MATLAB: Error Inner Matrix Dimensions must agree?? Help!

Question

So i am trying to solve for these 2 values but i keep getting an error aobut my matrix dimensions. i dont know why, i will attach what my x value is.

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Error using * Inner matrix dimensions must agree.

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ang_position=x(:,1);

ang_velocity=x(:,2);

ang_accel= diff(ang_velocity);

a_cmx= (ang_accel.*(L./2).*cos(ang_position))-((ang_velocity.^2)*(L/2)*sin(ang_position));

a_cmy= (-ang_accel.*(L./2).*sin(ang_position))-((ang_velocity.^2)*(L/2)*cos(ang_position));

Answer 1

Suppose x is of dimension nx2. Then from

 ang_position=x(:,1);
 ang_velocity=x(:,2);

both of these vectors are of dimension nx1.

Now check the next line of code

 ang_accel= diff(ang_velocity);

The above creates ang_accel which is of dimension (n-1)x1 because of diff reducing the length of the input vector by one. This creates the error at the line

 (ang_accel.*(L./2).*cos(ang_position))

because the two vectors, ang_accel and ang_position, are of different dimensions. You may just want to replace

 ang_accel= diff(ang_velocity);

with

 ang_accel= [0 ; diff(ang_velocity)];

so that the dimensions are correct (and zero seems valid, since there is no prior data to determine the acceleration for the first element).

Note also that you will have to update your final two lines of code so that the pair-wise multiplication can occur for all cases (missing from the final multiplication of each line)

 a_cmx= ((ang_accel*(L/2)).*cos(ang_position))-(((ang_velocity.^2)*(L/2)).*sin(ang_position));
 a_cmy= ((-ang_accel*(L/2)).*sin(ang_position))-(((ang_velocity.^2)*(L/2)).*cos(ang_position));

Note that for scalar multiplication, the period is not needed: I replaced

 ang_accel.*(L./2)

with just

 (ang_accel*(L/2))