Forces

How are forces computed with TimeStep?

Does it just does something like
F=ma
v=a.TimeStep+Oldv

Or does it do something more complicated?

If I apply a force every loop,must I include Time in the strength of my force in order to have the same results with differents computers?(I am using ogre as time-controller,so there is no fixed time step).

No there are various NxForceModes which you can use.

Thanks!So if i understand well,i should put this:
addForce(vector*t,NX_FORCE)
or
addForce(vector,NX_IMPULSE)
in order to get the same result with 2 computers running at different speed. Am I right?

Edit:the first one seems wrong to me,as t is the time from the loop.

The physics engine is designed so that the simulation will run at the same speed on any machine, regardless of framerate.

If you want to apply a force every frame, you should use NX_SMOOTH_IMPULSE (see below).

Paraphrase of the PhysX docs:

NX_FORCE has units of mass * distance / time^2
NX_IMPULSE has units of mass * distance / time
NX_VELOCITY_CHANGE has units of distance / time (so this effect is mass independent)
NX_SMOOTH_IMPULSE is a NX_IMPULSE but has the effect applied over all substeps. This is to be used for motion controllers that repeatedly apply an impulse.
NX_SMOOTH_VELOCITY_CHANGE is the same as NX_VELOCITY_CHANGE but the effect is applied over all substeps.
NX_ACCELERATION has units of distance / time^2, ie acceleration.

What exactly is substep? I have a general idea of what it is,but i don't know precisely.