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Homework Help: Mass attached to a spring rotating in a circle

  1. Oct 3, 2011 #1
    1. The problem statement, all variables and given/known data
    A mass of 2kg rotates at 1m/s in a horizontal circle on a table at the end of a spring with an elastic constant of 50N/m. If the original length of the spring is 2m, find the extension of the spring.

    Given - M=2kg, V=1m/s, k=50N/m, original length of x=2m
    find x'

    2. Relevant equations
    Ek + Ee = Ek' + Ee'

    Fc = mv² / r

    3. The attempt at a solution
    Im starting the question by finding the extension without taking in account for the attraction to the centre by the centripetal force. I know I will need to do this later.
    This is where im stuck

    Ek + Ee = Ek' + Ee'
    Since there is no energy when the thing is at rest I use
    0 = Ek' + Ee'
    0 = .5mv'² + .5kx²
    -.5mv'² = .5kx²
    SQRT [ (-.5mv'²) / (.5k) ] = x

    Problem is you cant sqrt a negative number.. Im stumped. I dont know where else to start
    please help guys
  2. jcsd
  3. Oct 3, 2011 #2


    User Avatar
    Gold Member

    I am not sure energy balance equation works here as you found out.

    Try finding the radial acceleration, the radial force, and then the spring extension.
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