spork
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And that was actually a homemade spork. What's the only tool worse than a spork?...
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Screwdriver
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You'll find no sympathy here. I buy all my tools spares or repairs. Been doing that for years, marvelling at my ingenuity in acquiring a toolset beyond my financial means.
Only problem is, everything I have is ever so slightly broken...
PhillipM
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Me.
spork
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Yes. I'm glad you mentioned that. A lot of people feel like the wind can't effect it if it's outrunning the wind. But it's not like you don't get wet if you're swimming downstream. You're immersed IN the water.
I do. There are tons of diagrams that show what the flow looks like when it passes through an "actuator disk" (aka propeller). I've attached one here. Just ignore the ground and superimpose this picture on the air. That's what happens. It leaves a wake of de-energized (i.e. slower air) behind it. And of course that wake of slow air "dissipates" as it is eventually entrained by the wind around it (which is then slowed slightly).
I'm not sure what you're saying.
Almost! Keep in mind that the wheels end up feeling a retarding force so they can turn the propeller. So the wheels ultimately apply force on the Earth in the same direction the wind would have when it hits the trees and buildings. You can't fight conservation of momentum - even with a vehicle that goes DDWFTTW.
spork
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That's "Bill Nye the Science Guy". He's the most famous scientist we have on this side of the pond! (well, not really, but he is a science communicator celebrity here). Interestingly, I got to know him when he attended a talk I gave on this very subject at the Google science conference. I'm pretty sure Kusenko didn't know that (not that it would change anything).
Screwdriver
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Yes I suppose what I am suggesting is that the amount of retardation the wheels impart to the ground is less than it would have been if the vehicle was bolted down. That difference being the energy imparted to the vehicle via the prop.
As for energy being dissipated locally, it has to come from somewhere. One system or the other is going to receive or contain less energy than it would have done had the vehicle not been there. Either the wind now contains less or the earth is receiving less since it has been deposited into the kinetic energy of Blackbird.
As for energy being dissipated locally, it has to come from somewhere. One system or the other is going to receive or contain less energy than it would have done had the vehicle not been there. Either the wind now contains less or the earth is receiving less since it has been deposited into the kinetic energy of Blackbird.
Screwdriver
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I can't make head or tails of that diagram, I will have a Google splurge tomorrow and reframe the question. I haven't slept for days during my current obsession in modelling the JWST. Now trying to figure out the dimensions of every single spar in the backplane. It will look amazing tho...
My confusion is still regarding exactly where the exchange of energy is being made (as posted above). A kneejerk (super tired brain) example might be what's it like living one side or the other of a gigantic windfarm? Ahead of the turbines, business as usual. Behind them, the wind has deposited some of its (admittedly gigantic) energy into turning those great big generators, so I suppose you could say it's a bit less windy (with fewer birds). Bad example and a potential source of confusion when exploring your device which is not a turbine (!) but just an observation regarding the conservation of energy.
spork
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Don't be so sure of that. With the vehicle bolted down, the propeller blades will be fully stalled. They'll produce a whole lot less lift that way (think of an airplane falling straight down out of the sky maintaining normal attitude (not altitude) vs. that same plane flying or gliding down. So with the vehicle bolted down, it won't actually provide that much impediment to the wind. It will provide some, and that will be transmitted to the ground through the wheels (and whatever anchoring is used). The wind that's not impeded will go on to hit trees and buildings, and the net momentum will be the same.
Energy is a different story. Energy is also conserved, but it's harder to do the equation for a problem like this since it all ends up as heat one way or another. As to where the energy comes from - you get to decide since energy is frame dependent. Pick the wind as your reference frame and the cart is ground powered. Pick the ground and it's wind powered.
Screwdriver
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I thought I was sure to be honest. But it's stupid o'clock and I am struggling to generate the curvature of a telescope mirror frame. It's a fair bit trickier than I had hoped!
Before my brain explodes, I am (relatively) sure the following is true: If you were to push an object though a static gas/fluid/atmosphere, it would require energy to be expended proportional to the relative speed. The situation is exactly reversed if you were to flow the same fluid/gas around the same static object at the same relative speed. The object would experience a force equal to that required to propel it (ignore the (many) practical barriers to that equality but I am pretty sure that is the guiding principle for wind tunnels etc.
