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Relative Velocity Help

  1. Jan 30, 2017 #1
    1. The problem statement, all variables and given/known data
    If the relative velocity of object B to object A is 10 m/s, and the relative velocity of object C to object B is 10 m/s, what is the relative velocity of object C to object A?

    2. Relevant equations
    These should be vectors just to give a heads up

    vbs= vbw+vws

    vba=vab

    These are the equations given in my book

    3. The attempt at a solution
    My first attempt was using the second eqaution since the question before it was similar but it did not have c and I used the second formula and I got it right.
    The second formula uses the idea of reference frames. So A and B have the same magnitude but different direction. So B and C is the same. So -10m/s -10m/s= -20 m/s. I was given this option and was told it was wrong. Then I made it an absolute value and it was wrong. So would it be -10m/s + 10m/s. Because if B is -10m/s, the opposite magnitude would be 10m/s and adding them together would give me 10? Or would I need more information to solve this.
     
  2. jcsd
  3. Jan 30, 2017 #2
    Convert B wrt A to A wrt B giving -10 then make point diagram of A B and C showing C 10 B 0 and A -10 meter per second this will give you the answer. Hope this helps.
     
  4. Jan 30, 2017 #3
    So was I on the right track making one of them -10m/s and the other one 10m/s. So at this point do I add them to find the relative velocity of C to A. Making it 0 m/s? I find this part really confusing
     
  5. Jan 30, 2017 #4
    No no you are not adding scalars, you are adding vectors afyer this step you have to go to A frame getting B = 10 and C = 20
     
  6. Jan 30, 2017 #5
    For relative velocity wrt to A you always have to be in A's frame or can you use common sence. Always remember this point. Good question.

    Cheers
     
  7. Jan 30, 2017 #6
    I still don't understand this. I find it hard to visualize.
     
  8. Jan 30, 2017 #7
    Then you need an offline physical world teacher to help you.
     
  9. Jan 30, 2017 #8

    haruspex

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    Do you mean vba=-vab?
    Why do that? The first equation applies immediately.
     
  10. Jan 30, 2017 #9
    Just ro help you visualise
     
  11. Feb 1, 2017 #10
    Yes
     
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