ThinAirDesigns
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ROFLAO. Genius argument - say something that has already happened, won't. Consistent at least.
JB
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Merged DDWFTTW in a “Real” Wind
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ROFLAO. Genius argument - say something that has already happened, won't. Consistent at least.
JB
Let's take an extreme case. Suppose you have two magnets stuck together. Each exerts a force on the other. You pick them up and walk across the room with them. Even in the ground's frame, did either do any work on the other? They exerted forces on each other, and they moved relative to the ground, but it seems obvious to me that no work was done.
What do you think?
I think I understand the second case. Not the exact numbers, but the general idea, yes.
I don't understand the first case. What is doing work on what, at the rate of (156 N)(9 m/s)? What do you mean by "effective power" and where does it come from? And why are the two things you add together, appropriate to be added together?
ThinAirDesigns
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I think you must be pulling our leg.
That's like saying that since the the Toyotas on the car transporter are chained down to its deck, no work was done as they traveled cross country from the dock to the dealer.
JB
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Gravity pulls straight down. It doesn't help you keep moving sideways when you jump. That's purely due to inertia.
Yes.
No one claims that the ground is like a treadmill which is sometimes off and sometimes on. The claim is that the ground is like a treadmill which is on for the duration of the experiment under consideration. The ground doesn't change speed, so neither should the treadmill.
No one claims that standing on a backward-moving treadmill is like standing on the ground with no wind. The claim is that standing on a backward-moving treadmill is like standing on the ground with a tailwind blowing whose speed is equal to the speed of the treadmill. When standing on a backward-moving treadmill, you feel a wind at your back, so there should also be a wind at your back when you're standing on the ground, in order for the two situations to be comparable.
Yes. And I've just explained what's different, and how to make them the same.
You can achieve the same effect by running on the ground, provided there's a tailwind as there should be.
Because treadmills are short, so the cart would run off the end before it got up to speed.
On a long enough treadmill, it would work fine without being held, if it would work that way on the ground in a wind.
The motion of a treadmill isn't perfectly smooth like the motion of the ground. I can easily imagine that this would cause the cart to bounce around more on the treadmill than on the ground.
I'm not convinced. You've given qualitative arguments here. Why not calculate some thrusts and torques and forces and such? You agree that the cart works on a treadmill, I think. Ok. Why does it work? The forward thrust of the propeller must be greater than the backward force of the treadmill on the cart's wheels. Calculate them. Then see if the calculation would be any different for a cart moving on the ground at the speed of the tailwind that's blowing.
No.
I don't understand.
Lots of work was done overcoming air resistence and rolling resistance and stuff like that. But I'm not talking about that.
I don't think any work was done by the force of the chains on the cars. Was some done? The chains were under tension, but they moved with the cars.
What did you mean by this?
(Here's the post in question.)
It seems you agreed with me then, but now you don't.
It doesn't seem to be defined well enough to give any kind of answer. But in the cart's case, it is a trivial matter that in the FOR where the ground is stationary, there is a force of 156 N (or 160) acting on the cart over a distance of 9 meters in one second. So that is 1404 (or 1440) Joules of work is done on the cart.
I delineated the specific numbers in a previous post.
There is work being done on the cart at that rate.
The effective power is derived from the "Engine Power" formula on that page previously linked. It is the amount of power effectively converted to thrust AFTER aerodynamic losses (turbulence, rotational losses etc.) are taken out.
It's the effective work done at the prop, and it gets added to the KE loss of the air to account for the total work done on the cart above.
That gets back to what we've said about energy being frame dependent. In the frame of the truck the chains do no work. In the frame of the road the chains do lots of work.
Think about it like this. You have a tree that's being held from blowing over with a chain that's anchored to the ground. No matter how hard that wind blows, the chain doesn't do work on the tree - because neither the chain nor tree moves - right? In the fame of the ground that's the case. That's just like the Toyotas on the car carrier if you are also on the car carrier. Nothing is moving, but the wind is blowing on the car - and the chain is keeping the car from blowing off the back of the car carrier.
But to someone standing on the side of the road, the cars on the carrier are being pulled through the air at 75 mph - by a chain. That chain is clearly doing work on the car.
Different frames - different energy accounting.
What did you mean by this?
(Here's the post in question.)
It seems you agreed with me then, but now you don't.
Nope - I think JB is agreeing with you. You told hyevpangostopher that he didn't prove the cart was using its K.E. He agreed that you're right and he pointed out that your linked post was exactly right as well.
-Say that three times fast....
Not a friggin' chance! That might cause him to appear before me.
By what? The air, I suppose. And what else (if anything)?
I'm still not getting it.
Ok, the "effective power" is the power that's available to drive the cart forward, i.e., to overcome air resistance and rolling resistance and/or to accelerate the cart. But what provides that power if not the KE that's continuously being lost from the air? I don't see why it makes sense to add them.
It's the propeller shaft power less aerodynamic losses. It gets added as it is doing part of the work.
It makes little difference.
I mentioned it earlier. Forgotten SZ?
http://talkrational.org/showthread.php?p=896564#post896564
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Spork said:
The truck's fuel is spent overcoming drag with the air, and with tyre losses. The tension of the chains has no effect on the fuel consumption, so Spork's reply is nonsense.
The chain has a static force between the car and the truck, and static force does no work, so not in any frame.
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tobermoryphil
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OK, Humber, £1000 in a brown paper bag, if you demonstrate your propless cart advancing against a treadmill.
Now, PUT UP, OR SHUT UP!
It's worth more than that, Tobermoryphil. I don't need the money, and I don't want yours just for the dubious pleasure of taking it from you.
If the 60 degree change in wind, and the efforts to hide that, do not tell you that you have been deceived, then I don't suppose anything will. There is not one trial that does not show the cart is worse ddw. TAD saw that too.
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Nothing will ever make humber demonstrate his propless cart advancing against a treadmill. Not for you; not for NALSA; not ever.
But I imagine you understand that. I think your offer, while probably legitimate, is made for the entertainment of others so we watch humber decline even a straight up offer of cash with no risk to himself.
Needless to say, humber couldn't demonstrate such a thing even if he wanted to. I'm honestly not sure if he even fully comprehends that; but every one else clearly does.
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