Since it requires energy to perform a task (lit. do work) then that energy is transferred from one system to the other. Whether that is the kinetic energy gained by Blackbird zooming about or heat energy/turbulence (yikes!) generated by holding Blackbird still within the flowing media (as measured by strain gauges in a wind tunnel). "The wind" will lose the same amount of energy as Blackbird will gain, whether or not it is easy or even practically achievable to measure exactly how those losses are manifest in something as ephemeral as a steady breeze.
My individual frame of reference has no bearing on the matter. Blackbird will acquire kinetic energy (plus heat with bearings and stuff) and that energy gain can be measured. The wind loses energy since our static object creates turbulence within the medium of the atmosphere and I can also hear it as an audible sign that work is being done and energy is being "lost" in sound waves/heat (whether the vehicle moves or not) and my personal frame of reference does not dictate which system is expending or acquiring energy.
The big issue here is that "the wind" is to all intents and purposes, an infinite energy source in that it is so vast, you'd struggle to measure any "losses". Though, as I said somewhat tongue in cheek, it may be a bit less windy on the leeward side of a giant windfarm.
Anyhow. I need to sleep before I collapse. I would like to understand the physics of this situation which is starting to look like it might change my current level of understanding by a country mile if my "seat of the pants" somewhat ill informed appreciation of these dynamics is so drastically incorrect.
spork
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Generally speaking, the force will be proportional to the square of the speed. The power (energy per time) will be proportional to the cube of the speed.
Yes - this is true. And this is Galilean invariance. And this part is important. The two aren't "equivalent"... they are the SAME THING. Velocity literally only has meaning relative to something else. That something else is usually any inertial frame we like. Generally here on Earth we choose the ground (which isn't really an inertial frame because the Earth is spinning - but close enough for the girls I date).
Couple of things... in engineering we generally use work and energy as interchangeable. But here's the part you're not going to like... Energy is not like Gatorade. No one really knows what it is. In the end, it's just bookkeeping. I *know* that sounds insane. But it's true. No one has ever seen or touched energy. We see the results of energy and the things we think represent energy. But think about the difference between kinetic, chemical, potential energy to name a few. What the heck is that all about!? Some of those are absolutely frame dependent, and some are not. But they're all the same thing in the big book of keeping your equations balanced. With a spring it kind of feels like the energy is "in the spring" when you compress it. But how about the potential energy of a weight that you've raised to some height? You may say it's 10 feet up, and I may say it's 100 feet up - because we didn't agree on the frame to measure it in. Does it "contain" two different amounts of energy? How about the kinetic energy of a speeding bullet? Same problem. You measure if from the ground frame (and it can kill you), and I measure it from the bullet train I'm riding on - and it's only going 5 mph faster than me. I could just reach out and catch that bullet. Does it "contain" two different amounts of energy? Nope.
So you really just have to get comfortable with the notion that energy is something we use to keep the books. You can't touch it or see it.
This is something I touched on in the last response. Any momentum lost by the wind interacting with the vehicle will be gained by the vehicle. Not true for energy. Energy IS conserved, but the interaction can be (and usually IS) a lot more messy. The wind can lose energy (be slowed down) by interacting with the vehicle. Some of that energy will be gained by the vehicle (it speeds up) and some will be lost to heat. If the brakes are locked - the vehicle gains no energy (except perhaps some of the heat energy), but still no energy is lost. It's just all turned to heat.
So here's another interesting (and sometimes tricky) situation. Your frame can change a whole lot of things. You might measure the vehicle to be going a different speed, have a different amount of kinetic or potential energy, have a different mass, or even have a different length depending on your frame. But what will *never* change is the result of the experiment. If the vehicle burns up - it will burn up regardless of the frame you choose. If it can fit through a doorway in your frame, it will fit through in any other frame.
But consider a hockey puck hitting the dasher board at 100 mph. It's got a lot of kinetic energy (1/2 * mass * velocity^2). But what if I'm measuring it from it's frame? It then has NO kinetic energy - until the dasher board comes along and hits it at 100 mph. But the *result* will always be the same.
By the way - I'm not sure if you really bargained for all of this, but I'm happy to do it if you care to go down these roads.
Yes!
