Merged DDWFTTW in a “Real” Wind

[spork]

No, I wouldn't say that the cart uses its own kinetic energy to accelerate. Not directly, anyway. But I think it might not be entirely incorrect to say that some of the cart's kinetic energy is (or, perhaps, would be) used up in turning the propeller. ("Would be", because once the propeller is turning, it allows the cart to gain more energy from the wind than is expended in turning it.) Of course, everything happens simultaneously, so the cart's kinetic energy doesn't ever actually decrease, but the tendency is there. Does that make sense?

To be honest - no that doesn't make sense. At least not to me. Any "tendency" the cart has to turn the prop by using it's own kinetic energy will HAVE to involve the cart slowing down - not speeding up.

 

[ThinAirDesigns]

No, I wouldn't say that the cart uses its own kinetic energy to accelerate. Not directly, anyway.

Nor indirectly.

But I think it might not be entirely incorrect to say that some of the cart's kinetic energy is (or, perhaps, would be) used up in turning the propeller.

The above statement is *entirely* incorrect when the BB is accelerating (the only mode we care about). With a fixed ratio transmission, if the propeller RPMs are increasing, then the cart must also be accelerating making the statement absolutely impossible.

Of course, everything happens simultaneously, so the cart's kinetic energy doesn't ever actually decrease, but the tendency is there. Does that make sense?

Nope -- not at all. Either the KE of the vehicle is used to spin the prop during acceleration or it isn't. In the case of the BB, it isn't. There is no 'tendency' whatsoever for it to happen because it can't happen.

JB

 

[69dodge]

To be more precise, for "tendency to decrease the kinetic energy of the cart", read "backward force of the ground on the cart". Does that make my meaning clearer? The net force, including the forward thrust from the propeller and the backward force from the ground, is forward, so the cart accelerates forward, but the backward force is still there.

I'm just trying to understand profpangloss's way of looking at this. Personally, I think it's easier to focus on torque and force, but he likes to focus on energy and power.

 

[spork]

To be more precise, for "tendency to decrease the kinetic energy of the cart", read "backward force of the ground on the cart". Does that make my meaning clearer? The net force, including the forward thrust from the propeller and the backward force from the ground, is forward, so the cart accelerates forward, but the backward force is still there.

The surface does of course apply a "backward" force on the cart. But the thrust from the prop is greater than that backward force plus the aero drag. But I don't see how that relates at all to using the cart's kinetic energy or momentum.

I'm just trying to understand profpangloss's way of looking at this. Personally, I think it's easier to focus on torque and force, but he likes to focus on energy and power.

Excellent. Please help us to see it too. I personally think it's more easily explained with energy and power, but I've offered both explanations.

 

[Llyricist]

To be more precise, for "tendency to decrease the kinetic energy of the cart", read "backward force of the ground on the cart". Does that make my meaning clearer? The net force, including the forward thrust from the propeller and the backward force from the ground, is forward, so the cart accelerates forward, but the backward force is still there.

I'm just trying to understand profpangloss's way of looking at this. Personally, I think it's easier to focus on torque and force, but he likes to focus on energy and power.

That's why the 600 W of prop shaft power is the result of the negative work of 750 J/s the prop and drive train do on the cart, after frictional and heat losses.

And the 483 W of effective power is what is left after aerodynamic losses at the prop.

Leaving a 100% energy transfer of 920 J/s from air to the cart system. Which covers all the frictional and heat losses of the cart system with 102 J/s left over to add to the KE of the cart system.

 

[ThinAirDesigns]

To be more precise, for "tendency to decrease the kinetic energy of the cart", read "backward force of the ground on the cart". Does that make my meaning clearer?

It does make your meaning clearer but it makes it no more correct. You can measure all sorts of forces in all sorts of places all over the vehicle, but in the end *none* of those forces will show that the vehicle can use it's own KE to accelerate (directly or indirectly). Can't happen - doesn't happen.

