How can I use lsim for a constant value input signal

I'm needing to plot displacement response with 3 different forcing functions and I'm having trouble getting the forcing function f(t) = 6 to work. I'm also having problems with my time vector as you'll see my code has it commented out(that's the only way I can get it to work for my other two functions), but that's another issue.

%%%%Assign variable values
m1=10; %kg
m2=4; %kg
k1=10; %N/m
k2=15; %N/m
k3=4; %N/m
c=2.4; %N-s/m
%
%%%%Assign transforms for X1(s) & X2(s) that were computed by hand
x1s = (m2*s^2+c*s+k3+k2)/(m1*m2*s^4+m1*c*s^3+(m1*k3+m1*k2+m2*k1+m2*k2)*s^2+(k1*c+k2*c)*s+k1*k3+k1*k2+k2*k3);
x2s = k2/(m1*m2*s^4+m1*c*s^3+(m1*k3+m1*k2+m2*k1+m2*k2)*s^2+(k1*c+k2*c)*s+k1*k3+k1*k2+k2*k3);
% Set transfer function LTI objects by brute force
sys1 = tf([0 0  4 2.4 19],[40 24 290 60 250]); 
%free resonse x1 = (4*s^2 + (2.4*s) + 19)/(40*s^4 + 24*s^3 + 290*s^2 + 60*s + 250)
sys2 = tf([0 0 0 0 15],[40 24 290 60 250]);
%free resonse x2 = 15/(40*s^4 + 24*s^3 + 290*s^2 + 60*s + 250)
%%%%%Assign forcing functions
f1=(-2*t); 
f2=(6);
f3=cos(3*t);
%%%%%%%%%
% PLOT
%%%%%%%%%
figure;
%%%%%Plot X1(s)/F1(s) & X2(s)/F1(s)
subplot(3,1,1);
%t=0:0.01:10;
z=lsim(sys1,f1,t);
plot(t,z);
title('X1(s)/F(s)& X2(s)/F(s) with f(t)=(-2t)');
xlabel('time (s)');
ylabel('Amplitude');
hold on;
% t=0:0.001:0.1;
z=lsim(sys2,f1,t);
plot(t,z);
hold off;
%
%%%%%Plot X(s)/F2(s) & X2(s)/F2(s)
subplot(3,1,2);
%t=0:0.01:10;
z=lsim(sys1,f2,t);
plot(t,z);
title('X1(s)/F(s)& X2(s)/F(s) with f(t)=(6)');
xlabel('time (s)');
ylabel('Amplitude');
hold on;
% t=0:0.001:0.1;
z=lsim(sys2,f2,t);
plot(t,z);
hold off;
%
%%%%%Plot X1(s)/F3(s) & X2(s)/F3(s)
subplot(3,1,3);
%t=0:0.01:10;
z=lsim(sys1,f3,t);
plot(t,z);
title('X1(s)/F(s)& X2(s)/F(s) with f(t)=cos(3t)');
xlabel('time (s)');
ylabel('Amplitude');
hold on;
% t=0:0.001:0.1;
z=lsim(sys2,f3,t);
plot(t,z);
hold off;

Answer 1

[Azzi Abdelmalek]

you said your signal is a constant 6 for example, then for each instant t, the value of your signal is equal to 6. If t=[0 1 2 3], the value of your constant signal should be [6 6 6 6] and not just 6

f2=6
z=lsim(sys2,f2*ones(size(t)),t);

[forkram]

Life saver! Many thanks!