1. The problem statement, all variables and given/known data An 8.0kg box slides a distance of 4.0m down a plane that is inclined at 30.0 degrees with the horizontal. The initial speed of the box is 2.5m/s, and the coefficient of kinetic friction between the box and the plane is 0.24. Calculate the final speed of the box. 2. Relevant equations Fg=mg Fn=mgcos(theta)? This is what my question is about Ff=mu(Fn) 3. The attempt at a solution I have solved for Fg, but my question is how do I know when to use sine and when to use cosine in the equation to find Fn? I have noticed sometimes I get the correct answer with one and sometimes I get the correct answer with the other. How do I know when to use which?