1. I have a spacecraft hovering over the Earth. Therefore the weight of the spacecraft is cancelled by the thrust, i.e. the spaceraft is at rest in respect to the surface of the planet. Since there's no net force, there's no centripetal force, and the ship should stop moving in a circular trajectory (along with the Earth's rotation) and go off on a tangent. But that does not happen in reality. Why? 2. An elevator lifts a ball at a constant velocity. Therefore, there's no net force on the ball, or the velocity wouldn't be constant. However, the ball is gaining energy (potential energy) since it gains height, so there is work being done on the ball. If the net force is zero, what force is doing the work? If you don't know the answer don't post.