# Interpretation of Lagrangian solution (complex numbers)

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• Mishal0488
In summary, The conversation discusses the development and solution of a Lagrangian system, with the speaker sharing their struggles and belief that they have found a correct solution. They provide a PDF with the derivation of the system and mention that they have ignored certain functions and components. They also mention using a holonomic constraint and substituting a result into derived differential equations. The speaker then shares a Matlab code and shows plots for their results, mentioning that they are not completely convinced of the accuracy. They ask for opinions and advice on how to further improve their solutions.
Mishal0488
Hi Guys

Finally after a great struggle I have managed to develop and solve my Lagrangian system. I have checked it numerous times over and over and I believe that everything is correct.

I have attached a PDF which has the derivation of the system. Please ignore all the forcing functions and damping for the system.

Essentially I have taken the holonomic constraint and represented (r+b) which I have taken as "B" as the subject of the formula, there after I am substituting the result into the derived differential equations.

The Matlab code is presented below, the "Statespace6" function is the representation of the differential equations while the main code is used to execute the code. There a number of plots developed (Please don't laugh at some of my spelling mistakes, I am going to fix these up later). The results do not make sense to me for the theta2 and "b" component of the system. In some instances there are imaginary numbers which are developed, which I am not sure how to interpret everything. Filtering in the results in most cases presents almost realistic results, however I am not completely convinced that the system is correct.

Further to this, the results which are understandable/realistic are developed when the same initial positions are imposed for "a" and "b" with regards to the radial displacement.

It would be nice to get an opinion of what I have done, and gather some comments, input and advice so I know what to look into next with regards to fixing my solutions.

 function xp = Statespace6(t,x,r,L,k,m1,m2) theta1 = x(1); theta1_dot = x(2); theta2 = x(3); theta2_dot = x(4); a = x(5); a_dot = x(6); A = (a+r); %B R1 = 2*A*cos(theta1)*cos(theta2); R3 = 2*L*sin(theta2)+2*A*sin(theta1)*sin(theta2); one = -A^2+L^2; two = A^2*(cos(2*theta1)*cos(2*theta2)+sin(2*theta1)*sin(2*theta2)); three = -L^2*cos(2*theta2); four = -2*A*L*sin(theta1); five = -2*A*L*(sin(theta1)*cos(2*theta2)-cos(theta1)*sin(2*theta2)); R2 = sqrt(2)*sqrt(one+two+three+four+five); B = 0.5*(R1+R2+R3); %B = 0.5*(R1-R2+R3) %b_dot R1_dot = 2*(a_dot*cos(theta1)*cos(theta2) - A*theta1_dot*sin(theta1)*cos(theta2) - A*theta2_dot*cos(theta1)*sin(theta2)); R3_dot = 2*L*theta2_dot*cos(theta2) + 2*(a_dot*sin(theta1)*sin(theta2) + A*theta1_dot*cos(theta1)*sin(theta2) + A*sin(theta1)*theta2_dot*cos(theta2)); one_dot = -2*a_dot*A; two_dot = 2*a_dot*A*cos(2*theta1)*cos(2*theta2) + -A^2*2*theta1_dot*sin(2*theta1)*cos(2*theta2) + -A^2*cos(2*theta1)*2*theta2_dot*sin(2*theta2) + 2*a_dot*A*sin(2*theta1)*sin(2*theta2) + A^2*2*theta1_dot*cos(2*theta1)*sin(2*theta2) + A^2*sin(2*theta1)*2*theta2_dot*cos(2*theta2); three_dot = L^2*2*theta2_dot*sin(2*theta2); four_dot = -2*a_dot*L*sin(theta1)-2*A*L*theta1_dot*cos(theta1); five_dot = -2*a_dot*L*sin(theta1)*cos(2*theta2) - 2*A*L*theta1_dot*cos(theta1)*cos(2*theta2) - 2*A*L*sin(theta1)*2*theta1_dot*sin(2*theta2) + 2*a_dot*L*cos(theta1)*sin(2*theta2) - 2*A*L*theta1_dot*sin(theta1)*sin(2*theta2) + 2*A*L*cos(theta1)*2*theta2_dot*cos(2*theta2); R2_dot = sqrt(2)*(one_dot+two_dot+three_dot+four_dot+five_dot)/(2*sqrt(B)); b_dot = (R1_dot+R2_dot+R3_dot)/(4*sqrt(R1+R2+R3)); %differential equations %theta1 theta1_dotdot = (-2*m1*theta1_dot*a_dot*A-9.81*m1*sin(theta1)*A)/(m1*A^2); %theta2 theta2_dotdot = (-2*m2*theta2_dot*b_dot*B-9.81*m2*sin(theta2)*B)/(m2*B^2); %a a_dotdot = (m1*theta1_dot^2*A-k*a+9.81*m1*cos(theta1))/m1; %b b_dotdot = (m2*theta2_dot^2*B-k*(B-r)+9.81*m2*cos(theta2))/m2; xp = [x(2); theta1_dotdot; x(4); theta2_dotdot; x(6); a_dotdot; x(8);%b_dot; b_dotdot;]; end  clc; clear; r = 1; L = 2; k = 353160; m1 = 3500/2; m2 = 3500/2; inc = 0.0001; SolverOptions = odeset('RelTol',1e-5,'AbsTol',1e-5); [t,y] = ode45(@Statespace6,[0:inc:2],[0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1],SolverOptions,r,L,k,m1,m2); theta1 = y(:,1); theta1_dot = y(:,2); theta2 = y(:,3); theta2_dot = y(:,4); a = y(:,5); a_dot = y(:,6); %motion plots xx = (y(:,5)+r).*sin(y(:,1)); yy = (y(:,5)+r).*cos(y(:,1)); figure(1) hold on plot(xx,yy) title('Motion plot for mass1') hold off xx1 = (y(:,7)+r).*sin(y(:,3)); yy1 = (y(:,7)+r).*cos(y(:,3)); figure(2) hold on title('Motion plot for mass2 using unfiltered data') plot(xx1,yy1) hold off figure(4) hold on L1 = plot(t,y(:,8)) L2 = plot(t,a_dot) title('radial velocities using unfiltered data') legend([L1 L2],'b dot','a dot'); hold off figure(5) hold on L1 = plot(t,theta1) L2 = plot(t,theta2) title('Rotational displacement using unfiltered data') legend([L1 L2],'theta1','theta2'); hold off figure(6) hold on L1 = plot(t,y(:,5)) L2 = plot(t,y(:,7)) title('radial displacement using unfiltered data') legend([L1 L2],'a','b') hold off figure(7) hold on L1 = plot(t,y(:,2)) L2 = plot(t,y(:,4)) title('Angular velocity using unfiltered data') legend([L1 L2],'theta1 dot','theta2 dot') hold off %filter data Fs = 1/inc; %sampling rate Fc = 1/(Fs/2); %cut off frequency [b1 a1] = butter(4,Fc,'high'); b_new = filtfilt(b1,a1,y(:,7)); figure(8) hold on L1 = plot(t,b_new) L2 = plot(t,y(:,5)) legend([L1 L2],'b','a'); title('radial displacement filtereddata') hold off Fc = 1/(Fs/4); %cut off frequency [b1 a1] = butter(4,Fc,'high'); b_dotnew = filtfilt(b1,a1,y(:,8)); figure(9) hold on L1 = plot(t,b_dotnew) L2 = plot(t,y(:,6)) legend([L1 L2],'b dot','a dot'); title('radial velocity filtered data') hold off xx1 = (b_new+r).*sin(y(:,3)); yy1 = (b_new+r).*cos(y(:,3)); figure(10) hold on title('Motion plot for mass2 using filtered data') plot(xx1,yy1) hold off %holonomic constraint i = size(t); i = i(1,1); for j = 1:i H(j) = ((r+y(j,5))*sin(y(j,1))+L-(r+b_new(j))*sin(y(j,3)))^2+((r+y(j,5))*cos(y(j,1))-(r+b_new(j))*cos(y(j,3)))^2-L^2; end figure(11) hold on plot(t,H); title('Holonomic constraint using filtered data') hold off for j = 1:i H(j) = ((r+y(j,5))*sin(y(j,1))+L-(r+y(j,7))*sin(y(j,3)))^2+((r+y(j,5))*cos(y(j,1))-(r+y(j,7))*cos(y(j,3)))^2-L^2; end figure(12) hold on plot(t,H); title('Holonomic constraint using unfiltered data') hold off 

