Error with matrix calculation

Question

Tobias 2024-5-2

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评论： Tobias 2024-5-3

% matrix calculation
M3 = [0 0 0 0 -1 0 0 0 0 0 1 0;
    0 0 0 0 0 1 0 0 0 0 0 0;
    0 0 0 0 0 0 0 0 0 0 -0.05 1;
    1 0 0 0 0 0 0 0 -1 0 -1 0;
    0 -1 0 0 0 0 0 0 0 -1 0 0;
    -0.15 0 0 0 0 0 0 0 -0.15 0 0.05 0;
    -1 0 1 0 0 0 0 0 0 0 0 0;
    0 1 0 1 0 0 0 0 0 0 0 0;
    -0.4*sind(20) -0.4*cosd(20) -0.4*sind(20) 0.4*cosd(20) 0 0 0 0 0 0 0 0;
    0 0 0 -1 0 0 0 1 0 0 0 0;
    0 0 (BD/2)*cosd(Angle_FBD) (BD/2)*sind(Angle_FBD) 0 0 (BD/2)*cosd(Angle_FBD) (BD/2)*sind(Angle_FBD) 0 0 0 0];
N3 = [mass_body1*Acceleration_G1x;
    mass_body1*Acceleration_G1y+mass_body1*9.81;
    mass_body1*radius_body1^2*alpha_1;
    mass_body2*Acceleration_G2x;
    mass_body2*Acceleration_G2y+mass_body2*9.81;
    (1/12)*mass_body2*radius_body2^2*alpha_2;
    mass_body3*Acceleration_G3x;
    mass_body3*Acceleration_G3y+mass_body3*9.81;
    (1/12)*mass_body3*radius_body3^2*alpha_3;
    mass_body4*Acceleration_G4x;
    mass_body4*Acceleration_G4y+mass_body4*9.81;
    (1/12)*mass_body4*BD^2*alpha_4];
X = M3/N3

It says that there is an error using "/" and that matrix dimensions must agree. I dont know exactly what this means, and can't find where the issue is. The error is in the line "X = M3/N3"

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Tobias 2024-5-3

M3 is A and N3 is B

Tobias 2024-5-3

I forgot a row with values in M3, but it still doesnt work. M3 is now 12 12, and N3 is 12 1

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Answer 1

James Tursa 2024-5-2

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在 MATLAB Online 中打开

We don't have all the variables necessary to run your code, but guessing that maybe you need element-wise divide instead of matrix divide?

X = M3 ./ N3

Note the period, so try ./ instead of just /

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Tobias 2024-5-3

thanks, it worked!

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Answer 2

Walter Roberson 2024-5-3

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When you use the / (matrix right divide) operator, the two operands must have the same number of columns. The resulting matrix will have a size that is the number of rows of the first operand, by the number of rows of the second operand.

A = rand(5,3); B = rand(7,3);
A / B
ans = 5x7
         0         0         0         0    1.8427    1.2240   -0.1906
         0         0         0         0   -0.3170    0.8279    0.7349
         0         0         0         0    1.1372    0.7934    0.5501
         0         0         0         0    0.2419    1.0085   -0.2503
         0         0         0         0    0.2136    1.2041   -0.0009
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You appear to be constructing an 11 x 12 matrix and dividing it by a 12 x 1 matrix. None of the operators \ .\ / or ./ would be suitable for that purpose.

It would not be an error to calculate

X = M3/N3'

which would then be an 11 x 12 matrix divided by a 1 x 12 matrix, which would give an 11 x 1 result.