OK, I think I understand the problem, I'm just a little confused on some pieces in the middle. Problem: Consider a wave packet formed by the superposition of two waves psi1=cos(1.00x-2.00t) <--1.00=k, 2.00=w psi2=cos(1.01x-2.03t) <--1.01=k, 2.03=w where x and t are measured in meters and seconds respectively. What is the Phase velocity? What is the group velocity? We know: Vp= w/k Vg=dw/dk -To find the Vp, can we simply say that Vp=2.00/1.00=2.00m/s? Now, to find the Group Velocity, we can use the suerposition principle and find the net wave by the sum of the two individual waves (psi1 + psi2) which gives us psi=2cos(1.00*1.01)x*cos(-2.00*-2.03)t = 2cos(1.01)x*cos(4.06)t thus Vg=dw/dk= -(2/1.01)sin(1.01)x*cos(4.06)t - (2/4.06)doc(1.01)x*sin(.06)t? Is this correct? Is the partial derivative correct? Thanks.