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Homework Help: Birds in uniform cirular motion

  1. Oct 16, 2008 #1
    Hi friends,

    1. The problem statement, all variables and given/known data
    Birds of prey rise up and down on thermals. A bird completes circle of radius 10m every 6s and rises vertically at a rate of 4.0 m/s. Assume uniform acceleration. Find

    a)speed of bird relative to ground
    b)magnitude of acceleration
    c)direction of birds acceleration
    d)angle between birds velocity vector and horizontal.

    2. Relevant equations

    ar = v^2/r

    3. The attempt at a solution
    I have got answers for a to c. a) 11.2 m/s b) 11 m/s^2 c) 0°

    But I cant really figure out how i can measure the angle.

    Could it be tan-1 = 4/11.2 = 19.65?

    God in the past few days I have been dreaming about angles.

    Please help me.
  2. jcsd
  3. Oct 16, 2008 #2


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    Homework Helper

    w = 2*π*f =v/r

    v = 2*π*r*f

    There is also the acceleration of 4 m/s2 in the z direction don't forget.

    As to your dreams, maybe try and forget the acute ones? There are plenty of others out there.
  4. Oct 16, 2008 #3
    Hi Pion, thanks for the reply.

    What do you mean by w = 2*π*f =v/r

    v = 2*π*r*f?
    What is w, n, and f?

    Is this how I get the angle?

    Also 4m/s is the velocity not the acceleration :-)

    Please help
  5. Oct 16, 2008 #4
    I got the answer but I did it in a different way.

    I separated the vector into its components vx and vy.

    vx = 4 vy = 10.47

    then: tan-1 = 4/10.47 = 20.something!

    Thanks anyway!
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