Solution
= Solution
A <wave crest> moves at the phase velocity, while the envelope of a narrow <wave packet> moves at the group velocity. Since $c_g>c_p$ for every nonzero wavenumber, the crests move more slowly than the packet.
= Solution
A <wave crest> moves at the phase velocity, while the envelope of a narrow <wave packet> moves at the group velocity. Since $c_g>c_p$ for every nonzero wavenumber, the crests move more slowly than the packet.