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## Homework Statement

Here is a position-time graph for a ball on a spring in SHM.

What is the phase of the particle at point a on the graph?

## Homework Equations

phase = ωt + ɸ

_{0}

## The Attempt at a Solution

First I found the initial phase, ɸ

_{0}. 0.5A / A = 0.5 = cos(ɸ

_{0}). Then ɸ

_{0}= -π/3. It's negative because the particle is moving to the right.

Now for point a, I figured this is at 0.75T, where T is the period.This is kind of hard to see so it could be wrong. But then:

phase = (2π/T)(0.75T) - (π/3) = (7/6)π or 210 degrees. However, the answer is -120 degrees.

I also tried with points b and c but they were wrong too. Can anyone help?

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