1. The problem statement, all variables and given/known data I seem to have problems finding time in SHM. To find the time after projection, I know that either x=asinωt or x=acosωt needs to be used, so since R is projected away from O, it means that it is moving towards the equilibrium position, therefore I used x=acosωt to find the time. 2. Relevant equations x=asinωt x=acosωt 3. The attempt at a solution x=acosωt -L/4 = 3L/4 cos (√2g/L)t t= (√L/2g) cos-1(-1/3) But, the correct answer should be t=(√L/2g) sin-1(1/3) Why can't cosine be used? Since it is moving towards the equilibrium position? Also, for the equations x=asin(ωt+α) or x=acos(ωt+α) I am not sure when is α needed to take into consideration. So far, I never encountered any questions needing to find α, but I would like to know when is α needed. Thanks very much in advance!