findroot problem

*To*: mathgroup at smc.vnet.net*Subject*: [mg56362] findroot problem*From*: bharrison at walla.com (bruce)*Date*: Fri, 22 Apr 2005 06:25:04 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

I'm new to mathematica and have been using it to solve differential equations. Today i did a simple equation and managed to plot it (plot looked correct) but when i tried (and tried and tried...) to use Findroot to find the root i keep getting error messages. This is my work (without the plot): mc = 1; mp = 0.01; l1 = 0.1; l2 = 0.4; g = 9.81; d = (mc l1 - mp l2)*g/(mc l1^2 + mp l2^2) 81.1862 simpletreb = NDSolve[{ϕ''[ t] == -d Sin[ϕ[t]], ϕ[0] == (3π/4), ϕ'[0] == 0}, ϕ[t], {t, 0, 1}] {{ϕ[t] -> InterpolatingFunction[{{0., 1.}}, <>][t]}} {{ϕ[t] -> InterpolatingFunction[{{0.`, 1.`}}, "<>"][t]}} Plot[ϕ[t] /. simpletreb, {t, 0, 0.5}]; *PLOT GOES HERE* FindRoot[ϕ[t] == 0, {t,0.2}] Findroot::nlnum : The function value (ϕ[0.2]} is not a list of numbers with dimensions {1} at {t} = {0.2} ************* I tried many things all of which came up with various errors. From the plot i know the root is between 0.25 and 0.3. If anyone can point me in right direction id greatly appreciate it!