It is late at night here and maybe your question will make more sense in the morning but what is z?
Also,
This is a physics question, it is probably a bit too advanced for this forum.
I would suggest that you post it in
physicshelpforum.com
This is a new forum, it is a spin off from
mathshelpforum.com
I have found both these forums to be full of very nice and knowledgable people.
They are likely to ask you what you have tried before they will help you. (They don't want to do your homework for you - they want to help you to understand)
I hope this information is useful to you.
+4 
+118654 
