Split Thread The validity of classical physics (split from: DWFTTW)

[Subduction Zone]

humber, an object will have different KE's for every different inertial frame of reference it is viewed from. KE is not absolute. Various members have posted links that showed that KE is relative, can you post even one that shows that it is absolute?

 

[humber]

I asked you to forget about treadmills. Did you not read my post carefully? I just want you to examine the scenario i provided and answer the questions.

Pretend it's a high school physics exam and you have to answer those three questions.

Forget about the rest of the thread for now. I never said any of the objects had zero KE, I only provided their mass and velocities. I asked YOU to provide me with the kinetic energy value.


If you do not do this, then I'll have confirmed my suspicion that you are not able or willing to read and think carefully.

No Spacediver. Dan_O tried this. Do you think that So_invictus' equation answers the question as to why all windblown objects do not get to winspeed? Doesn't it bother you that we do not make use of this in our transport systems? That after more than 100 years of mechanised travel, no one has noticed that there is a miraculous fall in drag at windspeed?

No, your question is not valid, because whatever result you obtain, has nothing to do with the treadmill. The problem is conceptual, one of logic and insight, not textbook examples. I don't answer for the same reason that you see the moves ahead when playing chess. This one leads to stalemate.
I have made that point maybe 10 times, so it is you that is not listening.
So, no answer. You can quit, or try something else.

If you like, cast it on the treadmill. Then I will take a look. You are behaving like Christian. He will not argue his case unless I first accept his premise as correct. I do not do this to you. I can take any treadmill view, and show by your own reasoning that it is false.

When I say I have built or designed something, been somewhere, that is the truth. Please don't treat me like an idiot.
Perhaps it is you that is unwilling to learn.

 

[jjcote]

Do you understand that the question of whether an object placed in a river will wind up moving at the speed of the water is the same question as whether an object given a shove into a large lake will slow to a stop?

 

[humber]

I asked you to forget about treadmills. Did you not read my post carefully? I just want you to examine the scenario i provided and answer the questions.

Pretend it's a high school physics exam and you have to answer those three questions.

Forget about the rest of the thread for now. I never said any of the objects had zero KE, I only provided their mass and velocities. I asked YOU to provide me with the kinetic energy value.


If you do not do this, then I'll have confirmed my suspicion that you are not able or willing to read and think carefully.

No Spacediver. Dan_O tried this. Do you think that Sol_invictus' equations answers the question as to why all windblown objects do not get to winspeed? Doesn't it bother you that we do not make use of this in our transport systems? That after more than 100 years of mechanised travel, no one has noticed that there is a miraculous fall in drag at windspeed?

No, your question is not valid, because whatever result you obtain, it has nothing to do with the treadmill. The problem is conceptual, one of logic and insight, not textbook examples. I don't answer for the same reason that a player see the moves ahead when playing chess. This one leads to stalemate. I have made that point maybe 10 times, so it is you that is not listening. So, no answer. You can quit, or try something else.

If you like, cast it on the treadmill. Then I will take a look. You are behaving like Christian. He will not argue his case unless I first accept his premise as correct. I do not do this to you. I can take any treadmill view, and show by your own reasoning that it is false.

When I say I have built or designed something, been somewhere, that is the truth. Please don't treat me like an idiot.
Perhaps it is you that is unwilling to learn.

 

[spacediver]

No, your question is not valid, because whatever result you obtain, it has nothing to do with the treadmill. The problem is conceptual, one of logic and insight, not textbook examples. I don't answer for the same reason that a player see the moves ahead when playing chess. This one leads to stalemate. I have made that point maybe 10 times, so it is you that is not listening. So, no answer. You can quit, or try something else.

I don't really care about treadmills and wind, and my physics education doesn't extend beyond high school. I just wanna understand how you deal with basic concepts in physics.

You seem unable to answer the questions, so I gather you really haven't a clue.

 

[humb]

The tread of the valid before cannot massless cart at terminal textbook miraculous. You frame observers windspeed along with belt at riverspeed, without a corresponding straw man. I am not fooling me. The hanging post will show my parrot adventure. Good? OK, then. This is established from numerous hopping perspectives, and references no academic from you still.

Ceci ne'st pas une ceinture. It is a model, but only substantially so. Once we are at windspeed, we must then get to windspeed, and then to windspeed, and so on. Even then it is wrong, and only notional.

I make as much as sense from the perspective of any frame. (I feel like I need a shower.)

 

[sol invictus]

<snip>

humber, let's make this very, very, very simple.

