I have two same errors every time i paste a code to check
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This is my task and here is code:% Load the x-y-z positions into the variable R
load datafile.mat
m = 1.2; % particle mass in kg
% Find the center of mass location in a row vector rcm
rcm = mean(R);
% Find the moment of inertia matrix I, a 3-by-3 matrix
Ixx = sum(m*(R(:,2).^2+R(:,3).^2));
Iyy = sum(m*(R(:,1).^2 + R(:,3).^2));
Izz = sum(m*(R(:,1).^2 + R(:,2).^2));
Ixy = -sum(m * R(:,1) .* R(:,2));
Ixz = -sum(m * R(:,1) .* R(:,3));
Iyz = -sum(m * R(:,2) .*R(:,3));
% Composite main inertia matrix I
I = [ Ixx, Ixy, Ixz;
Ixy, Iyy, Iyz;
Ixz, Iyz, Izz ];
% Find the principal moment of inertia
[~,II] = eig(I);
and the errors are
I tried many different methods to solve this but non of them worked :/, thanks for any help
4 个评论
Jakub
2025-11-9
In the problem statement, it's written that I should be stored in a variable IM:
% Composite main inertia matrix I
IM = [ ]; %matrix 3x3
Jakub
2025-11-9
Code NR1
% Load the x-y-z positions into the variable R
load datafile.mat
m = 1.2; % particle mass in kg
% Load the x-y-z positions into the variable R
% Find the center of mass location in a row vector rcm
rcm = mean(R);
% Find the moment of inertia matrix I, a 3-by-3 matrix
Ixx = sum(m*(R(:,2).^2+R(:,3).^2));
Iyy = sum(m*(R(:,1).^2 + R(:,3).^2));
Izz = sum(m*(R(:,1).^2 + R(:,2).^2));
Ixy = -sum(m * R(:,1) .* R(:,2));
Ixz = -sum(m * R(:,1) .* R(:,3));
Iyz = -sum(m * R(:,2) .*R(:,3));
I = [ Ixx, Ixy, Ixz;
Ixy, Iyy, Iyz;
Ixz, Iyz, Izz ];
IM = I;
[~,II] = eig(IM);
CODE NR.2
% Load the x-y-z positions into the variable R
load datafile.mat
m = 1.2; % particle mass in kg
% Load the x-y-z positions into the variable R
rcm = mean(R);
Ixx = sum(m*(R(:,2).^2+R(:,3).^2));
Iyy = sum(m*(R(:,1).^2 + R(:,3).^2));
Izz = sum(m*(R(:,1).^2 + R(:,2).^2));
Ixy = -sum(m * R(:,1) .* R(:,2));
Ixz = -sum(m * R(:,1) .* R(:,3));
Iyz = -sum(m * R(:,2) .*R(:,3));
IM = [ Ixx, Ixy, Ixz;
Ixy, Iyy, Iyz;
Ixz, Iyz, Izz ];
[~,II] = eig(IM);
Both are incorrect and there are such errors:
回答（2 个）
Hello,
Code looks almost correct, but the issue looks like with the definition of the moment of inertia matrix. Specifically, you did not subtract the centre of mass from each position before calculating the inertia tensor. The inertia tensor must be calculated about the centre of mass, not the origin.
You used R(:,1), R(:,2), and R(:,3) directly, which are positions relative to the origin.
Try using positions relative to the center of mass: R – rcm
Replace every instance of R(:,...) in your inertia tensor calculation with Rcm(:,...).
To know more about this topic, you can refer to:
Thanks
6 个评论
Jakub
2025-11-11
Hi, thank you for your answer, unfortunately it doesnt work too,
here is code my teacher gave, and we need to modify it,
% Load the x-y-z positions into the variable R
load datafile.mat
m = 1.2; % particle mass in kg
% Find the center of mass location in a row vector rcm
rcm = mean(R);
% Find the moment of inertia matrix I, a 3-by-3 matrix
Ixx = sum(m*(R(:,2).^2+R(:,3).^2));
Iyy =
Izz =
Ixy =
Ixz =
Iyz =
% Composite main inertia matrix I
I = [ ]; %matrix 3x3
% Find the principal moment of inertia, a 3-by-3 diagonal matrix of II
[~,II] = eig(I);
and i did so
% Load the x-y-z positions into the variable R
load datafile.mat
m = 1.2; % particle mass in kg
% Center of mass (row vector)
rcm = mean(R, 1);
% Positions relative to the center of mass
Rcm = R - rcm;
x = Rcm(:,1);
y = Rcm(:,2);
z = Rcm(:,3);
% Inertia components about the COM (point masses)
Ixx = m * sum(y.^2 + z.^2);
Iyy = m * sum(x.^2 + z.^2);
Izz = m * sum(x.^2 + y.^2);
% Products of inertia (negative sign convention)
Ixy = - m * sum(x .* y);
Ixz = - m * sum(x .* z);
Iyz = - m * sum(y .* z);
% Inertia matrix about COM in the given xyz frame
I = [ Ixx Ixy Ixz;
Ixy Iyy Iyz;
Ixz Iyz Izz ];
% Store also in IM if the template/tests expect IM
IM = I;
% Principal moments of inertia (diagonal matrix)
[~, II] = eig(I);
But the errors are still the same, and now i dont know where the mistake can be
Dyuman Joshi
2025-11-11
That definition is correct, however for Inertia tensor the definition used is correct. See here - https://en.wikipedia.org/wiki/Moment_of_inertia#Definition_2
Is it [x1 y1 z1; x2 y2 z2; x3 y3 z3]? or its tranpose? or any other way?
Also -
> What is the message/output when you click on Show Feedback?
> Are the output values supposed to be rounded or to be provided in a certain/specific format?
Jakub
2025-11-11
Hi
- Matrix R layout: R is 100×3. Each row is one particle position [x y z] in the given frame. Verified with: size(R) % returns 100 3
- Show Feedback message:
3.Formatting / rounding: No rounding required. I am returning raw double arrays:
In case rounding is not required, it would be better to check how answers/outputs are compared. It is likely that your teacher is using isequal to compare floating point numbers, which is not recommended -
isequal(0.3, 0.2+0.1)
A robust approach is to use tolerances to compare -
tol = 1e-4;
abs(0.3-(0.2+0.1))<tol
%or
isapprox(0.3, 0.2+0.1, RelativeTolerance=tol)
Have any of your classmates successfully solved the problem?
Jakub
2025-11-12
Jakub
2025-11-12
2 个评论
Dyuman Joshi
2025-11-12
Ixy = Izz;
Ixz = Iyy;
Iyz = Ixx;
This doesn't make sense at all.
I'd ask for a clarification from your teacher regarding this.
And it seems you had to do some rounding (and sorting as well!).
dpb
2025-11-12
Agree w/ @Dyuman Joshi -- the product of intertia terms in the intertia tensor are products, not the same as the moments around the major axes. I think your original solution is correct and this is the wrong answer given what we know here.
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