Find the time when a parabolic function starts to decrease.

Hello,

My task is to write a script that will find the time when a parabolic function (height, a function of time) reaches its max.

clear all
t=0;
for time=0:0.5:30;
    if height(time)>=height(time-0.5)
        t=t+0.5;
    end
end
disp(t)

height(time-0.5) wouldn't execute so here is my second code

clear all
t=0;
for time=0:0.5:30;
    e=0.5:0.5:29;
    time2=[0 e];
    while height(time)>height(time2)
        t=t+0.5;
    end
end
disp(t)

this code does not work either. How can i utilize max( ) function in here? And what is the best way to do this kind of loop?

Thanks!

Answer 1

[Walter Roberson]

What is height here? If it is a vector then you cannot use height(time) you can only use height(SomePositiveIntegerIndex) . But once you have found the appropriate positive integer index, you can use it to figure out what the corresponding time is.

[Roger L]

height is just a function of time, like a projectile function. if i plot t and f(t) its just an upside-down parabola. (both are 1x61 elements)

I can definitely get the max height using max(height(time)). but how would i solve it back and get t at max? quadratic formula? I could also do a loop of the derivative==0, but how do i incorporate time? I think there is an easier way with loops.

[Walter Roberson]

[maxheight, idx] = max(height(time))

where time is a vector. Then time(idx)

[Roger L]

thanks for that command, I haven't used it before. It gave me an index on the time vector corresponding to the seconds that I needed.