# How do you find the initial phase angle?

1. Jan 10, 2012

### nukeman

1. The problem statement, all variables and given/known data

A mass is hanging on a vertical spring, and oscillates in SHM with an amplitude of 10.0 cm, and period 0.500 s. The graph shows its motion as a function of time. At t = 0, the mass is found at x = -7.50 cm below the equilibrium position.

Assuming that x(t) = A cos (ωt + φ), find the value of the initial phase angle, φ.

2. Relevant equations

3. The attempt at a solution

How do I get started in finding the initial phase angle, φ?

2. Jan 10, 2012

### Delphi51

Set x(0) = -7.5 cm and solve for the angle. You know the A and the w, so only one unknown.

3. Jan 10, 2012

### nukeman

Im sorry I dont quite understand. It has to be x(t) = A cos (ωt + φ)

So dont I have to figure out A, ω, and t ? then just solve for φ ?

How come you just put x(0) = -7.5cm ? (I understand why you put x(0))

Thanks very much for the help!

4. Jan 10, 2012

### Delphi51

Sorry, I should have written x(0) = A cos(ω*0 + φ) = -7.5 cm.
You WILL have to calculate numbers for ω and A before you can solve for φ.