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Relative motion in a circle

  1. Sep 10, 2016 #1
    1. The problem statement, all variables and given/known data
    13.164.png

    2. Relevant equations
    a= omega^2*r

    3. The attempt at a solution
    a_A/B = a_A - a_B

    a_A = 10^2*r (-î)
    a_B = 10^2*r (+ĵ)

    a_A/B = 10^2*r (-î) - 10^2*r (+ĵ)
    => -200 î -200 ĵ

    why is the answer 200 î -200 ĵ (accelerate in +î direction) ???
     
  2. jcsd
  3. Sep 10, 2016 #2

    Student100

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    The question is asking for the acceleration of A relative to B. What's the acceleration at A and at B independently?

    Hint: Check the direction of acceleration vectors.
     
    Last edited: Sep 10, 2016
  4. Sep 10, 2016 #3
    Err... isn't my expression correct..?

    a_A = 10^2*r (-î). A is going -x, -y direction
    a_B = 10^2*r (+ĵ). B is going -x, +y direction
     
  5. Sep 10, 2016 #4

    Student100

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    No. Should your acceleration be tangential or radial to the disk? Remeber the disk has constant angular velocity.
     
  6. Sep 12, 2016 #5
    should be tangential cuz constant radius??
     
  7. Sep 12, 2016 #6

    Student100

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    Actually it should only have radial acceleration. Can you see why?
     
  8. Sep 12, 2016 #7
    because constant rotation?
     
  9. Sep 12, 2016 #8
    so the a_r for B is pointing from point B to Point O
    and for A it is pointing from Point A to Point O?
     
  10. Sep 13, 2016 #9

    Student100

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    Exactly, so what's your relative acceleration now?
     
  11. Sep 13, 2016 #10

    so the a_r for B is omega^2*r (-i)
    a_r for A is omega^2*r (-j)

    a_A/B = a_A - a_B = -omega^2*r (j) - -omega^2*r (i)
    rearrange = omega^2*r (i) - omega^2*r (j)

    Got it!! Thanks!
     
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