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Quick question on forces

  1. Nov 4, 2008 #1
    Hi everyone! Ok, so we have a bird which is flapping its wings. Avg speed of air displaced is v. Area of wings is A. Density of air is p.

    Thus mass of air displaced per second is vAp.

    This is ok.

    However, according to conservation of momentum, (vAp)(v)=mg, where m is mass of bird

    Also however, according to Newton's Third Law, Force everted by bird on air=Force exerted on air by bird, (vAp)(g)=mg ???

    Is there a mistake in my understanding???

    Thanks.
     
  2. jcsd
  3. Nov 4, 2008 #2

    Doc Al

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    Staff: Mentor

    So what's the problem with that?
     
  4. Nov 4, 2008 #3
    I mean like there are two different equations to ma when the bird is hovering in air.

    vApg=ma and v(Apv)=ma

    Which is correct?
     
  5. Nov 4, 2008 #4

    Doc Al

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    Staff: Mentor

    Check your units. Only one of those equations has units that match. (I assumed it was just a typo before. How did you get that highlighted equation?)
     
  6. Nov 4, 2008 #5
    Well, according to Newton's Third Law, Force everted by bird on air=Force exerted on air by bird, (vAp)(g)=mg

    I know this is wrong, but why is it?
     
  7. Nov 4, 2008 #6

    Doc Al

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    Staff: Mentor

    That's certainly true.
    Where did you get this from? It certainly doesn't follow from Newton's 3rd law. (Note that this equation makes no physical sense, since the units do not match across the equal sign.)
     
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