# Homework Help: Simple Harmonic Motion for a block-spring system

1. Jun 23, 2012

### obxer22

1. The problem statement, all variables and given/known data

If the phase angle for a block-spring system in SHM is π/5 rad and the block's position is given x = xm cos(ωt + phi), what is the ratio of the kinetic energy to the potential energy at time t = 0?

2. Relevant equations

PE = (1/2)*k*[(max amplitude)^2]*cos^2(ωt + phi)
KE = (1/2)*k*[(max amplitude)^2]*sin^2(ωt + phi)

Solve for KE/PE in that form.

3. The attempt at a solution

I took KE and divided by PE, giving sin^2(phi)/cos^2(phi). Plugging in pi/5 for phi gives us 1.20253316E-4/.99987974669. However, the system is saying this is wrong . I do believe that I solved for KE/PE correctly. Any help would be much appreciated. Thanks very much!

2. Jun 24, 2012

### ehild

Have you set your calculator to RAD? pi/5 is in radians.

ehild

3. Jun 24, 2012

### obxer22

I knew I was doing it right! my calculator was in degrees....i hate it when that happens...thanks for helping me figure that out!