Asked by Derek Gabaldon
on 22 Sep 2019

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 by KALYAN ACHARJYA
on 22 Sep 2019

Edited by KALYAN ACHARJYA
on 22 Sep 2019

Accepted Answer

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);

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Answer by MS
on 22 Sep 2019

Edited by MS
on 22 Sep 2019

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);

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