A teacher got switched from teaching chemistry, in which she is a subject matter expert, to teaching AP Physics 1, in which she is emphatically NOT expert. She is well aware that students in high-level courses like this tend to be "students who care," and she fears also that such students "can tell when a teacher has no idea what they are doing." This teacher is quite worried about the high-stakes of the AP Physics 1 exam; she feels behind already, like she can't give her students the education they deserve.
On one hand, it is entirely unfair - to this teacher AND to her students - for the district to put them in this situation. "Here, Greg, teach AP art and design!" Yeah, right. Not gonna go well.
However, for all kinds of systemic reasons, an enormous number of science teachers are forced into this same situation every year. I meet them in my summer institutes. What advice can I give?
Look... you can't fake physics. Students do and will know that you're not an expert. But, and this is important... THAT'S OKAY. I've had plenty of teachers just like you in the APSIs that I run, and they do very well - as measured over a three year period.
The suggestion is, be open and honest with your students that you are learning AP physics alongside them. Do every assignment with them. Put yourself in lab groups with them. You can use my tests - and take them yourself, with the students. Use the AP Classroom personal progress checks, just like the students do. Don't be the authority figure, because students will rebel. Instead, be the captain of their team.
Your goal this year should not be to know anything about physics now, or next week; but instead, to get a 5 on the AP Physics 1 exam in May, if you were allowed to take it. :-)
Then, have a three year outlook. That's how long it takes to become comfortable teaching AP Physics. You are not doing your students a disservice - you're giving them the education they deserve, because they deserve an ally who will do whatever it takes to learn physics alongside them. You have to learn sometime. And if this year's class gets the less-than-perfect you, so what, because you have a long career ahead of you; if you have a couple of rough years followed by decades of hard-earned expertise, well, you're serving the next generation more than well.
The students will respond to your humility, earnestness, and hard work. Okay, well, MOST of them will. That's okay, 'cause after 24 years I don't always get every student to respond to me. I have to be like a football cornerback, who usually prevents touchdowns, who cares deeply about preventing touchdowns... but who has to be able to recover instantly when he gives one up.
Oh, and do make extensive use of the AP Physics 1 workbook, available through your course audit. It's designed explicitly for people in your situation. You have students (and yourself) working through that, you'll be pointed in the right direction. I'd also say get the 5 Steps to a 5: AP Physics 1 book, but that'd be self-promotion and I don't do that, at least not very often. :-)