@Override
    public void prePhysicsTick(PhysicsSpace space, float f) {
    }
Callback from Bullet, invoked just before the physics is stepped. A good
time to clear/apply forces.

@param space the space that is about to be stepped (not null)
@param f the time per physics step (in seconds, &ge;0)
jme3-jbullet/src/main/java/com/jme3/bullet/BulletAppState.java