How do you find the initial phase angle?

[nukeman]

Homework Statement

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.

Homework Equations

The Attempt at a Solution

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

 

[Delphi51]

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

 

[nukeman]

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

Im sorry I don't quite understand. It has to be x(t) = A cos (ωt + φ)

So don't 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!

 

[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.