Lecture 04, 18.300, Thu Feb 10, 2022.
Summary
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 Do #007, SKIP #008--#009, continue with #010--#011.
 Notes:
 1) Went through Flood Waves equation and showed why, generically, the
    flux q = Q(A) is superlinear [dQ/dA grows with A].
 2) Replaced #008 by a derivation of the Klein-Gordon equation for
    a taut string on an elastic bed, using conservation of transversal
    momentum.
    Then showed how conservation of energy follows automatically.
    Note that mass and longitudinal momentum are trivial because there is
    no horizontal displacement in the infinitesimal amplitude regime
    --- this follows because then the longitudinal tension forces,
        T*cos(theta), are constant up to quadratic terms [here theta is
        the angle with the horizontal, so u_x = tan(theta)].
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