physics and intuition

Discussion in 'All Things Boats & Boating' started by Dave Gudeman, Jan 25, 2010.

  1. Dave Gudeman
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    Dave Gudeman Senior Member

    There have been several threads on this forum about two related topics:

    (DUW) Can you use wind power to drive directly upwind?

    (DDWFTW) Can you use wind power to go directly downwind faster than the wind?

    The answer to both questions is "yes", but a lot of people argue very strongly against this answer on the grounds that it is "like a perpetual motion machine" or "you are getting out more energy than you put in".

    The basic problem is that a lot of people, especially people who build or repair things (like boats) have very good physical intuitions. In other words, they can imagine a physical situation in their head and guess pretty well at what's going to happen. We are born with this ability and we develop it further over our life. Without good physical intuition, we couldn't throw and hit a target, we couldn't catch anything, we couldn't do simple things like lean two boards up against each other so they don't fall down. We sure couldn't play pool or table tennis. To do all of these things we have to be able to see what is happening and visualize what is going to happen.

    But physical intuition is just wrong in the case of DUW and DDWFW. One of the things that is built into human physical intuition is a sort of "conservation of motion principle". This isn't something discovered by physics. There are "geometric proofs" of mechanical principles dating back as far as the dark ages that just take his principle for granted. You don't need a physicist to tell you that when the moving queue ball hits the standing target ball, the total motion of the two balls after the collision is going to be somehow "the same" as the motion of just the queue ball before the collision. All that physics did is quantify this intuition.

    Our intuition tells us that the wind has a certain "amount of motion". It tells us that you can't get more motion out than you put in. It also tells us that you can't just reverse motion without something hard and massive to bounce against. Both of these are true intuitions, but they are being applied incorrectly to the DUW and DDWFTW situations.

    Physics tells us that there are actually two different laws that correspond to our notion of "conservation of motion". One is conservation of momentum and the other is conservation of energy. Neither of these laws is violated by the DUW and DDWFTW situations. Lets take each of the conservation laws in turn.

    (1) Conservation of momentum: the wind that hits the vehicle has a certain momentum change m. This momentum change must be balanced somewhere else. In the case of a boat doing DUW, momentum is transferred to the water by way of the propeller. There isn't any law that connects the momentum change of the wind to the momentum change of the water. It just depends on how much power we can put into the propeller, and that depends on how much power we can get from the wind turbine. There isn't any conservation law that says one must be less than the other.

    (2) Conservation of energy (the DUW case): Let's say the speed of the wind is w and the speed of a boat going into the wind is b. Let's say that the energy that the boat requires to go at speed b into a headwind of w is e1. If the boat has a wind turbine on it then the wind turbine is converting energy from the wind and pushing it to the propeller. Let's call this energy e2. The drag on the wind turbine is already calculated into e1. Now, what conservation principle is it that says e2 cannot be greater than e1?

    The drag on the boat can vary over a huge range depending on design, and likewise the energy extracted from the wind and transferred to the propeller can vary over a huge range depending on design. Can your physical intuition really account for all of these factors? It is true that the drag on the propeller increases with size and that some minimal size is needed to get the energy out that you need, but there is no law that says takes all of these factors into account and says that e1 is always greater than e2.

    Similar points apply to the DDWFTW case except that the turbine becomes the propeller and the propeller becomes the turbine.
  2. gonzo
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    gonzo Senior Member

    1) yes there is
    2)Energy can not be created
    You are mixing carrots and philosophy. Intuition and physical laws are not the same.
  3. Dave Gudeman
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    Dave Gudeman Senior Member

    1) yes there is what?
    2) why are you telling me something that I just finished saying?
    I know that intuition and physical laws are not the same. My whole point is that people are mistakenly thinking that their intuitions are reflected in physical laws.
  4. gonzo
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    gonzo Senior Member

    I think it is very difficult to tell what you said. It is rather incoherent. It would be easier to discuss if you had more focus and better grammar.
  5. TollyWally
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    TollyWally Senior Member

    The grammar's fine, the explanation is just a bit wordy is all. Shorter version; what you think you see is not always what you really see. Physics only affects what you really see not what you think you see. Si? :)

    I'd like to see someone with a better grasp of these things than myself plot horsepower harnessed vs hull resistance for various wind driven powering devices. Sails, turbine/prop gizmos etc.
  6. Tiny Turnip
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    Tiny Turnip Senior Member

    We often use mental 'models', or metaphors to help us understand things. When we witness something new, we try to make it fit a pattern, or model we know and understand. Sometimes, the new thing is so new to us, so counter intuitive, that it requires a new model or way of thinking.

