No, we can not consider work as a change of energy because thats not what it is. Work is the transfer of energy. If there is no energy transfer, then there is no work being done. Using your definition, if I throw a ball against the wall, the wall has done work because the balls energy has changed. That is just wrong.
I am being a bit sloppy with my work/power by using velocity. Since V=0, X is also zero. So in any event, Power=ForceVelocity or Work=ForceDistance and both are zero.
I understand this, but saying things like the power to turn the propellor comes from the ground is just wrong.
From the generator’s reference frame yes, but we aren’t in the generator’s reference frame. We are in the reference frame of the earth or ground. In that reference frame, there is no work being done by the ground, because there is no energy transfer from the ground to the system.
Yes, but there is no work done by the ground because there is no energy transfer from the ground to the system. In the reference frame of the earth, as I said previously, the velocity or delta X is zero at the application of the force. Therefore there is no work or power being done by the ground. The power to turn the generator is coming from the force you are applying along with the velocity of the car.
If you would humor me for just a bit I really would like you and I to leave the ‘what does work on what’ theory behind for a bit and just focus on what works. Thanks.
So you agree with my premise that given a steadily propelled platform, using a tire rolling across the ground hooked to a generator to produce electricity actually works. Excellent.
Let’s see if we can come to an agreement on just how much electricity we can produce based on the force exerted on our ‘steadily propelled platform’.
First, let’s make sure we agree on the definitions:
If you would humor me for just a bit I really would like you and I to leave the ‘what does work on what’ theory behind for a bit and just focus on what works. Thanks.
So you agree with my premise that given a steadily propelled platform, using a tire rolling across the ground hooked to a generator to produce electricity actually works. Excellent.
Let’s see if we can come to an agreement on just how much electricity we can produce based on the force exerted on our ‘steadily propelled platform’.
(all losses ignored for the moment)
Using the below definitions(with reference link), can we agree that if our platform is moving at 55ft per second and is pulled along by an available force of 10lbs, that we can load up the generator such it will produce 750 watts of electrical power for us?
Once more into the brink. treis I would be interested to know if you believe:
a) DDWFTTW is a sham, and the NALSA was duped / is complicit
b) DDWFTTW is real, but the explanations in this thread are all wrong
EDIT: I’m now a believer in b) if we restrict b) to my earlier explanations.
Now to the direct quotes:
Actually that is correct. A transfer of energy results in a change of energy. Delta E. If you throw a ball against a wall there is either: no change in energy due to a perfectly elastic collision (the wall does no work), or a change in energy due to an inelastic collision, in which case work is done: the wall applies a force to the ball over a distance due to the ball’s deformation.
No. It’s perfectly appropriate to mention any intermediary we would like in our energy transition. Like the Man of Steel, I derive my energy from our yellow sun. However, it is just as appropriate to say that the energy to power me comes from plants.
EDIT: This was just such a beautiful piece of snark, I’m leaving it in, but boy is my face red. On review, the error is revealed if I change the analogy to say that the energy to power me comes from poo.
Why not? There is no privileged frame of reference. But we can choose another if you’d prefer.
And here I need to issue an apology - we may be close to converging on understanding. I was working on a nice, semi-snarky reply, but during it I discovered that I nearly agree with you - there is no work being done by the ground - there is work being done to the ground.
What is happening from the ground’s perspective? At t = 0, something is in contact with the ground at point x = 0. (This something is a point on a wheel). Due to static friction this something is applying a force on the ground in direction = 1; thanks to Newton the ground knows to push back with an equal force in direction = -1. We know that the force involved is not zero because the wheel turns. A short time later, t = 0 + e, and something is applying a force on the ground at point x = 0 + ve. The forces are similar. For a whole bunch of these little instances, something is applying a force on the ground along a linear path from x = 0 to x = x1. And so, depending on what that force is, work is done to the ground by the something: W = Fx1.
