You can make the privious three plot in single description using for loop, define "a" in array. #Recomended. The error arises because of non equal length of t and v, as v result from diff. Hence I have ignored the last element of t to make them both equal. You can opt out the first one also, as per your requirements.
m=20; %Mass
k=30; %Spring constant
c=0.9; %Damping constant
A=1;
a=0;
g=c/(2*m); %Damping coefficient
wn=sqrt(k/m); %Natural frequency
wd=sqrt(((wn)^2)-(g)^2); %Damped natural frequency
t=linspace(0,40,1000); %Time value
x=A*exp(-g*t).*cos(wd*t-a); %x(t)
subplot(4,3,1),plot(t,x)
title('Question 1')
xlabel('Time')
ylabel('Displacement (m)');
grid on;
c=0.9; %Damping constant
A=1;
a=0;
g= c/(2*m); %Damping coefficient
wn=sqrt(k/m); %Natural frequency
wd=sqrt(((wn)^2)-(g)^2); %Damped natural frequency
t=linspace(0,20,1000); %Time value
x=A*exp(-g*t).*cos(wd*t-a); %x(t)
subplot(4,2,1),plot(t,x)
title('Question 2')
xlabel('Time (s)')
ylabel('Displacement (m)')
grid on;
c=0; %Damping constant
A=1;
a=0;
g= c/(2*m); %Damping coefficient
wn=sqrt(k/m); %Natural frequency
wd=sqrt(((wn)^2)-(g)^2); %Damped natural frequency
t=linspace(0,40,1000); %Time value
x=A*exp(-g*t).*cos(wd*t-a); %x(t)
subplot(4,2,2),plot(t,x)
title('Question 3')
xlabel('Time (s)')
ylabel('Displacement (m)')
grid on;
c=1; %Damping constant
A=1;
a=0;
g= c/(2*m); %Damping coefficient
wn=sqrt(k/m); %Natural frequency
wd=sqrt(((wn)^2)-(g)^2); %Damped natural frequency
t=linspace(0,40,1000); %Time value
x=A*exp(-g*t).*cos(wd*t-a); %x(t)
subplot(4,2,3),plot(t,x)
c=5; %Damping constant
A=1;
a=0;
g= c/(2*m); %Damping coefficient
wn=sqrt(k/m); %Natural frequency
wd=sqrt(((wn)^2)-(g)^2); %Damped natural frequency
t=linspace(0,40,1000); %Time value
x=A*exp(-g*t).*cos(wd*t-a); %x(t)
subplot(4,2,4),plot(t,x)
c=15; %Damping constant
A=1;
a=0;
g= c/(2*m); %Damping coefficient
wn=sqrt(k/m); %Natural frequency
wd=sqrt(((wn)^2)-(g)^2); %Damped natural frequency
t=linspace(0,40,1000); %Time value
x=A*exp(-g*t).*cos(wd*t-a); %x(t)
subplot(4,2,5),plot(t,x)
c=50; %Damping constant
A=1;
a=0;
g= c/(2*m); %Damping coefficient
wn=sqrt(k/m); %Natural frequency
wd=sqrt(((wn)^2)-(g)^2); %Damped natural frequency
t=linspace(0,40,1000); %Time value
x=A*exp(-g*t).*cos(wd*t-a); %x(t)
subplot(4,2,6),plot(t,x)
legend('0','1','5','15','50')
t=linspace(0,40,1000); %Time value
x=A*exp(-g.*t).*cos(wd.*t-a); %x(t)
v=diff(x)./diff(t); %Velocity
subplot(4,2,7),plot(t(1:end-1),v);
