Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Uniform Circular Motion

  1. Feb 19, 2009 #1
    1. The problem statement, all variables and given/known data
    A 0.150-kg ball on the end of a 1.10-m-long cord (negligible mass) is swung in a vertical circle.



    2. Relevant equations
    Total net force = mass*centripetal acceleration = mass*(velocity squared)/radius


    3. The attempt at a solution

    The question I have is that... I'm looking at the book Physics Volume I, 6th Edition...And for the radius, shouldn't it be 1.10m/2 to get the radius? On the book, they just have 1.10m for r...So I'm confused here since I thought 1.10 is the diameter of the cord, not the radius. Could someone clarify that?
     
  2. jcsd
  3. Feb 19, 2009 #2

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The radius is the distance from the centre of a circle to any point on the circumference. Now, when the ball is being swung in a vertical circle it is *always* on the circumference of the circular path, agreed? So, what is the distance between the ball and the centre of the circle?
     
  4. Feb 19, 2009 #3
    Shouldn't it still be half the length of cord still since the center of the circle would be at the middle, so r should be half of the length....no?
     
  5. Feb 19, 2009 #4

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If you were swinging a ball around on a piece of string, at which point would you hold the string?
     
  6. Feb 19, 2009 #5
    AHA, that makes a lot more sense. So r would be the same even if the ball was in a horizontal circular motion, right?
     
  7. Feb 19, 2009 #6

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Correct. If the radius changed during the motion, then the ball would not be moving in a circle.
     
  8. Feb 19, 2009 #7
    Alright! You're awesome. Thank you for your help! =]
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook