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Homework Help: Simple Harmonics - Kind of wierd

  1. Dec 5, 2007 #1
    [SOLVED] Simple Harmonics - Kind of wierd

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

    A 1.30 kg mass on a spring has displacement as a function of time given by the equation:

    [tex]x\left( t \right) = \left( {7.40\;{\rm{ cm}}} \right)\,\cos\:\left[ {\left( {4.16\;{\rm{ s}}^{ - 1} } \right)t - 2.42} \right].[/tex]

    Find the position of the mass at t = 1.00s.

    2. Relevant equations


    3. The attempt at a solution

    Ok so since this is a position equation, i figured that i would just plugin 1 for t and get the answer. But that seems to be wrong and i cant figure out why. (i did change 7.4 cm to m).

    I figured out other parts for this equation to be:
    T = 1.51s
    k = 22.5 N/m
    Vmax = .308 m/s
    Fmax = 1.67 N

    Any help would be great!

  2. jcsd
  3. Dec 5, 2007 #2

    Well, if you have a position equation that is only a function of time, then yes, you should be able to plug in the time and get the position.


    Is this an equation that was given to you, or is this an equation you derived from the problem? You may have made a mistake in an earlier step. What exactly did the original problem say?
  4. Dec 5, 2007 #3
    Its an equation given to me.

    Problem said exactly what i posted. Everything was given to me and everything is correct.

    When i solve this: i get .073965879

    Can anyone confirm this? I tried that and it was wrong. (my calculator is in degrees mode - i think thats right)
  5. Dec 5, 2007 #4

    Well, I get the same number in degrees mode, but you probably want to be working in radians, as this is the unit typically used for these types of things. Try radians.

    What answer are you looking to get?
  6. Dec 5, 2007 #5
    O ok it works in radians.

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