Ok, so I'm starting to read the next lecture (3, water waves) and I'm a bit confused at the beginning. I'm not sure I understand what the kinematic condition means - it seems to be getting something from nothing, a condition that seems to be just based on defining a surface "eta(x,t) = z". I could follow most of the rest of the reading okay. I was surprised at getting hyperbolic trig functions out of there into the dispersion relation, I'll definitely have to have another think at some point about what that means.