dont pannick dude. just think. what are your other grades, if they are still like high, you should be okay. Also look at the weighting of the units, if the module with the U is a small percentage it shouldnt matter too much to your overall grade. also, look around at other courses, this can help you decide if physics is really for you. When you go back to college or whatever have a word with your physics tutor. but mainly DONT PANICK