Physics: the science behind every other science

Your biology teacher explains cells. Your chemistry teacher explains reactions. Ask either one why a cell membrane lets some molecules through and not others, and the answer quietly turns into physics: forces, energy, motion.

That is not an accident. For centuries, all careful thinking about nature was one subject: natural philosophy. As knowledge grew, it split into living things (biology) and non-living things (physical sciences) — and physics sits at the base of the second half.

So what exactly is physics? One definition: the study of matter and energy, and the relationship between them. That single sentence covers motion, space, time, forces holding materials together, and how radiation interacts with matter.