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Why _doesn't_ this scheme work?

  1. Nov 3, 2012 #1

    Imagine an imaginary car with a huge windmill on top connected to its enginator. Assume windmills and enginator are 100% efficient (not possible, I know, but it's a thought experiment). Perpetual motion? What exactly causes it to fail? (I.e. why won't it self-accelerate indefinitely) I'd be more interested in a hint, than a full answer. And by "what exactly causes it to fail", I'm _not_ looking for answers like "thermodynamics says there's no PMMs", etc. but the precise reasons (or a hint as to them, actually) why this particular scheme fails.
    Last edited: Nov 3, 2012
  2. jcsd
  3. Nov 3, 2012 #2


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    (how do you imagine an imaginary car? :confused:

    i can only imagine a real car! :biggrin:)​

    in other words, it's a car with a propeller?
  4. Nov 3, 2012 #3
    Conservation of Energy.
  5. Nov 3, 2012 #4
    This answer is not helpful:
    <Morbo> Windmills do not work that way!! </morbo>

    This answer is:
  6. Nov 3, 2012 #5


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    Energy would be conserved but not gained. With a lossless system, the car could "coast" at constant speed. Perpetual motion usually refers to a device that somehow manages to gain energy, or at least gain energy sufficient to overcome losses, as opposed to an object being able to move at constant velocity when there are no external forces such as drag involved.
  7. Nov 3, 2012 #6


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    Actually, it would work, and it doesn't even have to be 100% efficient. We have been powering boats this way for millennia. It isn't terribly fast or reliable, but it is relatively cheap.

    Energy conservation is not violated as long as any energy gained by the car is less than the energy lost by the wind. That is enough to prevent perpetual acceleration.
    Last edited: Nov 3, 2012
  8. Nov 3, 2012 #7
    Er... - I think you miss-read the question.
    The OP proposes a turbine on the roof of the car presumably powered by the slipstream. Power from the turbine driving the car wheels. It's a PMM, not a sail-boat-on-wheels.
  9. Nov 3, 2012 #8


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    The OP didn't say it explicitly, but I'm pretty sure he's talking about a situation where there is no wind.
  10. Nov 3, 2012 #9
    In that case, the OP should consider how the vehicle could even start moving from rest.
    ( Unless it has an internal motor as prime mover )
  11. Nov 3, 2012 #10

    Vanadium 50

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    I think the OP is missing a trick. You put up several windmills, use one to drive the car, and the rest are pure profit.
  12. Nov 3, 2012 #11


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    It's not perpetual motion if there is true wind. Then you can go at any speed directly upwind or downwind, if you are efficient enough. Here some fun experiments:

    I guess you mean an argument that doesn't invoke the concept of energy? Assuming no true wind, infinite L/D ratio of the blades and ideal direct transmission to the wheels you can show with vectors, that there can't be any forward net force. Even if you pull it to a certain speed initially.
  13. Nov 3, 2012 #12


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    If you're wondering why it fails (assuming that you want to power the windmill purely from the slipstream, with no natural wind), the answer is that the wind generator causes drag, and the drag caused is always going to be greater than the output power could offset.
  14. Nov 4, 2012 #13


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    You are probably both right. But perhaps my comments could encourage the OP to think about energy a bit. As long as he is pulling energy from the already moving air then energy is conserved and he has a functioning vehicle similar to a sailboat. Otherwise he has a complicated paperweight.
  15. Nov 4, 2012 #14


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    I think the OP is looking for an explanation not based on energy conservation (which is a rather abstract "reason"), but for reasons more specific to the proposed machine. For a simple transmission you can explain it with force vectors, without invoking energy directly. The only assumption is that the aerodynamic force cannot have an angle greater than 90° to the relative airflow. But how to prove that? I guess momentum conservation is at least implicitly assumed here.
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