How does physics relate to the skeletal system?
The skeleton is a mechanical system of complex joints, beams, pivots and axes, the forces through which can be calculated with the laws of physics.
Obviously all sciences interrelate: physics leads to chemistry, chemistry leads to biology, and biology, arguably, leads to psychology, sociology and the social sciences. Therefore without physics the biological skeletal system couldn't exist, although this is quite a broad answer.
More specifically, you can view the skeletal system as a mechanical series of joints, beams, pivots, axes and so on, with forces acting in certain directions from the contraction of muscles which move bones and project the rest of the skeleton in a certain direction.
The skeleton is, or can be viewed as, a mathematical, though complex, system. Each movement and force can be calculated with trigonometry and vectors, tools used with and in physics.