I'm using my teacher's study guide to study for a test, and he has the answers on it. I think he's just wrong with one. Here's the question: A book weighing 20. N slides at a constant velocity down a ramp inclined at 30 degrees to the horizontal. What is the force of friction between the book and the ramp. I know the force of friction has to be equal to the force of the book sliding down the ramp, since it's sliding with a constant velocity. As far as I know, all I should have to do is sin30 = 20N/x That gives me 40N. The answers say 10N is the FF between the book and ramp. Did my teacher just mess up?