Merged DDWFTTW in a “Real” Wind

[humber]

Convince us

No, you do it. All you have given is an assertion. Spork's claim that energy if frame dependent, is factually wrong. All changes in energy, are the same from anywhere, otherwise the conservation of energy could be violated.

 

[humber]

profpangloss,

humber has previously argued that changing an observer's point of view actually does change the physical reality of the system. Now, he's claiming that others are making the same mistake.

You won't get away with that calumny, mountebank.

 

[jadebox]

I have explained why I believe they are not the same. You have only asserted that they are. Can you do better than that? Convince me.

As I understand it, you think the situations aren't the same because the earth is large compared to the treadmill. In that case, all you need is a bigger treadmill.

Imagine that you are in a car driving down a road. You can feel the wind blowing in the window and you can see the road below you.

Now, all of a sudden, the car is magically transported into a building and placed on a treadmill. The treadmill looks like the road and is moving at the right speed that it looks just like the road did to you before. In front of the car is a fan that's blowing air at you at just the right speed that the air blowing in the window feels the same as when you were outside. All around you is projected a moving image that looks just like what you saw outside. Would you be able to tell that the magical journey had taken place?

You might argue that you won't be able to feel movement in the car when it is on the treadmill. Well, the truth is that you can't feel movement when you are in the car on the road either. As long as the car is not turning or changing speed, you won't feel the movement of the car.

So, you won't be able to tell the difference because sitting still on a treadmill is equivalent to moving along the road.

-- Roger

 

[profpangloss]

To move on a treadmill, a vehicle must have a source of energy, and it cannot be the treadmill itself.

To move on a river, a vehicle must have a source of energy, and it cannot be the river itself.

These statements are equivalent to yours..

No they are not. In fact that is the main point of my argument; the road is the planet earth and it is not a treadmill or a river. The reason for that is the gravitational force that is associated with the planet will move everything along as the planet moves along. Energy is required to overcome the tendency to move with the planet and that energy does not come from the motion of the planet itself. That is unlike a treadmill that we have already agreed can give energy to the cart that is running on it. Since you want to claim the statements are equivalent to a vehicle moving on the surface of this planet, please go ahead and make the case for that.

It hasn't been needed to explain it at all. For testing something that is moving at or very near wind speed, a treadmill is ideal. It provides a consistency in speed and direction of the wind that is difficult to get anywhere else. To anyone with a basic knowledge of physics, the carts tested on treadmills conclusively demonstrate dwfttw travel.

For something that hasn't been needed at all, it sure was called into service quite fast! From what I gather in reading up on this, there are a few people out there with more than a basic knowledge of physics who dispute the validity of the treadmill as a conclusive demonstration of dwfttw travel, so your assertion does not stand up.

 

[tobermoryphil]

Humber- please explain exactly how a runner on a treadmill in still air experiences any physical differences from a runner on the road with a tailwind.

ETA: and we are still waiting for you to explain exactly how you can differentiate between the three aircraft carrier scenarios by jumping up and down.

 

[profpangloss]

My remark was directed at spork. Funny that you make 'blip'.

Oh sorry, I didn't know it was a personal message intended only for spork. I am just trying to keep up with the discussion and I did not understand this. Whatever blip means.

 

[jadebox]

The reason for that is the gravitational force that is associated with the planet will move everything along as the planet moves along. Energy is required to overcome the tendency to move with the planet and that energy does not come from the motion of the planet itself.

I think you need to read up on inertia and momentum.

Anyway, the treadmill doesn't represent the "motion of the planet itself." It represents the relative motion of the cart and the ground.

-- Roger

 

[profpangloss]

As I understand it, you think the situations aren't the same because the earth is large compared to the treadmill. In that case, all you need is a bigger treadmill.

Imagine that you are in a car driving down a road. You can feel the wind blowing in the window and you can see the road below you.

Now, all of a sudden, the car is magically transported into a building and placed on a treadmill. The treadmill looks like the road and is moving at the right speed that it looks just like the road did to you before. In front of the car is a fan that's blowing air at you at just the right speed that the air blowing in the window feels the same as when you were outside. All around you is projected a moving image that looks just like what you saw outside. Would you be able to tell that the magical journey had taken place?

