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Why is dr/dt= -V

  1. Jun 25, 2014 #1
    In the solution , it says we have dr/dt= -V (polar coordinates)
    How? i can't see how this can be possible , we know that r(t)=V/w(t), and that's it .
    Last edited: Jun 25, 2014
  2. jcsd
  3. Jun 25, 2014 #2


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    r gets smaller as the vertical string gets longer.

  4. Jun 25, 2014 #3


    Staff: Mentor

    You have to be careful when using equations in physics. You cannot just blindly plug in variables, you need to know what each variable means.

    In this problem V is NOT the tangential velocity so V is not equal to rw as it is in many circular motion problems. Here V is the velocity of the downward pull on the string.
  5. Jun 25, 2014 #4
    thank you , the differential equations gives me w(t)=2 if i use V=rw , i just presumed that since it's alays used in these kind of problems
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