Since China hosts and participates in IPhO, the official IPhO archive includes Chinese problems from years when China was the host (e.g., 1994 Beijing, 2000 Leicester, 2020 virtual). These are authoritative PDFs with full solutions.
One evening, he faced a notorious problem from the involving a sphere rolling down a moving hemisphere. The equations were a delicate dance of horizontal momentum conservation and non-slipping conditions: chinese+physics+olympiad+problems+pdf+top