1. The problem statement, all variables and given/known data A constant external force P = 160 N (diagonally with an angle of 30) is applied to a 20-kg box, which is on a rough horizontal surface. The force pushes the box a distance of 8.0 m, in a time interval of 4.0 s, and the speed changes from V1 = 0.5 m/s to V2 = 2.6 m/s. The work done by friction is closest to: A) 1040 J B) -1040 J C) -1170 J D) 1110 J E) 1170 J 2. Relevant equations W = F*d 3. The attempt at a solution I made a free-body diagram, but I am confused on how to find the frictional force to plug into the work equation. So far, I have F_P = 160cos(30°), and F_P - f = 20a. a = (2.6-0.5)/4 = 0.525 m/s^2, so 112.58 - f = 20(0.525) and f = 102.08. W = (102.08)(8) = 816.64 J, but none of those are on the answers, so where did I go terribly wrong?