You might argue that you won't be able to feel movement in the car when it is on the treadmill. Well, the truth is that you can't feel movement when you are in the car on the road either. As long as the car is not turning or changing speed, you won't feel the movement of the car.

So, you won't be able to tell the difference because sitting still on a treadmill is equivalent to moving along the road.

-- Roger

The more you talk about magical journeys, the more I doubt your science. This car you are talking about is powered by an engine and not the wind, is that right? If so, it is not what we are talking about here but even so it will not sit on the treadmill without restraints. It is not the same and also irrelevant.

 

[Michael C]

I do agree that energy is frame dependent, just as velocity is. I still disagree that vehicles can be powered by energy from the ground.

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?

It would be more correct to use the frame where the ground is not moving and the wind is moving and giving energy to the cart so it can move against the friction of the ground.

You keep saying "the wind is moving". I think you mean "the air is moving. "Wind" is a velocity difference between two things.

Why do you want to use a reference frame that does not correctly describe what is happening?

All reference frames correctly describe what is happening. The reference frame where the air is stationary also happens to be a convenient one to look at the working of the cart.

I think gravity is entirely relevant to this discussion of energy transfer. Gravity exerts a force on every object on earth, pressing it down with a force of Fg = mg. If you want to move that object with respect to the surface it is on, you must overcome this force, which in the horizontal direction is mgk where k is the coefficient of static friction between the object and the surface. The surface is not going to exert the force to overcome the static friction, that force must come from elsewhere, from something like the wind, for example.

If the coefficient of friction between the cart and the treadmill surface is similar to the coefficient of friction between the cart and the road surface, then the energy to move the cart with respect to the surface will be similar on the two surfaces. Since both the treadmill surface and the road surface are horizontal, gravity is acting in the same direction and with the same force on any object on one of these two surfaces.

That being the case, the cart on the road must get its energy from the wind, not the surface of the road. That is not true for the cart on the treadmill, which can get energy from the moving tread even in still air.

Once more: motion is relative and energy is frame dependent. The cart can only increase its kinetic energy if there is a velocity difference between the two media (air and surface) with which the cart is is contact. This is just as true on the road as it is on the treadmill.

 

[profpangloss]

I think you need to read up on inertia and momentum.

Anyway, the treadmill doesn't represent the "motion of the planet itself." It represents the relative motion of the cart and the ground.

-- Roger

That's right! The treadmill doesn't represent the motion of the planet but the road does! Maybe you are finally understanding what I am saying?

 

[A.T.]

The treadmill doesn't represent the motion of the planet but the road does!

The treadmill belt is moving relative to the airmass in the room.
The road is moving relative to the airmass in windy conditions.
No difference.

Maybe you are finally understanding what I am saying?

Yes we all understand that you don't understand the relativity of movement.

 

[jadebox]

That's right! The treadmill doesn't represent the motion of the planet but the road does! Maybe you are finally understanding what I am saying?

Oh, now I see why you are confused. You think that the "motion of the planet" has some bearing on the situation. It doesn't.

The road, cart, my imagined cars, and the treadmill are all moving along with the planet. This movement doesn't affect how the cart works or why the treadmill is a good analogy. The treadmill represents the movement of the cart over the road (or the road under the cart), not the "motion of the planet."

-- Roger

 

[CNY_Dave]

Humber- please explain exactly how a runner on a treadmill in still air experiences any physical differences from a runner on the road with a tailwind.

...

Well on the treadmill it certainly takes less effort to get up to speed...

Dave

 

[jadebox]

If so, it is not what we are talking about here but even so it will not sit on the treadmill without restraints.

Why would you think that?

-- Roger

 

[Jack by the hedge]

If you were on that carrier, it would take but seconds "to tell" by measuring the speed of the ship against the water. No problem at all.

Ah. So when you said you would jump in the air to tell the difference, what you meant was that you would measure the speed of the ship through the water. Right.

So, how do you measure the speed of a ship through the water by jumping in the air?

(The obvious inference is that you actually believe that if a person on a moving ship jumps in the air, that person will stop moving when their feet leave the deck. The further implication of your latest comment is that you think they stop moving relative to the ocean rather than, say, the seabed. If this is the case, can you describe the force which acts on the moving person to change their horizontal component of velocity?)