The net force, including the forward thrust from the propeller and the backward force from the ground, is forward, so the cart accelerates forward, but the backward force is still there.

True and still irrelevent to the statement that the KE of the vehicle is used to accelerate the vehicle (directly or indirectly). That backward force must be overcome and while accelerating it's not being overcome by any use of KE of any sort from the vehicle - period. Any statement to the contrary is 100% incorrect.

JB

 

[humber]

The cart doesn't use it's own kinetic energy to accelerate, so whatever Harold has shown in this regard is not relevant.

JB

PS: and the post you refer to is spot on, though again not related to the cart using it's own KE to accelerate.

What ddw cart?

 

[humber]

The surface does of course apply a "backward" force on the cart. But the thrust from the prop is greater than that backward force plus the aero drag. But I don't see how that relates at all to using the cart's kinetic energy or momentum.



Excellent. Please help us to see it too. I personally think it's more easily explained with energy and power, but I've offered both explanations.

When you do make and test a ddw cart, rather than something else, we will get back to you.

 

[humber]

I agree. Hummer has consistently thrown up nonsensical parts of objections but because of the forum format has never had to lay out his entire argument in a coherent way. His objections fight with each other and the data at every turn and laying them out end to end has never been required before.

I'd like to give him a chance to do so without interruption and see what comes of it.

Until then I will attempt the impossible dream. I'll likely fail, but goals are important.

He is worried, and should be. NALSA will not be happy to learn they have been be involved in a scam.

 

[69dodge]

OK. In that post you wrote: “I don't believe 9 m/s is the right speed to use here. That's the cart's speed relative to the ground. But the propeller exerts a force on the air, not on the ground. Since the propeller's thrust is moving the propeller through the air at only 2 m/s, only (160 N)(2 m/s) = 320 Watts of power is required.”

If the objective here was to move 72 kg per second of air @ 2.2 m/s, as a fan would, then 320 Watts would definitely be sufficient for that task. But the main objective here is to move the 295 kg cart @ 9 m/s over the ground. To do that requires Work = Force x Distance and in this case that is 160 N x 9 m = 1,440 Joules, or in terms of Power 1,440 J/s (Watts).

Work is force times distance, but the force and the distance have to be at the same place, so to speak. I don't think it's correct to multiply the force of the propeller on the air by the distance that the cart travels over the ground. You could use the 9 m/s speed of the cart over the ground, but then you'd also have to use the force between the cart and the ground, whatever that is.

Actually, now that I think about it, I don't believe any work is done at the wheel/ground interface, except that needed to overcome rolling resistance. Each wheel rolls along the ground without slipping; the point of contact with the ground does not move relative to the ground, so although there's force, there's no distance. But I am not especially sure about this. I welcome comments.

Michael wrote: "we have a large truck with the brakes off. We give it a push to set it rolling at, say 5 m/s. We throw a tennis ball in the direction of travel, at, say 10 m/s. The tennis ball hits the rear of the truck. We have now increased the kinetic energy of the truck by hitting it with something that has less kinetic energy."

True, but this does not contradict me at all. I said that an object with less kinetic energy can transfer that energy to another object with greater kinetic energy if work is done. Work was done in throwing the tennis ball at the back of the truck.

We're supposing that the air is already moving, so why can't we suppose that the tennis ball is already moving?

This is also a bad example because it does not reflect the situation we have with the wind moving slower than the cart.

Yes, I agree. This important point is the difference in speed, not the difference in kinetic energy

This is known as over unity. In Newtonian mechanics, there is no way to use energy such that it creates more energy than what you started with.

I certainly agree that we can't end up with more energy than we start with. But the wind contains energy, and that's part of what we start with. There's no reason why we can't get a lot of energy from the wind in exchange for expending a little energy setting things up right. For example, it takes energy to erect a windmill, but once it's up, we can gain that energy back, and much more, from the wind.

I assure you I have examined the treadmill in great detail. [...]

I have to go now. I will try to get to this part of your post later.

