Merged DDWFTTW in a “Real” Wind

[humber]

I used to do this in school English essay questions too.. if I can't answer the question, change the question and hope no one notices.

And of course, there is an engine in the ship. It could sail, but not ddw to WS.
Did they not mention that?

 

[humber]

Ahhh... So the Earth is fixed in space, because it doesn't have an engine.

[qimg]http://1.bp.blogspot.com/_kKjrNsGlZbE/TRi4JQLD_TI/AAAAAAAAA4Y/z_ouoCvnhvo/s1600/geocentric+theory.jpg[/qimg]

I expect it won't be long before you call on Zeus.

 

[temporalillusion]

And of course, there is an engine in the ship.

I'm buying stocks in tap shoes.

 

[profpangloss]

He means the air is affected in the same way. The still air in a room will be disturbed by an object moving up the treadmill in the same way the wind is disturbed by an object on the ground.

I have been reading this and am confused by this comment most of all. The still air is being disturbed by the spinning propeller on the treadmill, meaning that energy is being transferred to the air. Isn’t that right? How can that be the same with the wind, if the cart is wind powered? In that case, isn’t energy being transferred from the wind to the cart? Please can you explain how you see the two cases to be the same?

 

[spork]

I have been reading this and am confused by this comment most of all. The still air is being disturbed by the spinning propeller on the treadmill, meaning that energy is being transferred to the air. Isn’t that right? How can that be the same with the wind, if the cart is wind powered? In that case, isn’t energy being transferred from the wind to the cart? Please can you explain how you see the two cases to be the same?

That's a valid observation - but easily explained...

Energy is not an intrinsic property something has. Energy is frame dependent, like velocity. In the frame of the cart (at steady speed), the propeller does in fact put energy into the air. It does this by accelerating the air relative to the cart. In this case the cart gets its energy from the treadmill. Measured in the frame of the cart, a cart on the road would get its energy from the road.

When the analysis is done in the road frame, you'll see that the propeller is slowing the air down. In this case it's taking energy from the wind. Note that to an observer on the ground, the cart leaves a wake of de-energized air (slower wind).

Make sense?

ETA: all of the above doesn't suggest the two cases are different; only that the energies differ depending on the frame we choose for our analysis. We could do our analysis in the cart frame, the road frame, the wind frame, or any other frame whether the cart runs on the treadmill or the road. In each case, all the math would be identical if we chose the same frame in either case.

 

[Michael C]

I have been reading this and am confused by this comment most of all. The still air is being disturbed by the spinning propeller on the treadmill, meaning that energy is being transferred to the air. Isn’t that right? How can that be the same with the wind, if the cart is wind powered? In that case, isn’t energy being transferred from the wind to the cart? Please can you explain how you see the two cases to be the same?

Energy transfer is a question of reference frames: kinetic energy is frame dependent. If you look at the cart on the road from a reference frame in which the road is at rest, the energy is being transferred from the moving air to the cart: part of the air is slowed down by the passage of the cart.

Now look at that same cart from a reference frame in which the air is initially at rest. In this reference frame, the ground is moving. In this frame, the kinetic energy from the ground is transferred to both the cart and the air, causing the cart to accelerate in one direction and some of the air in the opposite direction.

For the cart on the treadmill it's the same thing: first look at this cart from a reference frame in which the top surface of the belt is at rest, then look at it from a frame in which the air around it is at rest.

The confusion arises because we naturally tend to use the frame of reference where the ground is at rest when we look at the cart on the road, whereas we naturally tend to use the frame of reference where the air is at rest when we look at the cart on the treadmill.

ETA: Hey, I see that spork beat me by two minutes. It looks like we agree, anyhow.

 

[69dodge]

I have been reading this and am confused by this comment most of all. The still air is being disturbed by the spinning propeller on the treadmill, meaning that energy is being transferred to the air. Isn’t that right? How can that be the same with the wind, if the cart is wind powered? In that case, isn’t energy being transferred from the wind to the cart? Please can you explain how you see the two cases to be the same?

