What is the bird's speed immediately after swallowing?

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  • #1
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[SOLVED] Momentum Question

Homework Statement


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?

Homework Equations



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

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.
 
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Answers and Replies

  • #2
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0.3(36) + 0.01(36) = (0.3 + 0.01)[tex]v_{f}[/tex]

Check the velocities that you entered.
 
  • #3
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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)
 
  • #4
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I know something is wrong with them, the problem is I can't figure out what.

According to my assigment:

if two objects are moving in opposite directions (either toward each other or away from each other), the relative speed between them is equal to the sum of their speeds with respect to the ground

so that's what I thought it was, which apperantly isnt.
 
  • #5
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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.
 
  • #6
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I'm assuming that the answer wants the bird's speed relative to the ground
 
  • #7
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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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