I'm not going to let my physics partner copy my stuff after I've turned it in
What is your reason for not letting your physics partner copy your stuff?
If my partner copies all the shit, I'm gonna get in trouble for letting him do it.
While it is wrong not to help someone out that was your partner, it is even more wrong to copy your stuff. You are right not to want him to get credit for your work, and I would have done the same thing to avoid any trouble.
What if I just copy his stuff instead? He's not very smart and I bet I could get away with it.
I am a little confused as to who was the one copying the work. Copying work either way is wrong, and you should both do your own work. It makes sense that you would not want someone else receiving the credit for the work you've done.