I took an intro Biology course last semester, so I know how you feel. The Department Of Education suggests that you spend 2 hours studying for every credit hour that you're taking, so a 4 credit physics course would be 8 hours a week. My Bio professor said that we should have 40 hours of study time in before every test that we had. Yes, that's right, 40 hours!
I made notecards for the study guide that he gave to us, got tutored for 2 hours a week and ended up with a B. I would suggest requesting tutoring services through Student Support Services (assuming that your college has them), and put more study time into the class since you're having trouble (as did I), and ask for help from your professor as much as possible.
Do you have to take Physics? I know most students take Bio over a "hard science" like Physics because they find it to be easier. Physics requires a lot of memorization of forumlas, which I'm terrible at!
So in conclusion, 7 hours of 'off and on' study time probably isn't adequate for a course that demands more attention like Physics. I had to study my ass off for a B. Good luck with your classes, what you're doing is very inspiring!