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Homework Help: Proof of w=(k/m)^(1/2)

  1. Feb 17, 2008 #1
    So I am attempting to prove the simple harmonic motion equations with calculus so as to develop a better understanding of shm problems and have more flexibility when it comes to solving them. I am having a bit of trouble understanding the proof of w=(k/m)^(1/2)

    Here is how I am doing it. I am taking the x(t)=A*cos(wt+θ) equation and then the second derivative of it to find acceleration

    a(t)=-A*w^2*cos(wt+θ)
    F=ma=-kx
    a=-kx/m
    -A*w^2*cos(wt+θ)=-kx/m
    so I am guessing that I can make a few assumptions to solve for the final equation. If I assume that the time t=0, thus we x=A, I can solve
    -x*w^2*cos(w(0)+0)=-kx/m
    x's cancel out
    w^2*cos(0)=k/m
    w=(k/m)^(1/2)

    Is this a valid way to prove the equation? Also, I am assuming that the original x(t) equation is correct, I do not know how to prove it. Is there a way to prove the original x(T) equation? Finally, I am preparing for the AP Physics C: Mechanics exam, so if I were to solve a problem using Calculus, for those of you familiar with the exam, would the graders allow me to start off a proof of an answer with x(t)=A*cos(wt+θ) as a given?
     
  2. jcsd
  3. Feb 17, 2008 #2
    Yeah you should be fine, BUT I would recommend recalling that :

    x[t] = A Cos wt + B Sin wt

    theres two parts depending on your initial conditions.
     
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