function xdot = xdot_bk(t,x,etc)
%
%  find xdot given t,x
%  etc contaings [b k load_type period amplitude]
%  load_type is 1, 2 or 3
%  period is for load_type 2 or 3
%  amplitude of load
%
b = etc(1);
k = etc(2);
load_type = etc(3);
amp = etc(5);
switch load_type
    case 1 %% constant load
        F = amp;
    case 2 %% sine wave
        period = etc(4);
        omega = %% FINISH THIS
        F = %% FINISH THIS
    case 3 %% square wave
        period = etc(4);
        F = sqwave(t,period,amp);
end
xdot = %% FINISH THIS