Re: Condition/constraint problem
- To: mathgroup at smc.vnet.net
- Subject: [mg40956] Re: [mg40915] Condition/constraint problem
- From: Bobby Treat <drmajorbob+MathGroup3528 at mailblocks.com>
- Date: Fri, 25 Apr 2003 08:06:38 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
The Derivative function is simply getting it wrong. (It's a bug.)
Here's a dissection that shows approximately what is happening:
vx1[t_] := 0.0015 + 0.0012 Sin[70 t] /; t < 5
vx2[t_] := 0.0015 + 0.0012 Sin[70 t]
fin1 = Dt[vx1[t], t]
fin2 = Dt[vx2[t], t]
plot1 = Plot[{fin1, fin2}, {t, 0, 2}, PlotStyle -> {Red, Blue},
PlotPoints ->
100];
{data1, data2} = Cases[plot1, Line[a_] :> a, Infinity];
Chop@Fit[data1, {1, Sin[70t], Cos[70t]}, t]
Chop@Fit[data2, {1, Sin[70t], Cos[70t]}, t]
-0.0401102 Cos[70 t]
0.084 Cos[70 t]
I experimented with adding other fit functions, and it seems clear
that, in fact,
fin1 == -0.0401102 Cos[70 t]
I have not yet guessed the formula that led to this, but it's clearly
wrong.
Bobby
-----Original Message-----
From: Bamboo <u8514501 at cc.nctu.edu.tw>
To: mathgroup at smc.vnet.net
Subject: [mg40956] [mg40915] Condition/constraint problem
Dear all,
I find a problem and don't know why. The input is as following.
If a condiction(constraint) is set to the function, vx1[t],
the derivative of vx1[t] is worng (fin1 is not equal to fin2).
Any help welcome.
vx1[t_] : = 0.0015 + 0.0012 Sin[70 t] /; t < 5
vx2[t_] : = 0.0015 + 0.0012 Sin[70 t]
fin1 = Dt[vx1[t], t]
fin2 = Dt[vx2[t], t]
Plot[fin1, {t, 0, 2}]
Plot[fin2, {t, 0, 2}]
Thanks,
Bamboo