Finding the angle of a pendulum in SHM

[t246335]

Homework Statement

A clock pendulum oscillates at a frequency of 1.5 Hz. At t = 0, it is released from rest starting at an angle of 20° to the vertical. Ignoring friction, what will be the position (angle) of the pendulum at the following times? (Hint: Do not confuse the angle of swing θ of the pendulum with the angle that appears as the argument of the cosine.)
Find the angle at t = 0.25 s

Homework Equations

T=1/f x=Acos(2pit/T) feta=2pi(t/T)

The Attempt at a Solution

T=1/1.5 T=.67sec feta=2pi(.25sec/.67)

 

[Delphi51]

Pay attention to that hint! In this question, the amplitude is 20 degrees and equation of motion gives the angle at other times:
θ = 20*cos(2πt/T)