1) there is an equation for the drag force. According to the wiki, it's F_drag = c(u) u², where c(u)>0 is a coefficient that is nearly constant (but may weakly depend on u) and the force is always in the opposite direction as u.

2) u is the velocity of the object relative to the fluid it is immersed in.

3) F_drag=m a assuming no other forces are acting, which means u will be changing with time unless F_drag=0

4) F_drag=0 if and only if u=0, using the equation above

5) therefore, any object placed into a fluid with non-zero u will approach u=0 (like 1/t, if you solve the equation for c a constant), and any object with u=0 will remain so.

Which, if any, of those steps do you disagree with?

 

[mender]

You must agree that going back with the belt is equivalent to a standstill in real wind.

I'd like to add a bit more detail just to make sure the statement is perfectly clear.

I agree that if the cart is going back with the belt at belt speed, the cart experiences the same force because of the relative movement between it and the surrounding air as it would when it is at a standstill outside at rest w.r.t. the ground while a tailwind with the same relative velocity as what the belt provided is blowing across the ground.

I agree that this is equivalent. I hope that from this point forward we can agree to this as a starting point for any further discussion if at some point we disagree.

Now for the next statement.

A small battery-powered car on the belt, it would first need to achieve beltspeed to get to zero wrt the ground, and then again, to get to windspeed wrt the ground.

Let's start with the battery powered car sitting on the ground outside. Add radio controls for convenience, and put an anemometer on top of the car. Add also telemetry to allow remotely recording of the speed of the wheels, the power draw of the electric motor and the anemometer readings. Let's use the now conventional 10 mph tailwind as a baseline.

The RC car is off. The tailwind is blowing at 10 mph. The anemometer faithfully records that and sends that result via telemetry to a laptop. It says that there is a wind blowing at 10 mph from behind the car. No wheel speed and no power draw.

What is the anemometer telling us? It tells us that when the car was at a standstill on the ground, the air was moving at 10 mph. Since air has mass, the anemometer is also measuring one of the components needed to compute the KE of the air w.r.t the car and the anemometer. For a given volume of air moving by, we can calculate the amount of KE in the air. Let's say that the air volume measured has the same mass as the car. Let's also say for convenience the KE as calculated has a nominal unit of one .

Start the car moving forward until the anemometer reading is zero. The wheel speed is now showing 10 mph and there is a specific amount of electric power being used to move the car at 10 mph.

Now what is the anemometer telling us? It tells us that when the car is moving forward at the same speed as the air, both at 10 mph, the air has zero KE. How can that be? Where did the KE go? Did the anemometer suddenly stop working? The car now has a KE of one. That came from the car moving along the ground, as we can plainly see.

Did the KE of the air somehow get exchanged for the car's KE? Could that possibly happen under the right circumstances? Maybe we'll get to that later. For now, let's do some more testing.

Let's pick up the car without changing the power setting (recorded) and take the car inside and place it on a treadmill, carefully adjusting the treadmill belt speed to match the car's wheel speed so that the car stays in place on the treadmill. Once we have successfully achieved that balance, we check the telemetry to make sure that the wheel speed of the car is exactly that same. The anemometer is reading correctly (still zero), showing that the air in the room isn't moving and therefore has no KE. Still no idea as to where the KE of the air went.

But now the car isn't moving, so it can't have any KE even though the wheels are still spinning and the power draw is exactly the same as it was outside at 10 mph in a 10 mph tailwind.

Now we lost the KE of the car too! How could we be so careless! Where did the KE of the car go? Or is this treadmill thing just an elaborate hoax to cleverly hide the KE to trick people?

I have independent supporting evidence that the cart is where it is because of low friction to the belt. I will include that.

The low friction of the RC car on the belt? That can't possibly be an issue; the telemetry confirmed that the conditions are exactly the same for the RC car when outside running at 10 mph across the ground in a 10 mph tailwind as when the car is maintaining station on a treadmill running at 10 mph in a room of still air. Yet all the KE disappeared! Did it slide away on the belt somehow?

Please include video evidence with the appropriate documentation.

 

[Modified]

No Spacediver. Dan_O tried this. Do you think that So_invictus' equation answers the question as to why all windblown objects do not get to winspeed?

They do, assuming they are not slowed by friction with the ground, or anything else.

Doesn't it bother you that we do not make use of this in our transport systems?

We do, it's called a hot air balloon.

 

[jjcote]

That after more than 100 years of mechanised travel, no one has noticed that there is a miraculous fall in drag at windspeed?

Except for bicyclists, maybe?