We can also consider that work is being done to our axle system, to the tune of W = (rF) * omega. For one revolution of the axle, W-to-the-ground = F2pir, W-to-the-axle = F2pi*r.
Where does this energy come from? As you previously maintained, it comes from the doofus pushing the device / the wind. Keeping our notional generator attached, the wind exerts a force equal to twice the back electromotive force. Limiting factors include maximum force the wind can apply (aerodynamics question) and the maximum static friction your wheel/ground interface can maintain. We can assume that energy available to the wind is virtually limitless.
If you then use your available energy to power a fan, you can push back against the wind, making it push on you even harder, sending you speeding down wind faster than the wind. There are probably limits here based on what your spinning propellor/fan does to your effective cross section into the wind. Aerodynamics makes my head hurt.
Yep. Gotta go with your analysis here. The energy transfer is from the wind - to the system - partially to the ground. The neat thing is that the work done to the ground gives you something to ‘push off against’ letting you extract more work from that lazy, lazy wind.
Again, my apologies for the earlier snark. It works, but not the way I initially thought it did.
This is correct. The point is that if we use the ground does work concept, you would need to add in that work to the analysis, which would result in an incorrect answer.
There is no privileged frame of reference, in the sense that you can do the math using any frame. There is a privileged frame of reference in the sense that we only care about what happens in the reference frame of the earth.
No there isn’t, at least in the reference frame of the ground.
No, the force is always applied where V=0 and thus there is never any change in distance at the point of contact. No delta x means no work.
No you can’t. Doing so will result in an incorrect solution because you must also account for the force on the axle over a linear distance.
Look, I’m not going to play answer 20 questions. I understand perfectly the underlying principal of using the wheels to turn the propeller. It is essentially no different from the compressor on a jet engine. The problem is that the wording is sloppy. I would be more interested in hearing about the principles behind the propeller producing more energy with a tailwind.
Tries - there is no “20 question” attempt here, there is only an attempt to understand what you mean by the following:
It is a simple fact that in the Blackbird, at all speeds the torque to drive the propeller is coming from the wheels. It is a fact that thrust of the propeller results in the acceleration of the vehicle. Those two facts appear to conflict with your statement.
Clearly the path I have taken in trying to understand it is frustrating you – perhaps you could explain in different words and we could go from there.
Ok, good. And thanks for sticking around, this has been a fun exercise, and your obstinate practice of being right has helped me work through a better understanding.
Not if it has infinite mass. It is a necessary catalyst for the transformation of energy as a portion of the ball’s kinetic energy is used to deform and heat the ball.
I think that the car or wind are also interesting frames of reference.
But, but, but… Thanks for pointing this out. I think this is where the conveyor belt hurt my understanding. If in the conveyor belt example we set our frame of reference on the belt itself, in that frame the belt does no work on the wheel.
So, in the end, the ground is a necessary component of the energy transfer but does no work (from the perspective of an observer on the ground). It does induce a rotation in the wheel. This means that the energy from the wind can be converted into rotational energy/potential energy harvested from the generator.
The trick seems to be that the wind is capable of providing a large amount of energy. As the frictional (rolling) force increases, the force delivered by the wind to propel an object at wind speed also increases, and work done by the wind increases.
The force on a sail is proportional to apparent wind speed. With our generator running we should travel at slightly less than wind speed. What I’m stuck on now is how this is sustainable when the propeller is turned on, which seems like it would result in an apparent wind speed against the direction of travel. Maybe the way to visualize it is that air moved by the propeller does work on the vehicle in the direction of travel, and that the body of wind in general does work on the air displaced by the propeller.
I’m a bit more confused now, although I’ve finally managed to wrap my mind around rotational friction not doing work.
Treis, it’s becoming pretty clear that you’ve just backed yourself in a corner here and just want your silly statement to go away through your silence. Since I’m the guy you said was either ‘crappy’ or ‘full of shit’, I’m not going to let you just slink off the topic quite so easy.
