Hi, I'm new to these boards. I hope I posted in the right category, (I believe collage is equivalent with gymnasium in Norway) Here's the problem: A rocket is fired of from the earth. The rocket's engine stops 85km above the earth's surface, then the rocket has a vertical velocity of 3,8km/s. How far can the rocket go? (how high) I'm uncertain about the criterias here. The rocket keeps going til the velocity is 0, right? Does the rocket only have potential energi at this point? If I try and figure it out with this, I only get rubbish. In the answer the book hints that you should use Ep = -yMm/r. But you don't know the mass of the rocket... So I guess you should find another expression equaling Ep. Anyways, I'm stuck. Thanks for the help!