Re: A simple ordinary differential equation

*To*: mathgroup at smc.vnet.net*Subject*: [mg98255] Re: A simple ordinary differential equation*From*: dh <dh at metrohm.com>*Date*: Fri, 3 Apr 2009 04:40:27 -0500 (EST)*References*: <gr21kh$lpg$1@smc.vnet.net>

Hi, look at: f[x_?NumericQ] = 1 - 1111*x'^2 you can not take the derivative of a number. Daniel I. Shechtman wrote: > What is wrong with this equation? > > Clear[sol] > f[x_?NumericQ] = 1 - 1111*x'^2; > sol = NDSolve[{x''[t] + (16/10^4)x'[t]^2*Sqr[ > f[x[t]]]/x[ > t]^2 - (1458/10^9)*f[x[t]]/x[t]^2 == > 0, x[0] == 1, x'[0] == 0}, x[t], { > t, 0, 100}, AccuracyGoal -> 8, > PrecisionGoal -> 8, WorkingPrecision -> > 30, MaxSteps -> Infinity]; > Plot[Evaluate[x'[t] /. sol], {t, 0, 100}] >