function vtb3_1(m,c,k,Fo,tf,flag)

%VTB3_1 Impulse response of a SDOF system.
% VTB3_1(m,c,k,Fo,tf,1) plots the response of the system to an
% impulse of magnitude Fo.  The input variables are the mass m,
% the stiffness k, and the damping c.  The total time of the 
% response is tf.
% VTB3_1(m,zeta,w,Fo,tf,2) plots the response of the system 
% described by the mass m, the damping ratio zeta, and the 
% undamped natural frequency w. 
   
clc
if flag == 1
  num=Fo;
  den=[m c k];
else
  zeta=c;
  w=k;
  num=Fo/m;
  den=[1 2*zeta*w w^2];
end

t=0:tf/200:tf;

x=step([num 0],den,t);

plot(t,x)
title('Impulse Response of the SDOF damped oscillator')
ylabel('Displacement x(t)')
xlabel('time  (sec)')