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**1. Homework Statement**

A 0.450 kg object attached to a spring with a force constant of 8.00 N/m vibrates in simple harmonic motion with an amplitude of 11.0 cm.

Calculate the maximum value (magnitude) of its speed and acceleration.[/B]

**3. The Attempt at a Solution**

I found both w and t.

[tex] \omega = 4.216[/tex]

[tex] t = 0.145[/tex]

To get t I solved this equation for t.

[tex] x(t) = Acos(\omega t + \phi)[/tex]

[tex]t = \frac{1}{4.216}cos^-1(\frac{x}{11})[/tex]

With that value of t, I solved for v. I got a negative number which is incorrect. However, if at the start I use sine instead of cosine for position i get a different value for t, and get the same answer for v, except positive, which is correct. But why would I use sine for position rather then cosine, and how would I know? Also why would it be positive or negative?