 

[profpangloss]

Oh, now I see why you are confused. You think that the "motion of the planet" has some bearing on the situation. It doesn't.

The road, cart, my imagined cars, and the treadmill are all moving along with the planet. This movement doesn't affect how the cart works or why the treadmill is a good analogy. The treadmill represents the movement of the cart over the road (or the road under the cart), not the "motion of the planet."

-- Roger

This is where you are wrong. The surface of the tread is not moving along with the planet. It has an external source of energy and moves independently of what the planet is doing and so it can transfer some of that energy to the cart. The road is moving along with the planet and in order for the cart to move on that road the cart requires an external source of energy, external to the road. The road cannot give any energy to the cart to make it move against the force of gravity and static friction. That is a major difference from the treadmill. It is you who is still confused, not me.

 

[profpangloss]

Well on the treadmill it certainly takes less effort to get up to speed...

Dave

That is also true for the cart! One more difference between treadmill and road.

 

[Modified]

No they are not. In fact that is the main point of my argument; the road is the planet earth and it is not a treadmill or a river. The reason for that is the gravitational force that is associated with the planet will move everything along as the planet moves along.

Gravity only exerts a force oriented toward the center of the earth, it doesn't act to "pull" things along with its rotation. That is, suppose that the core of the earth were rotating at an enormous speed relative to the surface. This would not tend to pull things on the surface in the direction of the core rotation, even though the core is providing significant gravitational pull. Forgive me if I'm stating the obvious, but it is not clear that you understand this.

Energy is required to overcome the tendency to move with the planet and that energy does not come from the motion of the planet itself.

Energy is required to overcome the tendency to move with the treadmill as well. Accelerating something on the treadmill will cause the treadmill motor to expend more or less energy to maintain speed, depending on the direction of acceleration. Accelerating something on earth in the east-west direction will slightly slow down or speed up the rotation of the earth, depending on the direction of acceleration. Either way, for a small vehicle relative to the treadmill size and a small vehicle relative to the earth size, the treadmill and earth are effectively not accelerated (any change in the treadmill speed or the earth's rotational speed are too small to matter), and the vehicle behaves and performs the same in either case.

For something that hasn't been needed at all, it sure was called into service quite fast!

Again, the treadmill was never used or needed to analyze the cart's performance, only to provide a very convenient and consistent testing platform.

From what I gather in reading up on this, there are a few people out there with more than a basic knowledge of physics who dispute the validity of the treadmill as a conclusive demonstration of dwfttw travel, so your assertion does not stand up.

There was one. If you know any physics professors, go ahead and ask them. I don't think you will be able to find any who disagree with the equivalence, other than on minor technical grounds (if the treadmill is too small and weak relative to vehicle size, there will be differences). The answer is in your own experience though. When you walk around on the deck of a ship that is moving at constant speed in calm water, or on an airplane that is cruising at constant speed and altitude, do you become disoriented and fall over? Do you have to alter your gait? No, you don't. You walk the same, and expend the same amount of energy, moving about on those moving platforms as you do moving around on the earth's surface.

 

[Subduction Zone]

profpangloss, I will be happy to run you through a series of yes/no questions that will show you that the cart on the treadmill is equivalent to the cart on the road at wind speed. I will tell you ahead of time that all of the answers are yes and only require explanation if you claim an answer is no.

Now I assume that you have seen spork's videos and will acknowledge that the cart can move forward on the treadmill in spork's den. Yes or no?

 

[jadebox]

This is where you are wrong. The surface of the tread is not moving along with the planet. It has an external source of energy and moves independently of what the planet is doing and so it can transfer some of that energy to the cart. The road is moving along with the planet and in order for the cart to move on that road the cart requires an external source of energy, external to the road. The road cannot give any energy to the cart to make it move against the force of gravity and static friction. That is a major difference from the treadmill. It is you who is still confused, not me.

You need to forget about the motion of the planet. It is not relevant.

The road, the cart, and the entire treadmill are all moving along with the planet. The planet is moving around the sun. The sun is moving around the center of the galaxy. All of that movement has no effect on the cart, treadmill, or road.

-- Roger