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# Query regarding solving differential equation

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Bharath Ram am 24 Mai 2018
Geschlossen: MATLAB Answer Bot am 20 Aug. 2021
I'm trying to solve equation of a mass damper system shown in the image. The x (LHS) is the unknown displacement which is to be solved from the equation.I know y and dy/dx values at each instant of time (obtained from experiments). While solving using ode45 I'm not able to update the value of y and dy/dx for each time interval. How else can I solve the equation to get value of x. I used central difference scheme but the answer is diverging from the expected values drastically, when marched in time. Thank you in advance.

### Antworten (2)

sloppydisk am 24 Mai 2018
I'm guessing you are trying to use ode45 with variable timestep, make sure to pass y and dy/dx as an interpolation function such that it is defined for any point in time. For example like this:
yInterp = @(thisTime) interp1(t, y, thisTime);
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Torsten am 24 Mai 2018
Bearbeitet: Torsten am 24 Mai 2018
The example
ODE with Time-Dependent Terms
under
https://de.mathworks.com/help/matlab/ref/ode45.html
shows how to solve your problem using ODE45.
Your y and dy/dt should take the role of f and g in the example.
Best wishes
Torsten.

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