Jump to content
Science Forums

Galilean relativity, or is there such thing as absolute motion?


Recommended Posts

The classic example of this question is a train moving on smooth straight tracks with all the windows covered. Is there any way to detect, with your casual human senses, whether the train is moving or still? The answer is generally taken as a firm "no." A more modern example would be if you're in a big airliner, in smooth air with all the window shades closed. Can you tell with your casual human senses that you're moving? Many of us from direct experience can say no. What about any advanced scientific equipment, is it possible even in principle if not in practice, to tell the difference?

(I should qualify this as straight-line and constant speed motion. When you get to a certain level of accuracy, you can detect a slight acceleration toward the center of the Earth as you follow the path of its curvature. So at this point, since I'm excluding accelerations, I have to approximate that away from the scope of my question, and say let's assume that the planet, as well as its gravitational field and atmosphere, are flat).

What does this mean for the concept of absolute motion, as something that may or may not effect any physical system (be it upsetting the experimental results in some subtle way, or event the concept of if such thing exists in reality)?

For example, wind tunnels have been in use to test aeronautical designs for over a century, where instead of the air staying still and the airplane moving, it's the airplane staying still and the air moving. Besides some things that have to be controlled for like effects from the walls, or scale effects like Reynolds number, I've never heard of which-thing-is-moving as being a factor to be considered. Is there any such issue with the veracity of wind tunnel data, or is the relative motion between the airplane and the air the only thing that has any physical effect?

Link to comment
Share on other sites

It's called Galilean relativity because it's been understood since the time of Galileo 400 years ago that there is no such thing as absolute motion.  The only thing that can have any physical effect is relative motion.

Wind tunnel data is just fine.

Link to comment
Share on other sites

1 hour ago, truthseeker said:

It's called Galilean relativity because it's been understood since the time of Galileo 400 years ago that there is no such thing as absolute motion.  The only thing that can have any physical effect is relative motion.

Wind tunnel data is just fine.

If there's no such thing as absolute motion, does that also mean that there's no such thing as absolute rest?

In one sense I accept wind tunnels as representing real fight as a layman, as that's undisputed engineering standard... but in another sense there's something deep seated in me that's not easy with it, and feels that if something is sitting still, then there's gotta be something fundamentally lacking! And I'm trying to get it all to agree.

Imagine that there's an air-driven turbine sticking out of the airplane powering an electric generator, should it produce the same amount of power if the airplane is in a wind tunnel vs really moving?

Link to comment
Share on other sites

Whoa! The rabbit hole suddenly deepened, to astronomic depths. Is this the same thing as the General Relativity-based precession of Mercury? I'm honestly not equipped to read your post, but this is the gist I'm getting. Are you bringing it up as actual matter that bears on whether wind tunnels are legit, or just reaching for great (and undetectable) depths of exactitude for sport? What is this a function of exactly, moving relative to the Earth's gravitational field? If that's what it is, I'll go ahead an exclude relativistic effects from the scope of my question, and limit it to classical mechanics.

So within this scope, are wind tunnels legit? What about treadmills? Is any experiment on a treadmill the same as its equivalent on normal ground (assuming the relative motion of air over the surface is the same, i.e., there's wind)? Is it possible to devise an experiment that can distinguish the difference? What about the source of the energy causing the motion of the air, or the surface, that ends up as this relative motion - does that matter? - whether it be the motor powering the moving treadmill under the still air, or the meteorological (ultimately, solar) energy powering the motion of the air over the still ground? Is this difference any more meaningful than the ultimate power source that ends up in the air turbine sticking out of the airplane in the wind tunnel vs. normal flight?

Link to comment
Share on other sites

Moderation Note: Some posts that were clearly out of place in this thread have been removed. They are not deleted but hidden awaiting moderator approval, which may or may not be given.

In the meantime, a discussion on frames of reference is always interesting and an important topic in Physics. You are certainly welcome to discuss this topic but please do not venture into the Closed topic on ddw.

