AS physics is not for the faint hearted... you've gotta enjoy physics and maths because so far, that's all the first module is (mechanics), unit two is more theory type stuff.
To be honest, all the A-levels are harder at AS compared with GCSE! Teachers, older students etc will tell you that it is a "big jump" and I didnt really believe it, but it really is, in everything.
The same with Bio, and Chem, its much harder at AS, and you begin to realise how much easier GCSE was only once you have completed them... its one of those things that you have to discover for yourself!
I've taken Bio chem phys math and english lit, (im at AS) so i know how the difficulty increases! Make sure you choose what 'you' are interested in, otherwise studying for a subject that you dont like becomes 100 times worse because of the added difficulty!
Hope this helps
Xe