Here was my statement regarding the wheels turning the propeller:
A:
Here was what you said about it:
B:
And later when I try to walk you through to show that you’re wrong:
C:
Just man up and admit that you’ve realized that your statement “B” is wrong. My original statement is true as you’ve now admitted in “C”, is not flawed in explanation in any way.
Heres my favorite though: Please support your assertion that the “wording is sloppy” in the following:
Yeah, I’m sure you’ll get right on that task. :rolleyes:
Please give me your infinite forgiveness for taking a whole 20 hours to respond to a post.
If the thrust of the propeller resulted in the acceleration than it would work regardless of whether it had a tailwind. Leaving out the tailwind, the most important part, makes it a bad explanation.
Yes hdc, it’s just nonsense to make the assertion that one frame is privileged because “we only care about the earth frame”. This is the domain of folks who come in throwing BS around like:
As everyone here knows, work and energy are frame dependent. Treis saying that the ground isn’t doing work on the car simply becasue HE claims to only care about the ground frame doesn’t make him any more correct.
The wheels always provide the torque to turn the propeller – frame independent. Depending on the frame of reference chosen (and as you point out there are excellent and instructive reasons to look at things from other frames), the power and work can come either from the wind, or from the ground.
I’m pretty sure you know this and have used more casual reference, but actually it would be more accurate to say that the force on the sail goes up proportional to the square of the apparent wind speed.
It’s sustainable because the vehicle is moving across the ground faster than it is moving through the air and thus the wheels can produce more energy than the prop needs to sustain. (see my full post @#20)
If I understand what you are saying here, once the vehicle accelerates past wind speed, the apparent wind is against the direction of travel. This apparent headwind must be overcome to sustain or accelerate and ultimately is one of the losses that limits the top speed of the craft.
Perhaps this is just semantics, but it might be more accurate to say that “the body of wind in general has already done work on the air displaced by the propeller”. After all, air the propeller is interacting with had already been accelerated to wind speed long before we got to it.
When the vehicle is at same speed as wind, to go even faster, something has to start turning faster first to affect other. If wheels start turning faster, how? If propeller starts turning faster, how?
Maybe the gearing can use the momentum to turn propeller faster, but that will cause braking effect.
Since both the prop and wheels are geared together using a fixed ratio transmission, one cannot start turning faster without the other also turning proportionally faster as well. This precludes your opening statement above from being technically true. I suspect however that this is just a minor wording issue on your part as the 2 questions following your statement are reasonable ones.
Since they are tied together, it initially becomes an issue of force, rather than one of speed.
At below wind speed, if the force of any tailwind drag combined with any forward force (thrust) of the propeller is greater than the retarding force at the wheels, the vehicle will accelerate and both the wheels and propeller will turn faster.
Exactly at wind speed and above wind speed, if the forward force (thrust) of the propeller is greater than any headwind drag combined with the retarding force at the wheels, the vehicle will accelerate and both the wheels and propeller will turn faster.
So, the answer to your ‘chicken or egg’ question that opens this post is that at exactly wind speed, the forward force of the propeller on the Blackbird is greater than the retardant force of the wheels and thus the vehicle accelerates and both the prop and wheels turn faster.
My previous post (#20) goes into the energy and force balance issues underlying the above answer. Happy to answer more questions as well or delve deeper if you wish.
As the Blackbird accelerates, every component of the vehicle gains momentum. At no time is there any momentum in one part of the vehicle being exchanged to make another part spin faster – it simply isn’t possible in the Blackbird with a fixed ratio transmission.
It’s the imbalance of forces that I described above:
“… at exactly wind speed, the forward force of the propeller on the Blackbird is greater than the retardant force of the wheels and thus the vehicle accelerates and both the prop and wheels turn faster.”
The root cause of the imbalance of course is the true tailwind which continues to blow unabated across the ground, even while the vehicle chassis itself is traveling at exactly wind speed and thus sees an apparent wind of zero.