grammatical number - Why is physics pluralised? Why do some countries use 's' and others 'z' in some words?

Possible Duplicates:
***zation vs ***sation?
What is the difference between a “singular noun” and a “plural noun treated as singular”?

My question came from looking at this one Why is "math" always pluralized in British English and singular in American English?. Why are words like Mathematics and Physics pluralised? On a side note i just noticied that i used an 's' instead of a 'z' in pluralised. Why do some countries use 's' and others 'z' in some words?