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Homework Help: Centripetal Acceleration

  1. Jul 3, 2009 #1
    1. The problem statement, all variables and given/known data
    A ball swings in a vertical circle at the end of a rope 1.50 m
    long. When the ball is 36.9° past the lowest point on its
    way up, its total acceleration is (-22.5i, 20.2j) m/s2. At
    that instant, (a) determine the magnitude
    of its radial acceleration, and (b) determine the speed and
    velocity of the ball.

    2. Relevant equations
    a = v2/r

    3. The attempt at a solution
    Centripetal acceleration resolved into two components, in x direction, -22.5 and y direction, 20.2. I would think that in order to get the net acceleration, just (22.5^2 + 20.2^2)^0.5. However, this yielded an incorrect solution. Why doesn't this work and what is the correct approach?
  2. jcsd
  3. Jul 3, 2009 #2


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    Homework Helper

    I think they give you the acceleration vector in x,y, but you also have a component from gravity in the vertical direction to take into account, don't you?

    So you have 2 ways to figure it.

    1) observe that tanθ = i/j and use the i component to resolve the unknown j or
    2) add 9.8 to the j
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