# Finding amplitude and phase angle from a graph?

1. Mar 4, 2009

### spj1

I have a question asking for the period, frequency, amplitude and phase angle from simply looking at a graph. I have the period and frequency, and 'think' i have the amplitude (high point - low point / 2), but I don't know how to get the phase angle. No equation is given so I can't simply consider the coefficients in front of sin and cos.

Can anyone point me in the right direction?

2. Mar 4, 2009

Can you post a picture of the graph? If you're comparing two signals, you can compare their peaks (or where they cross the x-axis), and use the difference to compute the angle.

I'm including a picture. The purple distances are both equivalent, and demonstrate the phase difference between two signals (sine waves).

If there's only one signal/line in your graph, they're likely referring to where your line crosses the x-axis, going in the positive y direction, compared to the origin of the graph, as demonstrated by the darker purple distance marking.

Digging around on the net I came across this, which might help you: http://www.intmath.com/Trigonometric-graphs/3_Graphs-sin-cos-phase-shift.php Make sure to read the comments about half way down the page regarding phase shift and phase angle.

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Last edited: Mar 4, 2009