# Maybe，the 6DOF model of simulink is wrong?

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songzhou Yu on 10 Jan 2017
Commented: Justin Beri on 18 Nov 2019
If the body 's force and torque of 3 direction is zero(figure 1)，the initial velocity of x direcion is 100, initial body rotation of y is 100(figure 2),I think this body will have an uniform linear motion，but，why it has motion in z axis(figure 3),I think the equations of motion is wrong.

Sergio Tamayo on 23 Jan 2018
This occurs because the tangent acceleration term (Omega X Vb), generates an acceleration on the Z axis. At t=0:
cross([0 100 0],[100 0 0])
ans =
0 0 -10000
Since no other accelerations are present (due to forces and moments), the acceleration along the Z axis is integrated, causing the displacement along the Z axis.
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Justin Beri on 18 Nov 2019
Hi Sergio, I think your answer is not entirely correct or needs further clarification.
I reproduced Songzhou's question.
As you can see there is indeed a displacement on the z axis (in the inertial frame). However, let's also tweak the numerical integration settings. I have reduced the absolute and relative tolerances.
Now we see that displacement is extremely small. So this leads us to think it is numerical error influencing the displacement.

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