in a TPS when is the best time to use a physics engine and for what general purposes. I'm devloping and I was wondering if i should create the game and then impl;iment a physics engine or should i be implimenting it as I write the initial code. Also can anyone tell me of any problems i might run into using jmephysics2 as I understand its not fully written in java and based on ODE.
I really depends on what you want. I think you need to ask yourself, "What do I gain by using a physics engine?". Are there aspects of your game that that require complex collision detection or interaction between objects that you cannot or do not want to simply approximate?
Most character movements can be handled using that standard ways of terrain following and checking for collisions while moving. But there are definite situations where a physics engine is the way to go.
Ask us more specific questions like "I want some boulders to roll down a hill and bounce off things. Is this best with a physics engine?". The answer is probably YES. "I don't want my characters to be able to walk through walls. should I use physics for this?" Probably not, because jME already provides the necessary information and with a little logic there is no need for physics, unless you already have it in use in other ways.
Throw us some specific ideas you might have, and we will be able to give you more meaningful advice.