# The Wave Equation

This demonstration illustrates the behaviour of solutions of the wave equation

u_{tt} = c^2 u_{xx}
u(0,t) = 0
u(l,t) = 0
u(x,0) = f(x)
u_t(x,0) = g(x)

In this example c=1, l=10, g(x)=0 and the intial amplitude consists of one bump centered on x=5. The demonstration plots the solution given by separation of variables that you have found in class. When the demonstration starts, the initial amplitude is plotted. By clicking the "Advance time" button, you instruct the computer to increase the time by an amount specified in the "time interval window". Separation of variables expresses the solution as a sum

b_1(t) sin(pi x/l) + b_2(t) sin(2 pi x/l) + ...

of modes. The demonstration also plots the values of the first six coefficients b_k(t). Note that every second coefficient is always zero. Why? Also note that each coefficient varies periodically in time. What is the period of each coefficient?

The demonstration below is an applet. Google Chrome, Firefox and Microsoft Edge no longer execute applets because of security issues with NPAPI plugins. Some browsers that still play applets are Internet Explorer, Safari and the Pale Moon browser [ windows, linux, Mac ].