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How do you find the initial phase angle?

  1. Jan 10, 2012 #1
    1. The problem statement, all variables and given/known data

    A mass is hanging on a vertical spring, and oscillates in SHM with an amplitude of 10.0 cm, and period 0.500 s. The graph shows its motion as a function of time. At t = 0, the mass is found at x = -7.50 cm below the equilibrium position.

    Assuming that x(t) = A cos (ωt + φ), find the value of the initial phase angle, φ.

    2. Relevant equations

    3. The attempt at a solution

    How do I get started in finding the initial phase angle, φ?
  2. jcsd
  3. Jan 10, 2012 #2


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    Homework Helper

    Set x(0) = -7.5 cm and solve for the angle. You know the A and the w, so only one unknown.
  4. Jan 10, 2012 #3
    Im sorry I dont quite understand. It has to be x(t) = A cos (ωt + φ)

    So dont I have to figure out A, ω, and t ? then just solve for φ ?

    How come you just put x(0) = -7.5cm ? (I understand why you put x(0))

    Thanks very much for the help!
  5. Jan 10, 2012 #4


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    Homework Helper

    Sorry, I should have written x(0) = A cos(ω*0 + φ) = -7.5 cm.
    You WILL have to calculate numbers for ω and A before you can solve for φ.
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