As for your question about the turbine in the wind tunnel, vs a turbine flying through the air, I agree with truthseeker that the turbine in a wind tunnel would generate the same power if it sees the same apparent wind (speed of the air relative to the turbine) as it would see when flying through the air.

Basically, the wind hitting the turbine, or the turbine hitting the wind, is a collision. An observer A, in the frame where the turbine is stationary, and the wind moving, must agree with observer B, in a frame where the turbine is moving and the air stationary, about the energy of the collision. Observing the same situation from different frames of reference cannot have any influence on the outcome of an experiment.

As for the treadmill belt running through still air, the result of the interaction between the belt and the air is also the same regardless of choice of reference frame.

However, this does not imply that a runner on a moving treadmill belt in still air, will experience the same result as he would if he were running on the ground outside in a wind, that has the same velocity of the belt. There have been many studies done on this subject, here is a link to just one: https://pubmed.ncbi.nlm.nih.gov/6656556/

This study concluded “The oxygen debt of the overground condition, means = 47.86 ml X kg-1, was 36% greater than the treadmill running condition” and this cannot all be attributed to wind resistance.

According to this study “Data suggest that the moving treadmill belt reduces the energy requirements of the runner by bringing the supporting leg back under the body during the support phase of running.”

The study does not mention a wheel rolling on the belt, but it is my contention that this increase in efficiency may well apply in this case also, although I have no data to support that contention. I came to this conclusion after realizing that the mechanics of a rolling wheel is not very much different than the mechanics of a runner. The runner places one foot down in front of the other, and according to this study “the belt brings the supporting leg back” instead of the runner having to expend energy to do that himself. A rolling wheel puts down a contact patch and normally must move by bringing that contact patch back and laying down another, in a continuous manner. It seems to me that on the treadmill belt, the belt will bring the contact patch back for the wheel, reducing the work done by the wheel in rolling. Indeed, this is how the belt forces the wheel to roll at all! It is impossible to say if the energy reduction is 36% more or less, but since the process is smoother and more continuous than the runner, I suspect the energy efficiency improvement is even greater than 36%.

In any case, what this study shows is the treadmill environment is not the same as the outdoor environment.

 

Link to comment
Share on other sites

1 hour ago, OceanBreeze said:

As for the treadmill belt running through still air, the result of the interaction between the belt and the air is also the same regardless of choice of reference frame.

However, this does not imply that a runner on a moving treadmill belt in still air, will experience the same result as he would if he were running on the ground outside in a wind, that has the same velocity of the belt.

 


I'm not sure if I'm piecing it together correctly, but if we're to liken a treadmill to a wind tunnel, then all relative conditions would have to be the same, no?

So if we start with an overground baseline, with no true/natural wind (i.e., air is not moving over the ground) then the runner experiences a relative/apparent wind the same as their running speed. I.e. they're running at 3 m/s, so the ground moves under them at 3 m/s, and simultaneously they feel a 3 m/s wind on their face, with the accompanying aerodynamic drag on their whole body.

To translate this to a treadmill, if the treadmill is set to 3 m/s, and the runner is keeping pace over it, on a normal treadmill they feel 0 m/s relative headwind. To make this fully equivalent to the overground situation, wouldn't there have to be a giant fan blowing a 3 m/s wind backward onto the runner's body? That's the only way that I can figure it.

I followed the link and it doesn't say if there was a fan blowing wind, and I'm fairly sure it was just a standard treadmill. If that's case, then the increase of efficiency is easily explained by the elimination of air drag. Am I going wrong anywhere here?

Link to comment
Share on other sites

If you read the paper, (there is a full version available at that link), you missed where it said that the 36% increase in efficiency “cannot be attributed completely to air resistance.” It goes on to say “data suggest that the moving treadmill belt reduces the energy requirements of the runner by bringing the supporting leg back under the body during the support phase of running” which is something the ground cannot do.