 

[CNY_Dave]

No Spacediver. Dan_O tried this. Do you think that Sol_invictus' equations answers the question as to why all windblown objects do not get to winspeed? Doesn't it bother you that we do not make use of this in our transport systems? That after more than 100 years of mechanised travel, no one has noticed that there is a miraculous fall in drag at windspeed?

No, your question is not valid, because whatever result you obtain, it has nothing to do with the treadmill. The problem is conceptual, one of logic and insight, not textbook examples. I don't answer for the same reason that a player see the moves ahead when playing chess. This one leads to stalemate. I have made that point maybe 10 times, so it is you that is not listening. So, no answer. You can quit, or try something else.

If you like, cast it on the treadmill. Then I will take a look. You are behaving like Christian. He will not argue his case unless I first accept his premise as correct. I do not do this to you. I can take any treadmill view, and show by your own reasoning that it is false.

When I say I have built or designed something, been somewhere, that is the truth. Please don't treat me like an idiot.
Perhaps it is you that is unwilling to learn.

Humber, you are missing the point that he is trying to build a 'common language' that you both can share, and thereby have a meaningful conversation.


Dave

 

[Modified]

However, three observers already say zero KE, so to reconcile all views, the KE must be in the belt, but not the cart.

Yes, and for the cart on the road, someone in the cart, on a plane moving at wind speed, and in a car moving at wind speed, would all say KE is zero, so the KE must be in the ground, but not the cart, right?

Once again, KE is not "in" anything. It is a relative measurement.

No, I get it, but KE is real in some frame for any object in motion, or it is not moving at all.

A KE measurement from one inertial frame is as valid as from another. There is no frame for which it is "real".

KE is energy, it cannot be destroyed.

In a sense, but you must use only one inertial frame for measurement to make that claim.

KE is not speed.

It is relative to the frame in which it is measured in exactly the same way that speed is.

 

[ThinAirDesigns]

I have not been so entertained on a thread in years as I am by Humb and Humber.

Classic and archive worthy work by both parties.

JB

 

[humber]

humber, let's make this very, very, very simple.
1) there is an equation for the drag force. According to the wiki, it's F_drag = c(u) u², where c(u)>0 is a coefficient that is nearly constant (but may weakly depend on u) and the force is always in the opposite direction as u.

OK

2) u is the velocity of the object relative to the fluid it is immersed in.

Of course.

3) F_drag=m a assuming no other forces are acting, which means u will be changing with time unless F_drag=0

Yes, F_drag = c(u) u²So, ma = c(u) u²

4) F_drag=0 if and only if u=0, using the equation above

If there is no drag, there is no drag. This is comedy, right?
Just in case not, you have taken the case of an object that is still, in still water (the other case where u =0 ) and added u = waterspeed to the result.
Magic!

5) therefore, any object placed into a fluid with non-zero u will approach u=0 (like 1/t, if you solve the equation for c a constant), and any object with u=0 will remain so.

Like, no.

Which, if any, of those steps do you disagree with?

You are first assuming your velocity, and going from there. With that premise, any equation for anything will probably give you the answer you want.
If the object is held in the stream, the relative velocity of the medium is at its greatest. As the object accelerates from rest, that velocity difference falls. However, the drag increase as the square of the velocity through that medium, so the applied force that accelerate the body is met by an ever increasing drag force. Driving force can never exceed the drag, so the object reaches terminal velocity below the speed of the medium.
This is why boats need ever more power to go faster against the stream, and why waterborne objects cannot reach waterspeed, and canoes drifting downstream have a bow wave.

The treadmill shows that the applied force to be inversely proportional to windspeed. It is at a maximum when going with the belt, falling to nearly zero at windspeed. However, the drag is (should be) the square of the velocity, so the cart cannot be reach windspeed. The model is wrong.

In your equation, you have jumped over the drag barrier to get your result. The treadmill does the same, by declaring windspeed to be still air in the same way as a still canoes in still water.

I think that you will need to provide evidence of objects traveling at the same speed as the medium to overcome my objections.

It is difficult to believe that you do not recognize that your claim breaks the laws of thermodynamics, let alone basic math.

 

[humber]

In river of uniform flow, given two objects of the same mass immersed in the water, which will have a terminal velocity closer to the velocity of the stream, one that has a very streamlined shape, or one that has a very draggy shape?

Ask a fish.

 

[ThinAirDesigns]

Ask a fish.

A goldfish perhaps?

JB

 

[sol invictus]

Of course.

Let the record reflect that you agree that u is the velocity of the object with respect to the fluid.

