fzero help

y=@(T) I*T*sind(A)-(0.5*G*(T^2));
root = fzero(y, 0)

abouve eqation is for a projectile that projected at an angle (gose in a kind of ar half oval shape)

want to find the time it hist ground but fzero only gives me the biginning which is 0

Maybe it just doesn't like buxZED... won't let me submit an answer to this one.

root = fzero(y,[eps, realmax])

any clue why?
can you post as a comment?

Hey Walter, answers stop working for me too :(

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??? Error using ==> fzero at 260
Function values at interval endpoints must be finite and real.

Error in ==> p34 at 26
root = fzero(y,[eps, realmax])
>>

root = fzero(y,eps, realmax)
gives me a - answer, imposible

Plot the function, look at the plot and pick a closer guess than zero.

@matt Fig
that was my inicial thought
but we are not alowed to use neumerical methods or guess work

You do realize that FZERO is a numerical root finder, right?

is there a way to force fzero to go to the next positive root?

Ummm, make it fzero(y,[eps,sqrt(realmax)/G]

Using fzero(y,eps,realmax) would cause realmax to be interpreted as an Option, and then it would use eps as the starting guess, not constrained from going backwards.

No, there is no way to force fzero to go to the next positive root. You can only exclude the area you already searched from the area fzero is to search next.

rt = 0;
d = .1;
cnt = 1;
while ~rt
rt = fzero(y,rt+d*cnt);
cnt = cnt + 1;
end

it works
but i want to understand what while ~rt means (what dose ~ say)

dose this stop counting as it finds the first root after zero?

~ is logical negation. It is true if and only if the thing being tested is exactly 0.

so dose this stops finding roots after it reached the 1st root after 0?

When rt is not zero, then ~rt is false. That would stop the WHILE loop.

y=@(T) I*T*sind(A)-(0.5*G*(T^2));
%root = fzero(y, eps, realmax)
rt = 0;
d = .1;
cnt = 1;
while ~rt
rt = fzero(y,rt+d*cnt);
cnt = cnt + 1;
end
fprintf('the root is %g\n' , rt)

gives me the root is 7.11215e-026

but its not posible as the projectile takes around 1.5sec to reach maximum hight, the root should be around 3

can you point me in the right derection?

is there a way to systematicaly increase the T value till Y become zero and get that value?

Change from:
while ~rt
to:
while rt<.1
or similar. It might help if you actually gave us some values for I, A and G.

it worked Matt
thank you so much
and also ~ would allow me to use matlab in many new ways
thank you again :D

Can I suggest using an initial guess of 2*I*sind(A)/G? Just sayin'...

Come on now, Matt! Where is the fun of solving the equation with your brain instead of making MATLAB do it?

Answer 1

The anwser is in the comments above

From Matt Fig

rt = 0; 
d = .1; 
cnt = 1; 
while rt<.1 
rt = fzero(y,rt+d*cnt); 
cnt = cnt + 1; 
end

Digging deep, Honglei! How did you come across this one from over a year ago?

I'm helping cleaning up some old, unanswered questions :)