Link to comment
Share on other sites

11 hours ago, OceanBreeze said:

If you read the paper, (there is a full version available at that link), you missed where it said that the 36% increase in efficiency “cannot be attributed completely to air resistance.” It goes on to say “data suggest that the moving treadmill belt reduces the energy requirements of the runner by bringing the supporting leg back under the body during the support phase of running” which is something the ground cannot do.

After bashing my head enough, I finally found the link to the full paper. (https://journals.lww.com/acsm-msse/Abstract/1983/15060/An_analysis_of_overground_and_treadmill_sprinting.7.aspx, then “Download” on the left, a little under the green bar)

As I suspected, it was a normal treadmill with no wind. They found a 36% difference in oxygen debt between treadmill and overground, but I have no idea how this translates to energy usage. (Might have been easier if they found a % difference in fuel flow or torque*RPM or something!) Anyway, they mention other studies that suggest a 3 to 5% contribution of air drag to energy expenditure. This other study gives a number as high as 13% at sprint speeds: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1331759/. But yeah, there still seems to be a gap to close. 

They mention all kinds of kinematic differences of various limb angles, etc. What’s the nature of them? They mention psychological factors, and it’s easy to imagine those. On the regular ground, the runners were free to simply sprint. Then on the treadmill, they weren’t free like that. The speed wasn’t set by a closed loop controller, it was simply set to move at each runner’s pre-recorded speed from the previous overground trials. Additionally, they had to ramp up to that speed (mechanical limitation) instead of the simple sprinting start. This, plus the default human Aristotelian mindset could have made subtle mental suggestions based not their knowledge of being still rather than moving, and this affecting their stride. One major pattern is that everyone’s stride lengths, squeezed toward the average when they moved from the ground to the treadmill.

About one of these differences, there’s one curious sentence about a third of the way down the sixth page: “It was hypothesized that this over-extension of the landing leg did not create the excessive breaking [sic] force that it would have incurred for the overground running condition because the moving treadmill belt brought the supporting foot back under the trunk rather than having the trunk move over the supporting foot.” Could this be an Aristotelian error on the writer’s own part? In accordance with what we’ve accepted about wind tunnels, I’m not seeing any difference between the foot moving under the trunk vs. the trunk moving over the foot. If there is any difference, what is it? How is this any different from the distinction between the airplane in the wind tunnel moving through the air, vs. the air moving over the airplane?

Besides the lack of wind (which they acknowledged accounts for some but not all of the energy difference) and the psychological factors, they mention one other potential mechanical difference: cyclic changes of the treadmill belt speed with each step. This, combined with the elasticity of the belt, could provide a springy type of energy storage and release that can help at key points at each step. For example, the forward landing foot exerts some forward force on the belt, which then stretches forward, and snaps back helping to propel the foot backward. Then the sentence in the second to last paragraph (“One plausible explanation…”) which is largely a repeat of the sentence I quoted a bit earlier, it’s unclear if it was talking about this type of cyclic elastic help, or the simple Aristotelian equivalence error.

So the question remains open, if we remove all these confounding factors…
- psychological effect of unnatural running environment
- cyclic belt speed change
- elastic belt effects
… by putting something on the belt not susceptible to them, like a car instead of a human. It’s unaffected psychologically, it simply propels itself forward agains the rolling resistance and the air drag, according to the power provided by the engine, and doesn’t impart any cyclic loads to the belt.

Then would there be any difference in these pairs of situations?

