issue with fzero and root finding

Question

Amanda 2017-10-3

0
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https://ww2.mathworks.cn/matlabcentral/answers/359407-issue-with-fzero-and-root-finding

回答： Matt J 2017-10-3

I am writing a function that computes the time that the rocket reaches some velocity. I am solving in using fzero. For my initial guess, I used 0. I know the domain for ln is 0, and infinity. I cannot make the domain until infinity, because the second bound needs to be finite. Do I need a second bound? I thought I could just have o, but I am not getting the correct output. Thank you.

% function tstar = fzero_rocket_example(u, g, q, m0, vstar)
%%Input
%      m0:  initial mass of rocket at time t=0
%      g:   gravitational constant
%      q:   fuel consumption rate
%      u:   velocity at which fuel expelled relative to rocket
%      vstar: upward velocity value for which tstar is to be determined
%
%%Output
%  tstar: the time that the rocket reaches the specified velocity, vstar. (scalar)
%%Write your code here.
%Compute time that the rocket reaches this velocity
tstar = 0;    % Initial Guess
% fzero(function, x0) - used to designe the find the real root of a single equation
% function = name of function being evaluated
% x0 = initial guess
vstar = fzero(@(vstar)u*log(m0/(m0-q*tstar))-g*tstar,0)