of force, on the off chance that we could walk on the Moon one day? There are two possible significant effects of Earths rotation - changes in weight measurements (commerce) and Coriolis force (navigation). Neither of these have been 'detected' by humans without using instruments.
ISS crew are subjected to a rotation rate of 1/90minutes (about 16 times the Earth rotation rate), which is way more than on the Moon. I have never heard of any sensations that are not visually generated.

"For Lack of a Better Term...Moon Motion"

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