I am doing a school project on the physics behind motion detectors. I have done some research but only found some info on "Smart detectors." Can anyone help me?? I have ordered a motion detector kit, and must put it together, but I also am required to know the science behind motion detection...

There isn't really much physics behind it. Most of the work is in the interpretation of the results once you've got it.

Most systems will work by bouncing radiation or sound off a scene and reading it into a sensor, or using the radiation that's already out there. Then it's up to software to interpret this to see if something is moving within it's error margins.

If you can shrink down your question a little bit, maybe I can give you a better answer.