 

[humber]

Wow, that's all very interesting. I wonder why it's so critically important for harold/yev/ppg to refuse to respond to SZ's questions?

I have a cart that says you don't have a clue how the TM works.
That cart, the amusing failings of yours, the NALSA data and the complete failure of any theoretical support, are going to leave you "thrashing about", Spork.

 

[3point14]

OK so as to avoid even the possibility of ground power, I would prefer to stay in the FOR where the ground is stationary and has no energy.

I'm afraid this statement really does show that you don't understand what's going on at all.

 

[Dr. Keith]

I have a cart that says you don't have a clue how the TM works.

No, you don't.

Let me repeat that: You don't. You don't have a cart. You don't have a cart that will travel up a running TM. You don't have anything.

You just flat don't and anyone reading this thread knows that you don't. It is patently silly for you to keep asserting this. It just doesn't exist. Never has, never will.

 

[humber]

Entertaining that humber's greatest insult is to call someone "literal" - isn't it? As an engineer, I take it as a compliment. Although to be fair, being insulted by humber should probably always be taken as a compliment.

You are no engineer!
I gave the example of where the collision is perfectly elastic, so KE is also conserved. The ball comes to a dead stop against the surface as all of the KE goes to the truck.
That leaves you to explain how the headwind does not slow the cart by the converse of that process, and gain KE from the cart.

Good luck with telling NALSA their data is wrong.

 

[Dr. Keith]

I'm afraid this statement really does show that you don't understand what's going on at all.

Quite the opposite, it shows that he knows what is going on but just can't admit it. Failure is not an option.

 

[Llyricist]

Work is force times distance, but the force and the distance have to be at the same place, so to speak. I don't think it's correct to multiply the force of the propeller on the air by the distance that the cart travels over the ground. You could use the 9 m/s speed of the cart over the ground, but then you'd also have to use the force between the cart and the ground, whatever that is.

Actually, if you're doing the analysis in the "ground's frame", it is force X distance over the ground.

What he is doing is mixing up energy required to do the work with energy balance. These are two separate issues.

In the first case I have 920 J/s KE loss of the air, plus 483 J/s of effective power spinning the perfect prop = 156 N X 9 m/s

In the second case I have an energy transfer of 920 J/s from air to cart system, with cart system losing 818 J/s in heat, friction and aerodynamic turbulence, and gaining 102 J/s in KE.

 

[Dr. Keith]

You are no engineer!
I gave the example of where the collision is perfectly elastic, so KE is also conserved. The ball comes to a dead stop against the surface as all of the KE goes to the truck.
That leaves you to explain how the headwind does not slow the cart by the converse of that process, and gain KE from the cart.

No, you don't.

Use your formula and run the numbers. The ball transfers KE to the truck. Yes, it is infinitesimally small, but that was the point of the example. It flew right over your tiny little head.

 

[CORed]

You are no engineer!
I gave the example of where the collision is perfectly elastic, so KE is also conserved. The ball comes to a dead stop against the surface as all of the KE goes to the truck.
That leaves you to explain how the headwind does not slow the cart by the converse of that process, and gain KE from the cart.

When humber gets really frustrated, he resorts to posting irrelevant gibberish.

 

[3point14]

Quite the opposite, it shows that he knows what is going on but just can't admit it. Failure is not an option.

He's obfuscating rather than failing to understand reference frames?

 

[profpangloss]

I'm afraid this statement really does show that you don't understand what's going on at all.

Isn't it easy to make empty statements that don't say anything at all? How about showing where my understanding is at fault, rather than just claiming it is? You see even the carteers are denying ground power and using the cart's own kinetic energy, but they are unable to show how the wind that is moving slower than the cart, and has considerably less kinetic energy than the cart, is able to contribute to the cart's kinetic energy. If you understand things so well, then why don't you explain it to all of us?

While you are at it, explain how the cart on the treadmill, running without the inertia from the tread, is the same as a cart on a road.

It should be a simple matter for you, since you understand it all so well!