I've come to the conclusion that i don't understand at all the concept behind inertia. How can we know when an object stays at rest or at constant velocity? (take for example a girl in a rope. She would tighten her grip on the rope until the tension balances the force of gravity (assume rope is frictionless). This means that she would then have no net force acting on her. Does she stop or does she continue at constant velocity? Why is this for any answer?) On the other hand, if we have a rocket and we drop from say, an airplane (this airplane is going at constant velocity). When the rocket starts to fall in free-fall (ignore all other forces) does the rocket continue with a horizontal velocity of zero or does go at the same horizontal velocity of the plane? For any of the answers, why is this? (and also, if we make a generalization to most cases) Thank you very much for your patience.