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Homework Help: Simple Harmonic Motion

  1. Nov 26, 2007 #1
    [SOLVED] Simple Harmonic Motion

    This is the question I was given on my assignment, I can't seem to get it, bad prof so I am trying to teach all of this to myself, but this stuff is a bit confusing. From what I have tried and gather from examples in mt text, sub in .120s and solve but that is not working.

    The position of a mass that is oscillating on a spring is given by (14.0cm)cos[(18.0s^-1)t].
    What is the speed of the mass when t = 0.120 s?

    Also, I don't think 18.0s^-1 is angular speed, I think its frequency but the book has nothing in it about it, got that from google.

    I am looking for a step in the right direction in solving this one, the equation I am using are:

    v = -wAsin(wt + theta)

    The solution I am getting is -13.4 m/s which is not right, any help would be great. Thanks.
  2. jcsd
  3. Nov 26, 2007 #2


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    Can you show how you got the -13.4 m/s? Are you working in radian mode on your calculator?
  4. Nov 26, 2007 #3
    Yeah, I am working in radian mode, this is going under what I was assuming the 18.0s^-1 is frequency so I converted it using:

    w = f2(pi)

    and then used that in the velocity equation, I don't think thats right, but just using the 18 as w doesn't work either.
  5. Nov 26, 2007 #4


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    I think the 18 s^-1 is the angular speed. Can you show what answer you get when you use 18?
  6. Nov 26, 2007 #5
    V = -(18 * .14)sin(18 * .120)
    = -(2.52)sin(2.16)
    = -(2.52)(.83138)
    = -2.095 m/s

    That answer is not right either.
  7. Nov 27, 2007 #6
    Yeah, I got it after, I was forgetting speed is a scaler so leaving out the negative solved the problem, thanks for your help.
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