Merged DDWFTTW in a “Real” Wind

[Jack by the hedge]

If you were in the car and not able to see out the windows, could you tell if you were in the wind tunnel sitting in place on the moving belt or if you were actually in a car on a road driving straight without changing speed or direction? If so, how?

Ooh! I know this one! Ask me! Ask me!

You jump up and down.

 

[CNY_Dave]

Please explain why. The comparison is with a road and tailwind. I assume the treadmill starts from rest, as does the runner on the road.

I was just being a smart-aleck, and pointing out a few subtleties.

If you get on a treadmill and turn it on, you do not need to accelerate your body mass.

If you go out on the road you do need to accelerate your body mass.

So getting 'up to speed' on a treadmill from a standing start takes less energy than doing so running on the ground.

As someone pointed out, jumping on a moving treadmill and running is equivalent to jumping out the door of a moving car and running- you do not need to do the work of accelerating your body.


What may be throwing some of the folks who don't see cart-on-ground vs cart-on-treadmill equivalence is that putting the cart on the treadmill while holding it from rolling back off the end is like pushing the cart on the ground up to windspeed.


Dave

 

[Myriad]

The car is attached to the frame. The belt moves, not the car.

Yet the tests work and are directly applicable to the car in racing conditions, that is, driving along the track.

Respectfully,
Myriad

 

[Dr. Keith]

Ooh! I know this one! Ask me! Ask me!

You jump up and down.

I swear this thread is such a slog sometimes and then you knock it out of the park with gems like this.

Nicely done.

Maybe even worthy of a WindGrin-like smiley. Nah, better keep it in check.

 

[profpangloss]

Well, let's say for example that in the ground frame the simple sail cart is moving at 3 m/s in the 5 m/s wind, in the same direction as the wind. So in a frame where the air is at rest, the sail cart is moving at 2 m/s in the same direction as the ground. What I want profpangloss to tell me is where he thinks the kinetic energy of the sail cart is coming from in this frame.

I have answered your question: the energy source is exactly the same in the second frame as it was in the first frame, the wind. The only thing that happens in changing the FOR is changing the viewpoint of the observer. In the first case, the observer is stationary wrt the ground, and the ground is taken as the reference from which all motion is compared. The observer sees the wind blowing past at 5 m/s and the cart moving at 3 m/s and of course the ground is stationary. The energy that is causing the cart to move is obviously coming from the wind.

Next the observer moves at 5 m/s along with the wind so the air is now stationary relative to the observer. The ground is moving relative to the observer at -5 m/s and the cart at -2 m/s all relative to our observer. But all that is relative to the observer and does not represent any change in the physical situation that existed in the original reference frame! The source of energy for the cart is still the same as it always was, the wind!

What you are in effect saying is that the choice of observer changes the reality of what is being observed. Do you really think that if you hop on a motor scooter and travel at the speed of the wind, that somehow changes the source of energy for the sail cart from wind to ground? Choice of reference frame can change the measured kinetic energy of things in those frames, but it won’t change the source of energy and that can be confirmed with such things as thermal imaging, for example. In a thermal image, the treadmill motor will always be identified as the energy source, no matter how fast the observer is moving.

In the same way, if you travel along next to a long treadmill, say a conveyor belt that has a cart running on it, do you think that because you have changed the way you are observing the treadmill it magically transforms itself into a stationary road? I have some breaking news for you: a simple transform of velocities is that and only that; it does not change a pumpkin into a coach or a treadmill into a road. No matter how you look at the treadmill, running, sitting, standing on your head, it is still a treadmill.

 

[profpangloss]

Yet the tests work and are directly applicable to the car in racing conditions, that is, driving along the track.

Respectfully,
Myriad

I'm sure the tests provide valuable information that can be applied to the real case of the car racing on the track, but it is a simulation and I don't think anyone will claim it is exactly the same because it isn't.

 

[humber]

Yet the tests work and are directly applicable to the car in racing conditions, that is, driving along the track.

