Problem 4: A string is stretched between the points x = 0 and x = 1 and is initially at rest along the x-axis. Its motion due to gravity is described by the solution u(x,t) of the following problem:
utt = c^2 uxx - g, 0 < x < 1, t > 0
u(0,t) = u(1,t) = 0, t > 0
u(x, 0) = ut(x, 0) = 0, 0 < x < 1
Here, the constant g is the gravitational acceleration.
(i) Find the time-independent solution v(x) of the above equation which satisfies the homogeneous boundary conditions as well.
(ii) Show that w(x,t) = u(x,t) - v(x) satisfies the wave equation wtt = c^2 wxx. Find the corresponding boundary and initial conditions for w and use the solution available to you to express w(x, t) as a Fourier series.
(iii) Use (i) and (ii) to find a formula for u(x, t).