    When first learning about sailing, I struggled with understanding how sailing into the wind was possible *at all*, despite explanations of pressing down on a bar of soap and making it sliding across the kitchen table. It wasn't until I added into the explanation the fact that the bar of soap needed to be wedge shaped that the resolution of vectors became clear. Of course, then further understanding of the foil effect, apparent wind and so forth means the model becomes more complex.

    When I first saw images of vehicles designed to accomplish DUW and DDWFTTW, they seemed very exciting, and defied belief. I am now reasonably comfortable with them, having thought about them a lot, and developed models for thinking about what is happening that I am comfortable with.

    I find what is being accomplished currently with dynamic soaring

    (399mph reported in December see

    harder to understand. I haven't yet developed a mental model of how DS works that I am totally comfortable with, though I do understand that the sailplane repeatedly dips in and out of the slow and fast moving bodies of air, through the windshear, and that action repeatedly accelerates it.
  7. Asleep Helmsman
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    Asleep Helmsman Senior Member

    I've had a lot of "practical" experience and I have no problems imagining a machine that will go straight into the wind.
    Simply stated: The available energy is obtained by the difference in the forces acting on an entity.
  8. gonzo
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    gonzo Senior Member

    Dave: Stating "yes" to your own question may be intuitive, but doesn't prove a thing. I can't tell if you are for or against believing that intuition is more accurate than physical evidence.
  9. DianneB
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    DianneB Junior Member

    With more than 30 years in the design field I have learned that my intuition usually tells me something that is later proven in the lab - it's like knowing the results before you understand the "why".
  10. Dave Gudeman
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    Dave Gudeman Senior Member

    I think that physical evidence is more accurate than intuition.

    I was just writing about my attempt to understand why intuition goes wrong in these cases. Understanding why intuition is wrong in one case may help avoid being mislead by intuition in other cases.
  11. Asleep Helmsman
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    Asleep Helmsman Senior Member

    I think for inventors in general, practice is essential.
    But if this discussion is about intuitive visualization, then I think the later in life you start to apply imaginative brain cycles to solving problems, the more you have to train your brain.
    Conversely you could be more open to thinking outside the box.
    Think about what Einstein accomplished without training. He saw a universe, different from the one everyone saw (well almost everyone), and then he created the math and the experiments to prove it.
    So I think that the ability to imagine brand new ways to do things could have very little to do with formal training, or formal training could be the inspiration a farm boy need to create the best vessel ever made.
    I believe you need to leave your slide rule behind and look for things in nature to inspire. Than come here and ask the math geeks their opinion.
  12. alan white
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    alan white Senior Member

    I believe that thought is geometric, which is why we understand music even if we aren't shown how a musical passage adheres to certain precise mathematical laws. Our minds hear the music, actually feeling a very sophisticated equation.
    I think there are levels of mathematical comprehension some people attain through practice and natural ability, similar to chemical crystallization, which allow a new kind of comprehension to take place. This new comprehension doesn't require the mind go back to zero to "see" the thing in question.
    Instead, the geometric "crystallization" has already instantaneously jumped to a level where sets of certain variables "hold" in an existing framework.
    This framework can be called up any time the problem at hand includes certain variables.
    I believe there are many such levels, and they are all like mathematical formulae, except they are felt by the intuition rather than being understood rationally.
    In other words, it has nothing to do with mathematical abilty as we think of it. Rather, it is a different way of reading mathematics (by feeling the geometry of the equation), and its value, besides assisting in the design of such a thing as a windmill-powered boat, is also how Beethoven or Mozart, and Picasso, or an olympic gymnast create their works.
  13. kistinie
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    kistinie Hybrid corsair

    Very good topic Dave.

    Diode is a good example of this paradox.
    So magnetic viscosity, is.

    Self - Diode - Condenser linked in O.U devices are also good candidate to this paradox.
  14. Asleep Helmsman
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    Asleep Helmsman Senior Member

    Absolutely: Think about the complex calculas your visual cortex does when you're driving your car.

  15. Submarine Tom

    Submarine Tom Previous Member


    I like what you've said. From my perspective, I believe I understand what you're saying.

    Let me ask you this:

    If you tether a helium balloon to your vehicle console so it is not touching anything, turn off all the ventalations systems and close the windows, then drive forward at a good clip and hit the brakes, where does the balloon go?

    I look forward to all the replies. Then, one day when a respondent happens
    to have a helium balloon handy, try it and let us know the result.

    1 person likes this.
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