Mathematica: Plot the derivative of a piecewise function

Mathematica: Plot the derivative of a piecewise function

Try using Plot[Evaluate[f[t]], {t, -1, 1}]

Plot is a bit picky when it comes to user defined functions.

With the given function you could use:

Plot[f[t], {t, -1, 1}, Evaluated -> True]

Evaluated -> True is to be preferred over Evaluate[f[t]].

Better is to follow nikies advice and define f differently:

Block[{t},
  f[t_] = D[moll[t], t]
]

See Scoping in assigning a derivative for an explanation.

Mathematica: Plot the derivative of a piecewise function

The pickyness of Plot comes from its Atttributes[Plot], which include HoldAll, so the unadorned f never gets evaluated. Force evaluation as ratatosk suggests.

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