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Homework Help: Circular Motion- Find the Velocity with tension, the radius, and the mass

  1. Oct 12, 2008 #1

    1. The problem statement, all variables and given/known data

    A 530 g ball swings in a vertical circle at the end of a 2.9 m-long string. When the ball is at the bottom of the circle, the tension in the string is 21 N. What is the speed of the ball at that point?

    2. Relevant equations

    2nd law statements

    a = V^2/r

    V= (circu.)/(period)

    3. The attempt at a solution

    I began by drawing a force diagram as if the ball were at the bottom of the cycle. Therefor I would have Tension facing up and weight down. (The tension vector would be larger than the weight vector). There would be no normal because there is no force to act upon the ball in the normal direction. Then I concluded that the second law statements were as follows:

    May= T-mg
    M(V^2/r)= T- mg (substitute in for a)
    Then I just plugged in the numbers:

    530(V^2/2.9)= 21- (530*9.8)
    V= 5.32

    However, this is not the answer. Where did I go wrong?

    Thank you in advance!

    Last edited: Oct 12, 2008
  2. jcsd
  3. Oct 12, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    All quantities should be in S.I. units ... :wink:
  4. Oct 12, 2008 #3


    Gosh darnit!!!! Now's the time where I just feel like crawling into a dark hole and dying.... thanks for the help! :smile: (And to think I've been changin to correct S.I. units all day....)

    So that means that every 530 in my equation above changes into .53 giving me a grand total of.......

    V= 9.3 m/s

    Thank you so very much!

    Have a great day!

    (By the way: LOVE the avatar!)
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