1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Science Advisor

    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook