float speed = _vel*_couple;
        float friction = _fric;// * (1.f + speed*speed / 100.f);
        float p = speed*float(_size) * friction;
        if(p > 0.f) {
            p = std::min(p, _vel);
        } else {
            p = std::max(p, _vel);
        }
        _power += -p;