# Runge-Kutta with a function that changes at each time step

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L'O.G. am 23 Aug. 2023
Bearbeitet: Sam Chak am 24 Aug. 2023
I previously asked a question about solving an equation dx/dt = u(x,t) where u(x,t) is a velocity. I want to solve for the positions x to obtain the trajectory x(t). Here, I have a starting position x0 and compute the velocity at that initial point, u(x0,t0). I want to use Runge-Kutta to determine the time evolution of the position based on the velocitie. I can calculate an arbitrary instantaneous velocity u(x,t) from the forces acting on a position x. I think I'm close, but need to modify the following to take into account the arguments I'm passing to the velocity function, and then calculate the time evolution of all points / particles (two in this case). How do I do that? I'm not sure how to pass the arguments into the velocity function for one thing.
Here's the simplified current version:
% Input
f = [0.05;0;0]; % Force
N = 2; % Number of points
% Initial positions
x1 = [-0.5,0,0];
x2 = [0.5,0,0];
deltaX = x2-x1;
c = 3; % Constant
x = [x1;x2];
% Define second force
fs = @(deltaX) c*deltaX;
% Initial values
t0 = 0;
tf = 2.5;
sol = ode45(@(t,x) u(x,t),[t0 tf],[x1(1) x2(1)]);
function v = u(x,t)
% Calculate forces on each point
fs_x1 = fs(deltaX);
fs_x2 = -fs_x1;
% Calculate net forces
f_k1 = -f + fs_x1(:);
f_k2 = f + fs_x2(:);
f_k = [f_k1(1);f_k1(2);f_k1(3);f_k2(1);f_k2(2);f_k2(3)];
% Calculate velocities
v = S*f_k; % I have a separate function for the calculation of S, which is a function of the positions
end
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Sam Chak am 23 Aug. 2023
Suggest you show math model of the dynamic system instead of the code so that we can understand the problem clearly.
Also show how the matrix S is calculated.
Sam Chak am 24 Aug. 2023
Bearbeitet: Sam Chak am 24 Aug. 2023
@L'O.G., have you shown the full equations of motion? It looks a 6th-order system.

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### Antworten (1)

Bruno Luong am 23 Aug. 2023
Bearbeitet: Bruno Luong am 23 Aug. 2023
Again everything is written in the doc of ode45 "For information on how to provide additional parameters to the function odefun, see Parameterizing Functions."
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