The two cases look the same if you're sitting in the cart. The ground (or treadmill) is rushing by, underneath you, and the air is also blowing past you, but more slowly than the ground.

Energy is frame-dependent. That means it depends on how you're moving. If you're standing on the ground when a wind is blowing, then, relative to you, the ground isn't moving but the air is, so the ground doesn't have energy but the air does. If you're moving along at the speed of the wind, then relative to you, the ground is moving but the air isn't, so the ground has energy but the air doesn't.

ETA: You people are too fast!

 

[spork]

ETA: You people are too fast!

We're all starving for adult conversation, and this seemed like a good opportunity.

 

[tobermoryphil]

Another analogy: I am at sea, out of sight of land. My yacht is still in the water; her sails are not raised.
I experience a 3 knot breeze.
How do I know if the boat is in a 3 knot tidal stream, and the air is still, or if the sea is still and there is a 3 knot breeze? (My GPS is not switched on).
Humber may jump up and down, but he has yet to tell us how this differentiates the options.
If I raise the sails, I will get a bit of thrust regardless of whether the plane surface (the sea) is moving below stationary air, or whether air is moving over the stationary plane surface.
The treadmill is another plane surface moving below stationary air; a cart on the treadmill, running at treadmill speed, in still air, experiences identical inputs to a cart running over the ground at the same speed with a tailwind also at the same speed, so that it too is in still air.

 

[profpangloss]

That's a valid observation - but easily explained...

Energy is not an intrinsic property something has. Energy is frame dependent, like velocity. In the frame of the cart (at steady speed), the propeller does in fact put energy into the air. It does this by accelerating the air relative to the cart. In this case the cart gets its energy from the treadmill.

Yes. I agree in this case the treadmill is the source of energy.

Measured in the frame of the cart, a cart on the road would get its energy from the road.

I don’t know of any examples where the road is a source of energy for vehicles travelling on it. By road I mean that it is part of the planet. The world is moving all the time but because of gravity everything including the atmosphere moves with it, except for local variations called winds that are not related to planet rotation. If everything moves with it, it cannot be source of energy to anything. This is different from the treadmill because gravity tries to hold things in place while the tread surface moves laterally to the force of gravity. I see where this case can be exploited to get energy to drive things like a cart on the treadmill. I don’t see similar situation with the road, which is the whole world.

When the analysis is done in the road frame, you'll see that the propeller is slowing the air down. In this case it's taking energy from the wind. Note that to an observer on the ground, the cart leaves a wake of de-energized air (slower wind).

Make sense?

ETA: all of the above doesn't suggest the two cases are different; only that the energies differ depending on the frame we choose for our analysis. We could do our analysis in the cart frame, the road frame, the wind frame, or any other frame whether the cart runs on the treadmill or the road. In each case, all the math would be identical if we chose the same frame in either case.

Sorry, but as I see it they are two different cases for the reason I have given.

 

[3point14]

Then the above suggests that I would need to teach you, but I am busy with the other seals at the moment.

There you go again with the insults, Humber


It's dreadfully telling that, when asked to provide evidence that you can actually, you know, do the maths, you default to insulting those who ask.

 

[ThinAirDesigns]

... I can’t do so without going over some tangential asides wrt how your question was phrased and the background of a similar exchange, ...

No RCP, it's a simple question that MichaelC keeps asking you and does not require any going into 'tangential asides regarding the background of similar exchanges'. ROFLAO

Here's the question:

"What is the direction of thrust of a propeller (any propeller, not just the one on Blackbird) in relation to the plane of rotation of the blades of the propeller."

And you say the answer it?

JB

 

[windgrins]

Duplicate Post

 

[windgrins]

Tober-A potential source of confusion

Another analogy: I am at sea, out of sight of land. My yacht is still in the water; her sails are not raised.
I experience a 3 knot breeze.
How do I know if the boat is in a 3 knot tidal stream, and the air is still, or if the sea is still and there is a 3 knot breeze? (My GPS is not switched on).
Humber may jump up and down, but he has yet to tell us how this differentiates the options.
If I raise the sails, I will get a bit of thrust regardless of whether the plane surface (the sea) is moving below stationary air, or whether air is moving over the stationary plane surface.
The treadmill is another plane surface moving below stationary air; a cart on the treadmill, running at treadmill speed, in still air, experiences identical inputs to a cart running over the ground at the same speed with a tailwind also at the same speed, so that it too is in still air.

