How to make a function proceed in time when t is unknown

Hi!

I have a function, for example speed as following:

v= v0 + a.*(t-t0);
If I have the input variables v0, a and t0 but t is unknown and I know that for this specific step the process will continue until v=-20 m/s. How can I make matlab plot the whole process with small time steps (dt) until the speed is -20 m/s?

I'm thinking about using a while function such as:

    while v>=-20
        v0=46;
        t0=2;
        a=-9.81;
        v= v0+a*(t-t0);
      end
It would be great if anyone could help me with that cause I am having real hard trouble to find how this should be done and trial and error does not seem to be the best solution!

Thanks

Answer 1

[Mischa Kim]

I suggest computing the time tend at which v=-20 m/s by hand, since it is so simple and then use

v0 = 46;
t0 = 2;
a  = -9.81;
t  = linspace(t0, tend, 50);

for ii = 1:length(t)
  v(ii) = v0 + a*(t(ii) - t0);
end

Alternatively, you can use your while loop instead

v(1) = v0;
t(1) = t0;
dt   = 0.1;  % or any other value 
ii   = 1;
while v(ii) >= -20
  ii = ii + 1;
  t(ii) = t(ii - 1) + dt;
  v(ii) = v(ii - 1) + a*(t(ii) - t0);
end

[Mischa Kim]

Yep, as I was suggesting, compute tend by hand. Or add another line

tend = (-20 - v0)/a + t0;

The problem with the while-loop is that you might not exactly hit the -20 m/s. You can, but this will make your algorithm much more complex.

[Maria Andersson]

When I run the code you suggested, I get the following error message:

Undefined function 't' for input arguments of type 'double'.

Error in Inl_Raket (line 21)
    v(i)= 46 + a*(t(i)-2);

Any idea why this happens?