Why won't this plot?

I'm new to Matlab and I'm trying to plot the velocity related to the function for a damped harmonic oscillator. I have been able to plot the function itself, but when I try to plot the velocity of said function, I receive this error:

Error using plot

Vectors must be the same length.

Error in damped_oscillator (line 102)

plot(t,v);

I'm not sure how to solve this problem. I've linked the original code. Any help is much appreciated!

Answer 1

[KALYAN ACHARJYA]

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);

Answer 2

[MS]

The two vectors have different sizes. Since you are using the diff function. From documentation:

Y = diff(X) calculates differences between adjacent elements of X along the first array dimension whose size does not equal 1:

• If X is a vector of length m, then Y = diff(X) returns a vector of length m-1.

Hence, the t vector has a size of 1 x1000 and the v vector has a size of 1x 999.

if you want, you can update t by removing the first element where the speed is not calculated:

t = t(2:1000)
plot(t,v);