I believe sympy.physics.mechanics.msubs(….) also does not go ‘inside’ the time
derivatives (?)
From: sympy@googlegroups.com <sympy@googlegroups.com> On Behalf Of Chris Smith
Sent: Thursday, September 18, 2025 3:13 PM
To: sympy <sympy@googlegroups.com>
Subject: [sympy] Re: Subs for variable without affecting it's derivative
You had to create dqdt manually (or else it would have evaluated to 0 already)
so instead of using `q` use `f(q)` where `f=Function('f')`. When you do this
something like `(q*Derivative(f(q),t)).subs(q,2)` it will now become
`2*Derivative(f(2),t)` instead of 0.
Else, as you say, the masking is easy: reps = {a:Dummy() for a in
eq.atoms(Derivative)};eq.xreplace(reps).subs(q,2).xreplace({v:k for k,v in
reps.items()})`
/c
On Wednesday, September 17, 2025 at 3:15:04 PM UTC-5 frank....@gmail.com
<mailto:frank....@gmail.com> wrote:
Hi all,
I constantly run into something like cos(q)*dqdt and I want to substitute a
specific value for q without it affecting dqdt say q = 0 but dqdt is non-zero.
Is there a way to do this without having to make a change of variables (ie dqdt
= u1 then subs for q =0)?
Thank,
Frank
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