differential equation - oscillation with changing mass

[Staphen]

Hi, can anybody give me a hint on how I get Mathcad to solve an equation like m(y(t))*y''(t)=F. The problem is that m(y(t)) depends for a certain time on the value of the last amplitude and therefore mathcad has to remember this value within the solve block somehow. Thank you

 

[GregLocock]

That's a bit like a rocket equation, I doubt it has a closed form solution.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Staphen]

Basically it is sort of a rocket equation but only for y´´>0. For the other half of the cycle m(y(t)) depends in the value of y(t). Considering F which discribes the propulsion it is a self oscillatory system with internal excitation. I managed to solve it in mathlab (simulink) but I can not use simulink anymore. Now I am trying to do my best in Mathcad.

 

[Staphen]

Sorry, I meant y´ instead of y´´

 

[GregLocock]

I must admit I have had little success with differential equations in Mathcad, not because they don't work, just from lack of need.

My approach with similar problems has been to write a time based simulation.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[PNachtwey]

Look at this
ftp://ftp.deltacompsys.com/public/NG/Mathcad%20-%20Ode%20Opt%20First%20Order%20no%20offset%202.pdf
I was experimenting with doing system identification to find the constants for ODEs. The first page is using a known ODE to generate data that must be identified using the the technique used on the second page.

Staphen will need to have four equations. instead of two.
One that computes m', y, y' and y"