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When you write:

dqdt[0].subs({qs[i]:z[i] for i in range(0,N)})

this returns another object, it does not change dqdt.

So, instead, you can write

dqdt[0] = dqdt[0].subs({qs[i]:z[i] for i in range(0,N)})

When you write:

dqdt[0].subs({qs[i]:z[i] for i in range(0,N)})

this returns another object, it does not change dqdt.

So, instead, you can writewrite:

dqdt[0] = dqdt[0].subs({qs[i]:z[i] for i in range(0,N)})