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Air resistance when moving in the same direction but faster

  1. Mar 13, 2015 #1
    Hi all,
    So after the wind almost blew me off my bike today, something occurred to me. It goes like this:
    We all know that if we bike in the same direction as the air does, then we will get a little push and it makes our journey easier. Then suppose we get a little adventurous (reckless people like me) and start moving faster than the air, Does it mean we will get air resistance instead of air push because we are hitting the air molecule in the front instead of being pushed by them? This just feels wrong to me but I don't see what is wrong with my reasoning, and I try to do a bit of search around, but I could not find such random question like this. I ask this question specifically because I have seen many boat and river and wind on homework and exams in the past, and they never take this factor into account. But again, I still feel that my reasoning is wrong somehow. Please give me your insights.
    Thanks
     
  2. jcsd
  3. Mar 13, 2015 #2

    Suraj M

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    Air resistance is nothing but opposition to relative motion. In your case you can actually consider the air around you at rest(as reference) and treat it as if your going at a velocity(=your velocitywith respect to the ground - velocity of the wind with respect to the ground.
     
  4. Mar 13, 2015 #3

    A.T.

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    Yes of course.

    You would have to be more specific about the boat scenario. For example, a conventional sail-craft cannot go dead downwind faster than the wind. But this rotor-sail-craft can:



    Note what happens with the red bands around time 3:00, when the cart passes wind-speed. The relative wind, and thus the air resistance on the cart change direction, as you suggested.
     
  5. Mar 13, 2015 #4
    Thank you both.
     
  6. Mar 13, 2015 #5

    Going to randomly pop in here and ask if you know the way this device functions -- I'm assuming it uses the momentum of the rotors to get across the dead zone of no wind speed. What bothers me is it keeps accelerating directly into a headwind, which I thought was impossible. Is it because each rotor makes an appropriate angle with the headwind so they deflect the momentum of the wind as if sailing crosswind, like two small ships sailing crosswind and in opposite directions while towing a large ship?

    I admit that my question is based on only casual knowledge of sailing a single sail vessel.

    Thanks!
     
  7. Mar 13, 2015 #6

    Suraj M

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    The acceleration is due to the wheels. It doesn't matter which direction the wind is blowing, it's(the wind) just used to move the object.
     
  8. Mar 13, 2015 #7

    A.T.

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  9. Mar 13, 2015 #8
    Maybe take it to the next level: Mach velocity, MV/C . Say that you're peddling your bike with the wind blowing against your back @ 20 mph. in a helpful manner. Now, if you speed up by any amount over 20 mph., you will begin to feel resistance as if you had turned 180 Degs. Now, any speed you generate in that direction will be against you. A real good exampl is a space capsule. With no resistance in outer space, no strain on the capsule. Coming back into the atmospherre of earth @ perhaps 25,000 mph. heat shields are needed to keep from burning the craft to cinders becaus of the friction of the air. So, just don't pedal your bike too fast. Hope that explanation helps a bit.
     
  10. Mar 16, 2015 #9
    Of course you will get air resistance when moving faster than the wind. The key thing is tyou will get LESS air resistance, so either it will be easier to maintain your speed or you will be able to go faster. Or a bit of both.
     
  11. Mar 16, 2015 #10

    A.T.

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    Right, you have less relative headwind, than you would have at the same cycling speed but without the true tailwind. That is also one way to explain why the DDWFTTW cart works only when there is true wind, but not if you push it on a calm day. The reduction of the relative headwind allows the propeller to produce more thrust for the same input power.
     
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