Sympy physics mechanics, second derivatives #21809
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Peter230655
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There may be a bug. I don't think we have any prior test cases of forces that are functions of accelerations. |
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Thanks!
My 'substitution' seemed to work, so I guess it was 'legal' in the frame
work of sympy.physics.mechanics?
Am Di., 3. Aug. 2021 um 14:00 Uhr schrieb Jason K. Moore <
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… There may be a bug. I don't think we have any prior test cases of forces
that are functions of accelerations.
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Thanks!
So, the safe way is to use the ,rhs' to do a substitution?
On Tue 3. Aug 2021 at 15:31, Jason K. Moore ***@***.***> wrote:
Yes, substituting should be fine. But if you need the mass matrix form,
you'll have to form that manually to make sure you capture the coefficients.
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In a model I am playing around with, the force on a body depends on the second derivative of a generalized coordinate.
Say, q = generalized coordinate
u = q.diff(t)
When I put it into the force list, like FL = [(Dmc, - k0*u.diff(t)*N.x)]
it seems that this force gets ignored, or set to zero when setting up the equations of motion. u.diff(t) was not in the dynamic symbols of the force vector, and also the result looked like it was ignored.
Then I created F = dynamicsymbol(‚F‘), and put this into the force list, like FL = [(Dmc, - k0FN.x)]
Now F appeared in the force vector, I substituted like force = force.subs({u.diff(t): rhs(..)})
Now all seemed to work fine.
I wonder what I did wrong, that my first attempt did not work.
Thanks for any help!
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