@Override
    public void physicsTick(PhysicsSpace space, float tpf) {
        rigidBody.getLinearVelocity(velocity);
    }
Callback from Bullet, invoked just after the physics has been stepped.

@param space the space that was just stepped (not null)
@param tpf the time per physics step (in seconds, &ge;0)
jme3-jbullet/src/main/java/com/jme3/bullet/control/BetterCharacterControl.java