(updated with formatting fix)
Hi all,
I am attempting to follow along a (very nice) example I got from On the Importance of Being Edgy—Electrostatic and Magnetostatic Problems with Sharp Edges—Wolfram Blog
I am having a problem with the “Getting the right format from Wolfram Alpha” part. Namely, in the attached cdf example there is a command:
In[2]:= WolframAlpha["equation of motion of a classical sinusoidal driven harmonic oscillator",{{"Result",1},"Output"}]
Out[2]= HoldComplete[{m Subsuperscript[\[Omega], 0, 2] x[t]+m (x^\[Prime]\[Prime])[t]==F Sin[t Subscript[\[Omega], d]-Subscript[\[Phi], d]]}]
followed by part of:
In[3] ... Join[ReleaseHold[%], {x[0] == x0, x'[0] == v0}
When I execute these command I get an error “Join::heads: Heads Defer and List at positions 1 and 2 are expected to be the same.”
When I look at the Head of the Out[2] i.e.
aa= WolframAlpha["equation of motion of a classical sinusoidal driven harmonic oscillator",{{"Result",1},"Output"}]
Head[ReseaseHold[aa]]
I get “Defer” rather than “List”
I can only proceed if I do
Join[First@ReleaseHold[%], {x[0] == x0, x'[0] == v0}
to get around the “Defer” head.
Similarly, with the next example where a function is defined:
\[Phi]LineSegment[{x_, y_, z_}] = WolframAlpha[ "electric potential of a charged line segment", {{"Result", 1}, "Input"}][[1]] /. {Subscript[\[Epsilon], 0] -> 1/(4 Pi), l -> 1, Q -> 1}
my output looks like:
ReleaseHold[ Hold[Log[( 1/2 + z + Sqrt[r + (z + 1/2)^2])/((1 - 1)/2 + z + Sqrt[ r + (z + (1 - 1)/2)^2])]/((4 \[Pi])/( 4 \[Pi]))] //. {r :> x^2 + y^2}]
which will not evalute when I call the function, i.e. \LineSegment[{1,1,1}] =
rather than the example’s output of
Log[( 1/2 + z + Sqrt[r + (z + 1/2)^2])/((1 - 1)/2 + z + Sqrt[ r + (z + (1 - 1)/2)^2])]/((4 \[Pi])/( 4 \[Pi]))] //. {r :> x^2 + y^2}]
and again if I use
\[Phi]LineSegment[{x_, y_, z_}] = First@ WolframAlpha[ "electric potential of a charged line segment", {{"Result", 1}, "Input"}][[1]] /. {Subscript[\[Epsilon], 0] -> 1/(4 Pi), l -> 1, Q -> 1}
things work out fine.
So, my question(s) boil down to a mix of : am I misunderstanding the use of ReleaseHold? is my “solution” just a kludge? did things change between v8 and v9 (I am running v9.0.1)
Thanks.