This is not a problem but more of a question, and I just don't understand why this is so... There is a book placed on top of a car and the car drives off. What are the two horizontal forces acting on the car? Now, I understand that acceleration is one of the forces but, how can garvity be considered a horizontal force??? I thought if it pulls things down it is a vertical force???? I know that I'm in college but I think nature forgot to provide me with common sense. please help. Thank you.