Tober,

The way this is worded is similar to what would happen in the real world but has a hidden source of confusion for the non-sailor/physicist.

There is no question about the equivalence of reference frames so that's not an issue.

On the real ocean or lake if a sailor were to notice a 3 knt change of wind over his sails, there could be only two reasons- either the air (wind) came up or he drifted into a three knot current. If one entered a current from still conditions (wrt ground) a sensitive accelerometer could detect that the boat was accelerated and not the air. If it was the breeze (most likely), the boat would not have an acceleration and the sailor could assume the wind change. In fact this is how the inertial navigation systems on submarines deal with current changes underwater. Once the delta of air vs water is established, and the sailor is in steady state conditions, there is no longer a way to detect a difference without outside references until another acceleration event occurs, i.e. GPS, a distant piece of land, the stars, etc.

It is important in thought problems to be clear about the conditions of the test and what is being stated. From your example, anyone here should be able to infer you meant static conditions but it doesn't seem so obvious after all to everyone. It seems for clarity we have to sort these kinds of things out early.

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!

Regards,

WindGrins

 

[Modified]

I don’t know of any examples where the road is a source of energy for vehicles travelling on it.

A sail or a windmill that is on the ground accelerates both the air and the earth. If in an east wind, then the earth's rotation is slowed. Whenever you begin to walk or drive east, you accelerate the earth and slow its rotation. It's just that the earth is very big, so the change is very small. For normal-sized vehicles, we can treat the earth as effectively constant velocity, just as we can the treadmill.

The source of energy in the road and treadmill examples is the difference in the speed of air and surface. In both cases, the cart causes the surface to accelerate by effectively zero, and accelerates a good quantity of air and slows it with respect to the surface.

 

[quarky]

Local to me 800Euro up.
My folding bike is a simple one. It has a hinge in the middle of the spar.
The rest may be folded, but most of the time that is not necessary. I can carry on to the train, unfolded, with one hand. It is light, agile and fun to ride, and cost 169Euro. I don't see how the supposed advantages of recumbents are actually realized.
.

But they do, and even in The Netherlands, you have to live with that. What you don't want is some recumbent blocking the ever-narrower cycle paths, and weaving all over the place every time it starts off. I hate the damn things.


It is better Quarky, because without those big trucks, you would find that things cost a lot more at the supermarket. They are very efficient per ton/mile. Perhaps its the cars that should go.

I can certainly relate to your opinions on this. I'd say we're in an awkward transition period regarding transportation. Around here, for instance, there's lots of Amish. I admire them, but I pity the horse and buggies using our roads.

 

[ThinAirDesigns]

I don’t know of any examples where the road is a source of energy for vehicles travelling on it.

In the frame of the wind approaching a sailboat, the earth (water) is the source of energy for every sailboat ever launched.

In the frame of the wind approaching the land yacht or ice boat, the earth is the source of energy for every such craft ever set in motion.

In the frame of the wind approaching the wind turbine, the ground is the source of energy for every wind generator ever installed.

The Blackbird is not different - in the frame of the earth, the energy comes from the air. In the frame of the air, the energy comes from the earth.

The truth is that in all of the above, both mediums (earth and air) are slowed relative to each other.

JB

 

[A.T.]

I expect it won't be long before you call on Zeus.

Only if I would actually buy into your nonsense:

So you think that if you are in a car, you can't tell if road is moving or the car is moving? Well you can, because it's the car that has the engine.

What if the engine is not in the car, but on the road and pulls the car with a rope. Now the road has the engine. Who is moving according to humberphysics?

