Why is physics useful for medicine?

Physics is a very powerful approach to understanding the world around us. One goal of the physics courses that I teach is to hone analytical skills through the rigorous approach to problem solving characteristic of physics.  Physics is good practice for developing the ability to tackle complex problems by making simplifying assumptions and systematically breaking these problems into smaller more manageable pieces. The subject matter itself is also relevant to medicine. Our bodies—our skeletons, muscles, organs, circulatory and nervous systems, the lenses in our eyes, and cochlea in our ears—all obey the laws of physics. Their motion and electrical properties are a direct application of the ideas of my courses, and I try to make these connections explicit whenever possible. The instruments used in medicine frequently originate in physics as well, for example, diagnostic imaging; like x-ray machines and MRI. 

See the following articles from Physics World (and links therein) for more on the interplay between physics and medicine:

The impact of physics on biology and medicine (written for the 100th anniversary of the American Physical Society), H. Varmus, Physics World, 9/3/99.
Physics in Medicine, Physics World, 11/1/98.