Good advice above from MAJOROFSTLO
Physics is a great major that can provide you opportunities for graduate school to further delve into physics or cross over into engineering. The military likes a STEM education because technology is integral to the mission, and a STEM education teaches a structured way of thinking and problem solving.
Keep in mind that if you commission and go on active duty either within the AF or the Army, it is unlikely (unless you branch into the Army Corps of Engineers, or become an AF engineer with an engineering AF specialty code and go to Wright-Patt or the AF Research Lab) you will get to directly exercise your degree. If you go directly into weapon system acquisition, you can use your physics or engineering degree, albeit in an indirect manner.
For the Army, your major task as a 2LT will most likely will be leading people and employing tactics (infantry, armor, artillery, etc.). In the AF, you could be leading people (Aircraft maintainers, cops, etc.), but you will most likely be learning how to implement your weapon system (aircraft, UAV, missiles, AWACs, etc.).