Hello, 

I am having some doubts on how to use subs() in sympy. My aim is to do 
lagrangian mechanics which involves differentiation.The code below is a 
cooked up one but I guess it would serve the purpose. 

Please look at the code below

from sympy.physics.mechanics import *
import cvxpy

q1, q2 = dynamicsymbols('q1 q2')
q1d, q2d = dynamicsymbols('q1 q2', 1)
L = q1*q1d + q2*q2d
print L.subs(q1, 2).subs(q2, 1).subs(q1d, 3).subs(q2d, 7) 

# EXPECTED ANSWER: L = 2*3 + 1*7 = 13
# ACTUAL ANSWER: L = Derivative(1, t) + 2*Derivative(2, t)

Please let me know how  to modify the code to get the expected answer

Thanks 
Ash

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