Respectfully,
Myriad

Remove the car and replace it with a flat board placed a few inches above the belt, and mounted to the frame.
Turn on the belt, and measure the pressure differential (down force) created by the laminar flow produced by motion of the belt. Nothing whatsoever to do with travel.
The car is there, so the a real under-car profile is used. Done in F1 for decades.

 

[windgrins]

Humber: "I got Nothing."

All the energy come from the source that compresses it and not from clamps.
A confusion of force and work.
Another blinding error on par with the PE free balloon

With the problem stated below:

A good example of this is the NB Balloon and Potential energy issue.

The question is does an NB Balloon in the atmosphere have PE wrt the ground.
The underlying (hidden) assumptions are:

1. the NB Balloon by definition weighs the amount of the air it displaces so has no forces acting on it other than gusts-This of course why we call is Neutrally Buoyant.
2. The balloon will remain intact for the purpose of the discussion.
3. The balloon is not rising or sinking once in steady state so it has no vertical or horizontal motion wrt to the air column it is occupying.
4. for the purpose of the discussion, we will assume that the density of air doesn't change with height over the short heights assumed for the problem nor does the balloon expand or contract-thus being always NB in the scope of the problem.

(That's a lot of non-stated assumptions after all but it expected that any reasonably bright physics student can figure these out. The devil is in the details so scientists go to great pains with explanations to rule out outside influence.)It's also worth noting that when solving a problem for a specific case with a set of assumptions, one should not generalize the principle outside of the assumptions because they don't always work outside of the assumptions (either stated or unstated.)

Once these things are stated, it becomes clear that if one were to physically move the balloon up (say 10 meters) that a volume of air, the same size and weight of the balloon would take the balloons place since it is a gas and can flow to occupy the abandoned space. One would also assume, for the purpose of the problem, that this would be done very slowly to avoid adding significant energy from drag by fast motions. Since the air to occupy the initial space had to come from somewhere and the air in the space that the balloon moves to has to go somewhere and the height of the atmosphere doesn't change, it is correct to assume the air from the new location is effectively exchanged for the balloon. Since both volumes have the same mass and density, no real work is done so no PE change. This holds true from the surface up to some arbitrary height that fits in scope of assumptions so for all practical purposes the NB balloon has no change in PE by moving it vertically so we say it has no PE.

Now, This sounds like a 9th grader's science class explanation and most posters tend to be a bit more savvy so we don't normally explain things like this. Clearly, the lack of clarity gets some posters into trouble. Since these generate heated arguments, these kind of explanations, however, seem warranted. No one is obligated to provide this but depending upon one's purpose (brainteaser or explanation), expect the inevitable kerfluffle.

So Humber,

Is your rebuttal something rational, scientific, or logical or rather what you seem to be saying a lot:

"Bilge"---translation----"I got nothing."?

It seems like you have this for all your answers lately. Faking it very badly again Humber!

Regards,

WindGrins

 

[3point14]

I love your one word answers such as the one above. It can be translated as:

"I have no rational, logical, or scientific answer that disputes what you alleged. So I'll just disagree in a condescending way." or

"I got nothing".

WindGrins

QFT

 

[3point14]

I'm sure the tests provide valuable information that can be applied to the real case of the car racing on the track, but it is a simulation and I don't think anyone will claim it is exactly the same because it isn't.

I will!

Because it is!!

I can get a fellow who works at McLaren to confirm it if you like?

 

[A.T.]

Then you tell me: in the frame of reference in which the air is at rest, where is the kinetic energy of the simple sail cart coming from?

I have answered your question: the energy source is exactly the same in the second frame as it was in the first frame, the wind.

So you agree that the wind is a source of energy in a reference frame where the air is at rest while the ground moves.

That means that the car on the treadmill is also powered by the wind in the reference frame of the room.

 

[Jack by the hedge]

What you are in effect saying is that the choice of observer changes the reality of what is being observed.

No. On the contrary. What he is saying is that changing the reference point from which velocities and kinetic energy are measured does not change the reality of what is being observed. The results work out exactly the same.

Motion is relative. Kinetic energy is relative.

