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[Stephen23]

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I can't solve this error

clear
G = 6.67408*10^-11;
M1 = 5.9722*10^24;
x1x = 5*10^6;
x1y = 0;
M2 = 3.3*10^24;
x2x = -12*10^6;
x2y = 0;
ft = 15000;
dt = 1;
t = 0:dt:ft;
V = zeros(1,length(t));
Vx = 0;
Vy = 7058.6;
Px = 13*10^6;
Py = 0;
r1x = 0;
r1y = 0;
r2x = 0;
r2y = 0;
while ft>=t;
    
    r1x = Px - x1x;
    r1y = Py - x1y;
    r2x = Px - x2x;
    r2y = Py - x2y;
   
    
    r1mag = sqrt(r1x^2 + r1y^2);
    r2mag = sqrt(r2x^2 + r2y^2);
    ax = -G(((M1/r1mag^3)*r1x)+((M2/r2mag^3)*r2x));
    ay = -G(((M1*r1y)/r1mag^3)+((M2*r2y)/r2mag^3));
    
    Px = Px + Vx*dt;
    Py = Py + Vy*dt;
    Vx = Vx + ax*dt;
    Vy = Vy + ay*dt;
    
    plot(P);
end

That's my code an I keep getting the following error:

Index exceeds the number of array elements (1).
Error in AnotherTry (line 35)
    ax = -G(((M1/r1mag^3)*r1x)+((M2/r2mag^3)*r2x));

How can I solve this?

Thanks.

p.s. If the V = zeros etc.. part looks odd, that's normal. I've been experimenting.

Answer 1

[DGM]

This needs to be a test which returns a scalar logical, but t is a vector.

while ft>=t   

G is a scalar, and you're trying to address the 98 billionth element of it.

ax = -G(((M1/r1mag^3)*r1x)+((M2/r2mag^3)*r2x));

are you sure you don't mean to be multiplying?

ax = -G*(((M1/r1mag^3)*r1x)+((M2/r2mag^3)*r2x));

[DGM]

These are two unrelated problems.

First, since t is a vector, so is the result of the test. The exit condition won't work correctly because of this.

size(ft>=t)
ans =
           1       15001

Normally, you'd want this to be an expression which evaluates to a scalar logical output. If it's a vector, it will only exit if all 15001 elements are true.

Second, you're setting

ax = -G(index)

where said index is

((M1/r1mag^3)*r1x)+((M2/r2mag^3)*r2x)
ans =
   9.8596e+10

Which makes no sense. G has only one element.

[Walter Roberson]

MATLAB has absolutely no implied multiplication. G(something) is indexing, not multiplication.