Problem set solution updates

Hi everyone 

Apologies for the slight backlog in solutions -- they're all there now. I have also corrected a rather clumsy answer to P31, part (b) (in PS 9). 

I hope the solutions have in general been useful in helping you learn and understand a bit of GR. 

Also, exam season is now upon us, so here are some unsolicited pieces of advice for dealing with the weeks ahead, in particular when studying for GR:  

-- Get plenty of sleep; both in general and before the exam. It's a seriously underrated commodity, especially among physics students. It helps with literally everything. 

-- In a similar vein, don't forget to eat regularly and take walks (the weather in Norway isn't quite on your side with this one but at least it's not freezing).

-- If you aren't focusing or understanding something, take it easy on yourself. Exams really up the pressure you feel and focusing can sometimes be tough, and on top of that GR is hard. Some lack of motivation and ability to `do things' is expected, so don't sweat some dips in productivity. 

-- Just try your best, whatever that may be at the moment.

Good luck for studying and for all the exams ahead!

Jake