It doesn't matter what you choose for your reference frame - stationary with respect to any part of your system or indeed moving with respect to all of it -so long as you are consistent, the maths will work out to give the identical result.

Did you read that chapter yet?

 

[humber]

So you are saying that the compressed gas in the cylinder which started out uncompressed has no energy once the clamp has squeezed it? If one opened a petcock on the cylinder and let the compressed air out, it couldn't do any work?

That is the release of the energy stored by the work of compression.

Gee, why did the manufacturer put an electric motor on my air compressor?

So it does work on the gas, so compressing it.

It sure seems to me that it move things around a lot (dust, leaves, air) when I let the compressed air out of the nozzle.
Or, are you really saying:
"I got nothing."
Regards,
WindGrins

You don't have an electronics degree.

 

[3point14]

That is the release of the energy stored by the work of compression.


So it does work on the gas, so compressing it.


You don't have an electronics degree.

I thought we agreed that we weren't going to deal with qualifications as it means you have to pony yours up.


Don't worry, look, we'll see if we can find you a textbook that really breaks it down into tiny pieces and build it up from there so you can understand. We're here to help.

 

[windgrins]

That would be quite cool!

I will!

Because it is!!

I can get a fellow who works at McLaren to confirm it if you like?

That would be quite cool, Pi! I would like to hear if they do things like simulate bumps with the treadmill by hydraulic actuators or something cool as well as tilt the table for side force.

Do you think you could get that person to write something you could post?

That would be educational. I think it is cool that they go to this much trouble to replicate the flow fields around the wheels. Do they as well actually use the engine to generate the "stance" of the vehicle from torque effects or just use the tie downs as a power source?

Thanks,

WindGrins

 

[humber]

With the problem stated below:

A good example of this is the NB Balloon and Potential energy issue.

Sure. Fill the balloon on the first floor.
Climb up to the 50th and repeat. So you think they have the same PE?
Is that none for each, or does the one on the first floor have the same PE as the one on the 50th?
You must think that NB balloons have the property of "ballooniness". 'specially the red 'uns.

 

[Jack by the hedge]

Sure. Fill the balloon on the first floor.
Climb up to the 50th and repeat. So you think they have the same PE?
Is that none for each, or does the one on the first floor have the same PE as the one on the 50th?
You must think that NB balloons have the property of "ballooniness". 'specially the red 'uns.

Swing and miss.

Underlying assumption 4. "for the purpose of the discussion, we will assume that the density of air doesn't change with height over the short heights assumed for the problem nor does the balloon expand or contract-thus being always NB in the scope of the problem"

In other words - You got nothin'.

 

[Subduction Zone]

Sure. Fill the balloon on the first floor.
Climb up to the 50th and repeat. So you think they have the same PE?
Is that none for each, or does the one on the first floor have the same PE as the one on the 50th?
You must think that NB balloons have the property of "ballooniness". 'specially the red 'uns.

Attach a weight to the NB balloon so it is no longer neutrally buoyant. Let it fall. Analyze how much energy is released. Hey!!! That energy is from the weight only! Where did all of humber's PE go?

 

[humber]

Swing and miss.

Underlying assumption 4. "for the purpose of the discussion, we will assume that the density of air doesn't change with height over the short heights assumed for the problem nor does the balloon expand or contract-thus being always NB in the scope of the problem"

In other words - You got nothin'.

Assume that. Still go it all.

 

[windgrins]

Humber: "I got nothing"

That is the release of the energy stored by the work of compression.


So it does work on the gas, so compressing it.


You don't have an electronics degree.

Just a moment ago you were saying there was no energy in the compressed gas from the forces applied to the piston! You seem to be able to flip pretty quickly when it doesn't fit your story!

Think what you like about my credentials. I have yet to state any! It only matters what my employer believes when I presented them. If you think my signing a post as EEgrins is stating I have a credential, think what you want. I know what credentials I have. As I've stated, credentials aren't all that important but the facts, clarity, and logic are. See if you can supply some.

I have no clue how any degree you may hold relates to physics but I think you should return it for the reason of "knowledge due-never received"!

Regards,

WindGrins