A) Driving forward on the ground at X m/s. There is no natural wind, so there is a X m/s relative headwind.
vs.
B) Driving on a treadmill at X m/s, with a relative headwind of X m/s provided by a fan

and

C) Sitting still on the ground (brakes locked) with a natural (and relative) tailwind of Y m/s
vs.
D) Resting on a moving treadmill belt (brakes locked) moving backward at Y m/s. Y m/s relative tailwind provided by movement through the room. (No fan)
 

Edited by EfisCompMon
fixed link
Link to comment
Share on other sites

From personal experience, running on a treadmill set to some speed is definitely easier than running outside and averaging that same speed.  My guess is it mostly has to do with the "tailwind" and how elastic the treadmill surface is.   Your feet spring off it in a way that's pretty different from pavement, and that might save significant energy.  Maybe the fact that it's exactly the same all the time and you don't have to watch where you're going (I typically run outside on busy streets or tracks with lots of other people) helps too.

Edited by truthseeker
Link to comment
Share on other sites

On 1/21/2022 at 1:26 PM, OceanBreeze said:

Moderation Note: Some posts that were clearly out of place in this thread have been removed. They are not deleted but hidden awaiting moderator approval, which may or may not be given.

In the meantime, a discussion on frames of reference is always interesting and an important topic in Physics. You are certainly welcome to discuss this topic but please do not venture into the Closed topic on ddw.

Discussing a physics topic that's been around for decades, is the subject of two recent Veritasium videos with over ten million views, reveals interesting features of reference frames and energy, and has been used as a physics olympiad test question in multiple countries, is off-limits?  That's bizarre.

 

Link to comment
Share on other sites

13 hours ago, truthseeker said:

Discussing a physics topic that's been around for decades, is the subject of two recent Veritasium videos with over ten million views, reveals interesting features of reference frames and energy, and has been used as a physics olympiad test question in multiple countries, is off-limits?  That's bizarre.

 

The posts that I removed had absolutely nothing to do with this topic. Basically, they fall into the category of sh1t posts. I actually thought you would appreciate having them removed so this thread can stay clearly on topic. It find it bizarre that you are complaining about this.

But OK, since you like sh1t posts so much, I will return them all to this thread. Enjoy!

Link to comment
Share on other sites

4 hours ago, OceanBreeze said:

The posts that I removed had absolutely nothing to do with this topic. Basically, they fall into the category of sh1t posts. I actually thought you would appreciate having them removed so this thread can stay clearly on topic. It find it bizarre that you are complaining about this.

But OK, since you like sh1t posts so much, I will return them all to this thread. Enjoy!

I think you confused one thing for another. Truthseeker was talking about the legitimacy of that other topic that you said is banned; and not about Autodidactocrat's wall of text about particle physics. I would venture that everybody appreciated having that removed. I know I did.

Anyway, back to the original discussion, I'm curious about your thoughts about the the thing I said:

Quote

 

So the question remains open, if we remove all these confounding factors…
- psychological effect of unnatural running environment
- cyclic belt speed change
- elastic belt effects
… by putting something on the belt not susceptible to them, like a car instead of a human. It’s unaffected psychologically, it simply propels itself forward agains the rolling resistance and the air drag, according to the power provided by the engine, and doesn’t impart any cyclic loads to the belt.

Then would there be any difference in these pairs of situations?

A) Driving forward on the ground at X m/s. There is no natural wind, so there is a X m/s relative headwind.
vs.
B) Driving on a treadmill at X m/s, with a relative headwind of X m/s provided by a fan

and

C) Sitting still on the ground (brakes locked) with a natural (and relative) tailwind of Y m/s
vs.
D) Resting on a moving treadmill belt (brakes locked) moving backward at Y m/s. Y m/s relative tailwind provided by movement through the room. (No fan)

 

 

Edited by EfisCompMon
Link to comment
Share on other sites

1 hour ago, EfisCompMon said:

I think you confused one thing for another. Truthseeker was talking about the legitimacy of that other topic that you said is banned; and not about Autodidactocrat's wall of text about particle physics. I would venture that everybody appreciated having that removed. I know I did.

Then he should not have included my moderation note in his post, as that did make it appear that removing those posts was what he was objecting to.

