Actually, sophomore year is the year where you can sort of go by unnoticed. AP physics is a hard class for most people. I wouldn't take the low grade too seriously though- the school year is just getting started. Most likely, there are not very many grades in the gradebook yet. Once there are more assignments, your grade will even out. If it doesn't, I would talk to your teacher. They will probably be more than happy to meet with you to figure out what isn't working for you and how you can get back on top of things. As long as you go in with an open mind and make it clear that you want to work with the teacher to make a plan and you will see it through, I can't think of a teacher who wouldn't agree to help you