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2D mass spring damper square lattice

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Equations of motion for a 2D mass spring damper system and code for modelling on matlab needed.
  3 Comments
William Rose
William Rose on 24 May 2021
The equation of motion for a M-S-D system may be written as
where is a position vector, is frictional drag, is spring force, and is the external force (if any). may have 1 or 2 or 3 dimension (or more, I suppose). We assume that the spring pulls the object directly toward the origin, with a force proportional to the distance from the origin, and that frictional drag is the direction oppostite to the velocity, and proportional to the magnitude of velocity. These assumptions can be written as
and
Therefore we get the equation
In the 2-D case, we have, in the x-direction,
which looks totally normal. It involves the x-components of the spring force and the drag force. But here's where it gets tricky and then simple again. What are the equations for and ? It turns out the equations are
and
So we end up with
You get a similar result for the y-component:
So we see that for this special case - a linear mass-spring-dashpot system - the x- and y-parts do not affect each other, i.e. the equations of motion are totally separable. These two equations are what you need to solve. Since they are not coupled, you can solve them separately, in either order.

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Accepted Answer

William Rose
William Rose on 27 May 2021
The system in the image you attached is obviously a lot more complicated. Masses m1 and m3 have 6 attached elements and therefore 6 forces. Each force may have x- an y-components. Masses m2 and m4 have 4 attached elements and therefore 4 forces. For m1:
where are the x and y components of the spring force from the wall acting on m1. are the x and y components of the dashpot force from the wall acting on m1. And so on. You will need geometry and calculus and physics to express each spring force and each dashpot force in terms of the x,y coordinates of the masses. If you want to solve the system in Matlab, you will need to convert the second order differetial equations above to coupled first order differential equations. Each mass will have four state variables. For m1, the state variables are . Likewise for the other three masses. So the state vector for the whole system will have 16 elements. This is explained well, and with examples, in the Matlab help on solving ordinary differential equations.
The attached document shows how each of the twelve forces above (on the right hand side of the equations) can be expressed in terms of the state varibles , etc. It also explains how to compute all the other forces that you need to solve the system.

More Answers (1)

William Rose
William Rose on 24 May 2021
The equation of motion for a M-S-D system may be written as
where is a position vector, is frictional drag, is spring force, and is the external force (if any). may have 1 or 2 or 3 dimension (or more, I suppose). We assume that the spring pulls the object directly toward the origin, with a force proportional to the distance from the origin, and that frictional drag is the direction oppostite to the velocity, and proportional to the magnitude of velocity. These assumptions can be written as
and
Therefore we get the equation
In the 2-D case, we have, in the x-direction,
which looks totally normal. It involves the x-components of the spring force and the drag force. But here's where it gets tricky and then simple again. What are the equations for and ? It turns out the equations are
and
So we end up with
You get a similar result for the y-component:
So we see that for this special case - a linear mass-spring-dashpot system - the x- and y-parts do not affect each other, i.e. the equations of motion are totally separable. These two equations are what you need to solve. Since they are not coupled, you can solve them separately, in either order.
  1 Comment
Muhammad Ali
Muhammad Ali on 26 May 2021
@William Rose thanks alot. I understant it for a single mass moving in x and y direction but, how would it work for a system which I have attached in this comment. Please have a look. Thanks.

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