Re: Re: Re: Condition/constraint problem
- To: mathgroup at smc.vnet.net
- Subject: [mg40983] Re: [mg40976] Re: [mg40938] Re: Condition/constraint problem
- From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
- Date: Sun, 27 Apr 2003 03:18:32 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
This seems to be just an accuracy problem due to the very rapidly oscillating nature of the function. You need much more accurate input, and even then the answers won't be exactly the same: In[1]:= vx1[t_] := 3/2000 + (3*Sin[70*t])/2500 /; t < 5 In[2]:= vx2[t_] := 3/2000 + (3*Sin[70*t])/2500 In[3]:= Derivative[1][vx1][2.`50] Out[3]= -0.016616340216276107118073957504289560620917139256026603482\ 40545`46.5497 In[4]:= Derivative[1][vx2][2.`50] Out[4]= -0.016616340216358530300150292495099072037728048646002646485\ 71143`47.1588 Andrzej Kozlowski Yokohama, Japan http://www.mimuw.edu.pl/~akoz/ http://platon.c.u-tokyo.ac.jp/andrzej/ On Saturday, April 26, 2003, at 04:26 pm, Bobby Treat wrote: > Your explanation implies there IS no value for vx1'[t], but Mathematica > does compute one, when t is numeric. It's simply wrong. > > vx1[t_] := 0.0015 + 0.0012 Sin[70 t] /; t < 5 > vx2[t_] := 0.0015 + 0.0012 Sin[70 t] > vx1'[2.] > vx2'[2.] > > 0.00793433 > -0.0166163 > > Bobby > > -----Original Message----- > From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de> To: mathgroup at smc.vnet.net > To: mathgroup at smc.vnet.net > Subject: [mg40983] [mg40976] [mg40938] Re: Condition/constraint problem > > Hi, > > der derivative is complete right, since > Condition[] has no derivative > > Dt[vx1[t]] evaluates to vx1'[t] > > until you can tell Mathematica how to > find out > a) what the function value of vx1[t] for t>5 may be > b) to compute the derivative for t==5 > c) determine when the symbol t in vx1[t] may be >5 > > > Regards > Jens > Bamboo wrote: >> >> 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 > > >