Quote

Anyway, back to the original discussion, I'm curious about your thoughts about the the thing I said:

 

 As far as the other topic, I don’t give a damn about videos or the fact that the question appeared on a physics exam. In my opinion, it had no business being on that exam paper as it is a controversial, non peer-reviewed claim with very little scientific evidence to back it up. I said if someone can produce a paper from a respected peer-reviewed journal, or a convincing outdoor demonstration, such as outrunning a NB balloon, then my opinion will change. The fact none of that has happened in the “decades” this has been kicking around the Internet, from one forum to another, often with heated arguments, does nothing to convince me of it’s correctness. And when I see an “analysis” that completely neglects to include rolling resistance, I tend to believe it is crackpottery.

As for your questions, in both instances, you are trying to equate two different experiments. That has nothing to do with Galilean relativity which only speaks to observing one and the same experiment from two or more different frames of reference. There is no law in physics that says the results of these two different experiments will be the same. You would need to make sure you have accounted for every possible parameter to make the two different situations exactly the same. That is the problem with using the treadmill experiment to make a claim about the outdoor cart. It borders on absurdity to compare a cart that weighs a few hundred grams, running on a treadmill indoors, to a cart that weighs a couple of hundred kilograms, running on the ground outside. Do you really think the situations are even close to being the same? I don’t, and I don’t accept the treadmill demo for anything other than a demo that a small toy cart can advance up a running treadmill belt.

 

 

Link to comment
Share on other sites

I didn't mean to mire this in connections to some other topic that seems to have quite some back story, and even heat, associated with it. My apologies if things took a turn in that direction.

But back to my question about the various situations...

1 hour ago, OceanBreeze said:

As for your questions, in both instances, you are trying to equate two different experiments. That has nothing to do with Galilean relativity which only speaks to observing one and the same experiment from two or more different frames of reference. There is no law in physics that says the results of these two different experiments will be the same. You would need to make sure you have accounted for every possible parameter to make the two different situations exactly the same.

... where you say I'm "trying to equate two different experiments," as best as I can try to see, I'm not really seeing a difference between A vs B, or between C vs D, and... they are actually equal? Is there is any difference that would affect a physical system in those situations, and if so then what is it? Is it possible to design an experiment that can distinguish whether it is installed in A vs B, or in C vs D? I can't think of any. It seems that if you observe A through a camera (or "frame," if you will) that moves with the car, then you have exactly B? Same for C/D. Best as I can tell, any experimental result (within each pair) would indeed necessarily turn out the same. Is there any unaccounted-for parameter that would make them different?

(Assuming of course the fan diameter is wide enough, and flow uniform enough, for the size of the experiment to be totally immersed in it... same as the controlled conditions in wind tunnels)

Edited by EfisCompMon
Link to comment
Share on other sites

The cart advancing up the treadmill is Galilean equivalent to (so exactly the same as) the same cart going directly downwind faster than the wind.  Its not doing it outside and it's very small and light and the wind is perfectly steady, but it's doing it.

Link to comment
Share on other sites

1 hour ago, truthseeker said:

The cart advancing up the treadmill is Galilean equivalent to (so exactly the same as) the same cart going directly downwind faster than the wind.  Its not doing it outside and it's very small and light and the wind is perfectly steady, but it's doing it.

And the very small advancement before they run out of space is not a limitation, either. There are at least a few different people who built circular/turntable treadmills, like this one: https://www.youtube.com/watch?v=VkPW9ql6t_A with unlimited space, and show the cart merrily advancing forward, and fast. And in case the Galilean equivalence is not equivalent after all, there's this really neat one that is a circular wind tunnel, where the ground stays still and the air moves. https://www.youtube.com/watch?v=simRdTslucs https://www.youtube.com/watch?v=GC-EoGbdyzE (first link is a disassembled view and explanation, the second one is actually running)

 

 

 

Link to comment
Share on other sites

Guest
This topic is now closed to further replies.
 Share

×
×
  • Create New...