1. The problem statement, all variables and given/known data ...when she pulls the ball down 2.5cm from equilibrium and releases it from rest, it oscillates at 5.5 Hz. What is displacement y as functions of t? 2. Relevant equations Y= Acos(omega t+phi) 3. The attempt at a solution I'm almost certain I should instead be using sin to characterize the displacement or add in a phase angle of pi to the cosine function since its released from bottom. Is this a correct assumption?