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Homework Help: Quick question about a simple harmonic motion problem

  1. Jan 17, 2010 #1
    1. The problem statement, all variables and given/known data

    The position of a particle is given by x = 4.6 cos (pi)t, where x is in meters and t is in seconds.

    (a) Find the maximum speed and maximum acceleration of the particle.

    I've already solved these by finding the first and second derivatives of the above x.

    v = -4.6(pi)sin(pi*t)
    a = -4.6(pi^2)cos(pi*t)

    (b) Find the speed and acceleration of the particle when x = 2.3 m.

    I've figured out the speed by using the position equation to find the time where x = 2.3m. This is at 19.098593s. I solved that, and took the absolute value to find its speed.

    Now, I'm stuck on acceleration. I only have one more try left on my homework and I need to make sure it's correct. I solved for acceleration the same way and got -22.7 m/s^2. I took the absolute value of this and plugged it in, and it's incorrect. Was I supposed to take the absolute value? Is -22.7 m/s^2 actually the correct answer?
  2. jcsd
  3. Jan 17, 2010 #2


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    Homework Helper

    Yes, because acceleration is a vector quantity and can be negative.
  4. Jan 17, 2010 #3
    Great, it was correct. Thank you!
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