1. The problem statement, all variables and given/known data At t = 0 a block with mass M = 5 kg moves with a velocity v = 2 m/s at position xo = -.33 m from the equilibrium position of the spring. The block is attached to a massless spring of spring constant k = 61.2 N/m and slides on a frictionless surface. At what time will the block next pass x = 0, the place where the spring is unstretched? 2. Relevant equations phi=phase angle angular frequency= sqrt(k/m)=w x(t)=Acos(wt+phi) 3. The attempt at a solution After working through the givens, I got my equation for the harmonic motion to be x(t) = 0.66 cos (3.4987t - 2pi/3) Where the angular frequency is sqrt(61.2/5)=3.4987, I got the amplitude from conservation of energy to be .6607 meters and the phase angle is -2pi/3. The next step, if I am not mistaken, is to solve for when the spring is at its equilibrium position, or when x(t1)=0. Here are the steps for what I did... 0=0.6607 cos (3.4987t - 2pi/3) 0=cos(3.4987t - 2pi/3) cos-1(0)=3.4987t - 2pi/3 pi/2= 3.4987t -2pi/3 7pi/6=3.4987t t=1.0476 seconds which, according to the homework website, is not correct! Can anyone see where I made my mistake? Thank you!!