If there is no drag, there is no drag. This is comedy, right?

Do you or do you not agree that F=0 only when u=0?

Like, no.

I solved the differential equation, humber. That's what equations are for, see? They tell us what the solution is.

I'll break it down for you. The equation is a=d_t u = (c/m) u². The most general solution to that equation is (ct/m + k)^-1, where k is a constant determined by the initial conditions.

In other words:

You are first assuming your velocity, and going from there.

wrong again. I just gave you the most general solution. Do you disagree that's the correct solution?

With that premise, any equation for anything will probably give you the answer you want.

No, it gives me a very specific functional form. And guess what - all of them go to zero as t gets large (for totally obvious reasons), just as I said.

If the object is held in the stream, the relative velocity of the medium is at its greatest. As the object accelerates from rest, that velocity difference falls. However, the drag increase as the square of the velocity through that medium, so the applied force that accelerate the body is met by an ever increasing drag force.

You just contradicted yourself in two consecutive sentences. The "velocity difference falls" and then "the drag increases". The drag is the velocity difference squared - if the velocity falls, the drag falls too, because it's the square.

 

[humber]

humber, an object will have different KE's for every different inertial frame of reference it is viewed from. KE is not absolute. Various members have posted links that showed that KE is relative, can you post even one that shows that it is absolute?

It is not "absolute" Subduction Zone. Again, if there are two objects in the air at the same velocity, they can be said to have zero relative KE, but that is meaningless. It says that objects motionless to themselves have zero KE.
It's not wrong, but it can never be the basis of a practical idea.

From the ground, they can each have KE, right? So zero relative KE does not mean zero KE, from all frames?

Observer A is on one of the flying masses. B is on the ground. Now you know that all views are equivalent, so they must agree. Yes, both say "relative to me" but they must also agree. How? The KE cannot appear or disappear with the viewer. It's energy, it cannot be destroyed.

So, how? If needed, ground observer B can use that energy to do work. But to do that work, say, to break a window, the object must lose velocity wrt to that observer. Right? It must decelerate to give up its KE.

The only way that A can do the same, is to slow the object to extract its energy. The only change that "frame" brings about, is the relative means of extracting that same KE. KE is a property of the body, it goes with it, but you see it from different velocities or "frames".
Zero KE is true, but so what? The "frames" idea seems to be an elaborate and forced way of saying Newton is right.

When I said the ground is a reference, I did not say absolute. The Earth is enormous, so its gravity is huge. You have to live with that (though it seems to be entirely ignored) from all frames. It seems pointless to take another view, just to do the the calculations backwards. It is against your natural inclinations and is a lot of trouble, for no gain.

You all seem to break your own rules. On the treadmill, I MUST take the view from the belt. Who's making the rules then? The ground frame IS valid. Equivalency says it must be!
You can certainly tell me that the cart is "like" it is at windspeed, that despite being still, it models that behavior, but NOT that I must be at windspeed to see it. That's just plain silly. Does it/I stop if I close my eyes?

I can accept the claim that the cart is as if it were at windspeed, yet examine it from the ground view. That is allowed.
It does not mean that the belt view is wrong , but it can't contradict the ground view, yet it does. That's one reason why the treadmill is wrong .

 

[Subduction Zone]

Humber, you are almost getting it. Every different inertial frame of reference can have a different KE for a particular object within that frame. Yes for the guy moving relative to the Earth and taking the kinetic energy of the Earth from his point of view may seem silly or pointless, but it very often has realistic uses in physics. For example in orbital mechanics when two different space ships, say a lem and a command module want to dock it is nice to know the different KE;s from both points of view. Remember an inertial frame is one from which there is not going to be any change in velocity. So the moving guy looks at how much energy it would take to bring the Earth to his velocity sees an insurmountable amount and decides he must change his own relative speed, effectively taking himself out of his frame of reference. The lem pilot looks at his remaining fuel and hopefully docks with the command module, but in this case the command module if needed can dock with him. The cart placed on the treadmill and held in place until its propeller is up to speed looks around and says "Hey, this is just like if somebody pushed me up to speed with a tailwind the speed of the treadmill!"

 

[spork]

The tread of the valid before cannot massless cart at terminal textbook miraculous. You frame observers windspeed along with belt at riverspeed, without a corresponding straw man. I am not fooling me. The hanging post will show my parrot adventure. Good? OK, then. This is established from numerous hopping perspectives, and references no academic from you still.

I'm almost certain this is humber posting under another name. I refuse to believe anyone else is capable of this .... whatever this is.