= Solution
Place the <carbon> and <sulfur> nuclei on a line separated by $r$, with masses $m_{\rm C}$ and $m_{\rm S}$. The <center of mass> lies between them and closer to the heavier <sulfur> nucleus. In the <linear rigid rotor> approximation, $r$ remains fixed while the <molecule> rotates.
\Image[/past-exam-of-the-chemistry-a-course-of-the-university-of-cambridge/2021/paper-1-carbon-monosulfide-rotor.png]
{title=Original <carbon monosulfide> rotor diagram showing masses, bond length and distances from the <center of mass>}
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