Return only the positive values to be used in subsequent eqns

Hello all,

I am trying to get matlab to only display, and assign the positive value attained from solving the syms of eqn... to be used in subsequent equations. It keeps displaying both. I have tried a couple of other ways Please help

Here is copy paste code if you are feeling super nice.

g= -32.3;
syms v t 
eqn3 = v > 0;
eqn4 = t > 0;
eqn1 = 3 ==  v  * cos (45) *t;
eqn2 = t  == (2 * v * sind(45))/g ;
eqns =[eqn1,eqn2]

sol = solve([eqns],[v,t]);
vS = vpa(sol.v)    
tS = vpa(sol.t)

sol = solve([eqn3,eqn4],[v,t])
vpos = vS(vS>=0);
tpos = tS(tS>=0);

vpos
tpos

I have also tried it this way

g= -32.3;
syms v t 
eqn3 = v > 0;
eqn4 = t > 0;
eqn1 = 3 ==  v  * cos (45) *t;
eqn2 = t  == (2 * v * sind(45))/g ;
eqns =[eqn1,eqn2]

sol = solve([eqns],[v,t]);
vS = vpa(sol.v)    
tS = vpa(sol.t) 
if vS == vSpos(vS>=0)
    vSpos = vS(vSol>=0);
    vS= vSpos
end

vS 
tS

Answer 1

[Alan Stevens]

Why not use

g = 32.3;
v = sqrt(3*g/(2*cosd(45)*sind(45)));
t = 2*v*sind(45)/g;

which will give you positive values immediately?

Incidentally, you probably want cosd(45), not cos(45).

[Susan Chestnutt]

The idea is to be able to have the user input a target distance.... where I'm using the value 3 and have the program calculate the v and t. I just started using matlab 1 week ago so I am doing peicewise to make sure I understand the different parts of the code.

This is a more complete idea of what I was going for...and the code was retuning errors surrounding solving the system ...so I started with the basics of sys solving in a new live script.

Projectile mini 1

Set up the parameters

y_o = 0;
x_o = 0;

dt=.001;
t = 0:dt:1.1;
g = -32.2;

a = zeros(15,75)
y = zeros(1,length(t))
x = zeros(1,length(t))
vy = zeros(1,length(t))
x_t = 
x (1) = x_o;
y(1) = y_o;

syms v a t 
S = solve (x_t == x + v  * cos (a) *t, y == v *t *sind (a) +1/2 *g*t^2, t == (2 * v * sind(a))/g) 

Simulate the trajectory

vy(1:end) = v * sind(a);



for i = 2 :length(t)
    vy (i) = vy(i-1) + g * dt;
    x (i) = x (i-1) + v * cosd(a) * dt;
    y (i) = y(i-1) + vy(i) * dt + 1/2 * g * dt.^2;
    
    
end

Plotting the balls path

plot (t,y)
title ("Height of the ball")

[Susan Chestnutt]

Nevermind, I figured it out......

If I get points for answering my own question let me know and I will post. If you are curious and can contact me directly I would be happy to post ans