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Homework Help: What is the bird's speed immediately after swallowing?

  1. Nov 12, 2007 #1
    [SOLVED] Momentum Question

    1. The problem statement, all variables and given/known data
    A 300 g bird flying along at 6 m/s sees a 10 g insect heading straight toward it with a speed of 30 m/s (as measured by an observer on the ground, not by the bird). The bird opens its mouth wide and enjoys a nice lunch.

    What is the bird's speed immediately after swallowing?

    2. Relevant equations

    [tex]p_{1}[/tex] = [tex]p_{2}[/tex]

    3. The attempt at a solution

    I know relative speed is the sum of the bird's and bug's speed since they are headed in opposite direction, and 30 + 6 = 36 so

    [tex]m_{bird}[/tex][tex]v_{bird}[/tex] + [tex]m_{bug}[/tex][tex]v_{bug}[/tex] = ([tex]m_{bird}[/tex] + [tex]m_{bug}[/tex])[tex]v_{final}[/tex]
    0.3(36) + 0.01(36) = (0.3 + 0.01)[tex]v_{f}[/tex]

    [tex]v_{f}[/tex] = (0.3(36) + 0.01(36))/0.31 = 34.87 m/s

    but this is wrong, probably because the relative speed. Any suggestions? Thanks.
    Last edited: Nov 12, 2007
  2. jcsd
  3. Nov 12, 2007 #2
    Check the velocities that you entered.
  4. Nov 12, 2007 #3
    Its been a while since I did one like this, but I don't think yo should use the relative speed like that. You have the bird accelerating massively, which is impossible.

    I'd say its more like (0.3kg)(6m/s)-(0.01kg)(30m/s)
  5. Nov 12, 2007 #4
    I know something is wrong with them, the problem is I can't figure out what.

    According to my assigment:

    so that's what I thought it was, which apperantly isnt.
  6. Nov 12, 2007 #5
    You're right..... it is wrong because of the relative speed.

    Don't use relative speed. The bird has a velocity of 6m/s and the bug has a velocity of -30m/s

    Use these and you should get your answer.
  7. Nov 12, 2007 #6
    I'm assuming that the answer wants the bird's speed relative to the ground
  8. Nov 12, 2007 #7
    God dammit...

    I tried doing that earlier in the day using that method... now I relased I put the decimal in the wrong place... *sigh*

    Thanks for the help, I would have been stuck on this for hours otherwise
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