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A frictionless block of mass 2.35 kg is attached to an ideal spring with force constant 310 N/m. At t=0 the spring is neither stretched nor compressed and the block is moving in the negative direction at a speed of 12.6 m/s.

1.Find the amplitude.

I got this without a problem. I used the formula A = square root of [tex]x_i^2[/tex] + [tex]v_i^2[/tex]/[tex]\omega^2[/tex]. [tex]\omega = \sqrt{k/m} = 11.49[/tex] rad/s. Plugging in known values results in 1.10 m, which is correct.

2.Find the phase angle.

Here's where I don't know why my answer is not correct.

I use the equation [tex]v_i = -\omega * A * sin(\omega*t + \phi)[/tex]. I know the initial velocity, I know the angular frequency, and I know the amplitude. I'm solving for the angle in radians.

[tex]-12.6 = -(11.49)(1.10)(sin(\phi)[/tex] This is at time t, so [tex]\omega * t[/tex] = 0.

I get [tex]\phi = 85.5 ^\circ[/tex]. Converting it into radians, it's 1.49 (rad).

This isn't right, though.

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# Homework Help: Finding the phase angle in simple harmonic motion

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