A block is projected up a frictionless plane with initial speed v0 = 3.50 m/s. The angle of incline is theta = 32 degrees. (a) How far up the plane does the block go? (b) How long does it take to get there? (c) What is its speed when it bets back to the bottom? I read through the solution to this problem, but there are a few things I don't understand: (1) Why did they use the positive x direction to be down the incline? (2) It said the positive x direction is in the direction of the acceleration. How is acceleration down when the problem is talking about the block moving up the plane? (3) It said the x component of Newton's second law is mg sin theta = ma . I don't understand how they got this.