No! In the ground frame, the puck put a lot of energy into the dasher board. In the frame of the puck (the frame before it hits the board), the board comes along and smacks the puck and puts energy into the puck (to the extent we can say something "contains" kinetic energy). And you might say "come on now - dasher boards don't come along and hit a puck at 100 mph!". But they really do. It's absolutely no more accurate to say the puck is moving and the dasher board is stationary than the other way around. If you're standing at the equator, you're moving at about 1000 mph just because the Earth is rotating. Or you're moving at many thousands of mph since we're orbiting the Sun. Or you're moving at a whole butt load of mph because our solar system is orbiting the center of the Milky Way galaxy. Or you can toss all of those out and pick an inertial frame that's going any speed you like, and let that define your motion. I know this sounds like stupid mind games, but it's really not. Inertial motion is completely relative (rotational motion is not. We CAN say how fast something is rotating. That's real).
Sure. And for a problem like this, we sometimes simplify things by making assumptions. We might assume the wind is steady from the ground, up. We might assume it is steady infinitely far upstream and downstream, etc. Sure, you'll never really find such conditions in the real world, but wind gradient at the ground doesn't really have to be studied to understand the nature of every problem.
Well, it can be really fun (and sometimes challenging) to update your intuitions. And we can certainly go down that road if you care to. But I also don't want to make people think you have to understand quantum physics to understand how this cart works. I'm happy to work through more intuitive explanations - just depends on what people want. But I will admit, this brain-teaser does tend to bring even the most educated folks down these challenging roads. There's no new physics. But it is sufficiently counter-intuitive that it does require you to carefully re-think some of the things you're pretty sure you know.
Munkul
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Rick, do you give engineering based lectures? You're certainly above average in being able to articulate and teach this stuff.
Designing and building the Blackbird is one thing... being able to explain it to everyone else is a totally different kind of gift!
Designing and building the Blackbird is one thing... being able to explain it to everyone else is a totally different kind of gift!
spork
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I've given talks on occasion. Usually on Blackbird, but I've also given talks on some other things. I sure appreciate the vote of confidence, but you are WAY outnumbered by the people that claim I don't know how it works or how to explain it.
If you've got an hour and 20 minutes of your life to squander...
If you've got an hour and 20 minutes of your life to squander...
Screwdriver
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Aha! Ok. First thing bright and early and i can see where my confusion is now. Thanks for the considered reply and I will have to dedicate some time to digesting it and finding some more questions for you.
Meantime, here is the crux of the matter:
My confusion is due to the age old dichotomy between physics and engineering. A physicist wants to know why something happens, and an engineer wants to know how to make something happen. In reality and for a person such as yourself, you need a working model which can be applied in real world situations to solve classical Newtonian problems. I am more of a keyboard warrior who is keen to appreciate the theoretical or philosophical meaning of a dynamic situation.
In the real world it makes perfect sense to assume we are operating in an inertial reference frame, when, unless we jump off a roof and ignore wind resistance, we are not. But in engineering we can safely ignore the microscopic differences a physicist would insist on so that their theoretical model can be scaled to include lit. relativistic effects etc. In other words, we do not need General Relativity to drive a car.
However, if we ignore (let's say) the "reality" of the physics in a situation in order to be able to make sound engineering decisions then the concept of "energy" becomes moot. I think that's why I can't fathom where your energy is coming from (which system is causing entropy to increase) because we have already chosen to ignore the conceptual framework which would define it.
Much as I would like to discuss the concept of entropy, in the engineering world, our macrostate might as well include the earth, all the wind and the incredible Blackbird.
PhillipM
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Theres an even more fun one with bikes in that we havent quite worked out how they stay upright either. Its not gyroscopic precession, its not mechanical or pnuematic trail - you can build a bike with negative trail and ride it fine - but more seems to be the feedback loop from pnuematic trail to the bars that we build. Because if you make a bike with zero trail its a nightmare although you can still manage it.
Even that theory falls over when you can send a bike at speed with no rider and it will balance and correct itself though
Even that theory falls over when you can send a bike at speed with no rider and it will balance and correct itself though
Screwdriver
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Haha, yeah, remember that one. Good old countersteering in action. Riding a bike is a bit like balancing a pole on your hand, if it falls one way, move the bottom the other. It pivots around the cog until the system is balanced. If a bike starts to fall one way, the front wheel falls the same way and due to the offset in the headstock pivot (caster and/or trail) , steers the bike upright.
Hell of a concept to play with on a motorcycle where, mid bend, you want to say tighten or loosen your line around a corner. Lots of beginners don't realise they already countersteer so it's a shock to them when you tell them to push or pull the bars the "wrong" way. On a motorcycle the effect is incredibly subtle, you can literally just apply a tiny amount of pressure to the bars while cornering to perform fairly drastic corrections.
