1. The problem statement, all variables and given/known data A 2800-kg space vehicle, initially at rest, falls vertically from a height of 3300km above the Earth's surface. Determine how much work is done by the force of gravity in bringing the vehicle to the Earth's surface. **I read my textbook's method and I got confused. Please take a look at my questions at the bottom of this post. 2. Relevant equations See part three. 3. The attempt at a solution Here is my textbook's method. I am confused: 1) Why do we have to use the negative vector form of the universal law of gravity? 2) The distance changes from 3300 km to 0 km (Earth's surface). All of the numbers between 0 and 3300 are positive, so is that why ##d \vec l ## is positive? 3) Where did ##d \vec l = dr \vec r ## come from? 4) During the integration ##\vec r ## vanishes. Why is that? Thank you.