#### Attachments

• img20211008_13472537.pdf
2.5 MB · Views: 103
Re: post #5 in the thread https://www.physicsforums.com/threa...ndulum-connected-through-a-rigid-bar.1007459/
For comparison please find attached Lagrangean of three coordinates x,y,##\theta##.

Last edited:
Mishal0488
Thanks, I will digest proposal tomorrow.
I need to also apply a dynamic load which is always parallel to the springs and horizontal. The application of such may become a bit difficult with the proposed coordinate system.

## 1. What is a Lagrangian solution?

A Lagrangian solution is a mathematical method used to solve problems in classical mechanics. It involves using the principle of least action, which states that the path taken by a system between two points is the one that minimizes the action, or the integral of the Lagrangian function.

## 2. What are complex numbers?

Complex numbers are numbers that contain both a real and an imaginary part. They are written in the form a + bi, where a is the real part, b is the imaginary part, and i is the imaginary unit (√-1). Complex numbers are commonly used in mathematics and physics to represent quantities that have both magnitude and direction.

## 3. How are complex numbers used in the interpretation of Lagrangian solutions?

In the interpretation of Lagrangian solutions, complex numbers are used to represent the position and velocity of a system in a two-dimensional space. The real part of the complex number represents the position, while the imaginary part represents the velocity. This allows for a more efficient and elegant way of solving problems in classical mechanics.

## 4. What is the significance of the imaginary part in the interpretation of Lagrangian solutions?

The imaginary part in the interpretation of Lagrangian solutions represents the velocity of a system in a two-dimensional space. It is an essential component in the calculation of the action, which is used to determine the path taken by the system. Without the imaginary part, the Lagrangian solution would not be able to accurately describe the motion of a system.

## 5. Are there any limitations to using complex numbers in the interpretation of Lagrangian solutions?

While complex numbers are a powerful tool in the interpretation of Lagrangian solutions, they do have some limitations. One limitation is that they cannot be used to represent physical quantities, such as mass or energy. Additionally, the use of complex numbers can make calculations more complicated and may not always be necessary for simple problems in classical mechanics.

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