looking back at a problem that puzzled the heck out of me. integrate this: dv/(v-vt) = -k/m (dt). The book says you get: ln ( (vt-v)/Vt ) = -k/m t the -k/mt part is fine, because you integrate from 0 to t. The ln part is tricky though. When I integrate, I get ln(v-vt), vt is just a constant. If you do the derivative of my anwser or the books they are the same. How did the book arrive at this anwser and not mine.