 

[Michael C]

I don’t know of any examples where the road is a source of energy for vehicles travelling on it. By road I mean that it is part of the planet. The world is moving all the time but because of gravity everything including the atmosphere moves with it, except for local variations called winds that are not related to planet rotation. If everything moves with it, it cannot be source of energy to anything. This is different from the treadmill because gravity tries to hold things in place while the tread surface moves laterally to the force of gravity. I see where this case can be exploited to get energy to drive things like a cart on the treadmill. I don’t see similar situation with the road, which is the whole world.

Three people answered your first question, all saying the same thing: energy is frame-dependent. If we choose any reference frame where the road (or ground, or call it what you want) is not at rest, then this road/ground has kinetic energy. Take for instance a very basic sail cart, just a thing on wheels with a square sail, set to run in the direction of the wind, slower than the wind. From the frame of reference where the air is stationary, where is the energy for the motion of this cart coming from? It cannot be coming from the air, since in this frame the air is stationary and therefore has by definition no kinetic energy. In this frame, the energy is coming from the ground.

People can get in a muddle when they talk about "the wind" as though this only involved the air and not the ground. Remember that all motion is relative: it makes no sense to talk about "the velocity of the air" without stipulating velocity relative to what. In fact when we say "the velocity of the wind is x km/h in such-and-such direction", we usually mean that the velocity of the air is x km/h in such-and-such direction relative to the ground. This is the exact same thing as saying that the velocity of the ground is x km/h in the opposite direction relative to the air: the two statements are equivalent.

Gravity is irrelevant to our discussion of energy transfer, since we are only looking at motion perpendicular to the force of gravity. The cart does not move up or down, so gravity cannot add energy to it nor subtract energy from it. The only importance of gravity in this case is to ensure that there is a traction force between the wheels of the cart and the surface on which it runs. Gravity fulfils this function equally well on the road and on the treadmill.

 

[profpangloss]

Three people answered your first question, all saying the same thing:

If something is wrong, it will still be wrong no matter how many people believe in it, so please don’t try to convince me with numbers, just arguments, thanks.

energy is frame-dependent. If we choose any reference frame where the road (or ground, or call it what you want) is not at rest, then this road/ground has kinetic energy.

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.

Take for instance a very basic sail cart, just a thing on wheels with a square sail, set to run in the direction of the wind, slower than the wind. From the frame of reference where the air is stationary, where is the energy for the motion of this cart coming from? It cannot be coming from the air, since in this frame the air is stationary and therefore has by definition no kinetic energy. In this frame, the energy is coming from the ground.

Yes, that is a way of looking at things, but what purpose does it serve when you know beyond any doubt that the cart in this case is being powered by the wind? You can say the ground has kinetic energy, but then so does a the side of a barn coming at the cart at whatever speed the cart is moving. What happens when cart meets the barn? The kinetic energy of the barn wall works to stop the cart’s motion, not to make it keep moving in the same direction. I say the kinetic energy of the ground works the same way, to slow down the sail cart, not to keep it moving. So this frame is not very useful for explaining the cart’s motion over the ground. 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. Why do you want to use a reference frame that does not correctly describe what is happening?

People can get in a muddle when they talk about "the wind" as though this only involved the air and not the ground. Remember that all motion is relative: it makes no sense to talk about "the velocity of the air" without stipulating velocity relative to what. In fact when we say "the velocity of the wind is x km/h in such-and-such direction", we usually mean that the velocity of the air is x km/h in such-and-such direction relative to the ground. This is the exact same thing as saying that the velocity of the ground is x km/h in the opposite direction relative to the air: the two statements are equivalent.

No. I think that everyone understands what wind is. People can get into a muddle when you try to use different reference frames that only confuse the situation. Why do you want to confuse the situation when it can be very straight forward? That is what I want to know.

Gravity is irrelevant to our discussion of energy transfer, since we are only looking at motion perpendicular to the force of gravity. The cart does not move up or down, so gravity cannot add energy to it nor subtract energy from it. The only importance of gravity in this case is to ensure that there is a traction force between the wheels of the cart and the surface on which it runs. Gravity fulfils this function equally well on the road and on the treadmill.

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. 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.

On that basis, I still maintain the two cases are not the same.