1. The problem statement, all variables and given/known data The total force needed to drag a box at constant speed across a surface with coefficient of kinetic friction Uk is least when the force is applied at an angle theta such that a.sin(theta) = Uk. b.cos(theta) = Uk. c.tan(theta) = Uk. d.cot(theta) = Uk. e.sec(theta) = Uk. 2. Relevant equations F = ma Ff = Uk * Fn = m*g*Uk Fx = F *cos(theta) Fy = F*sin(theta) 3. The attempt at a solution First the answer is d. cot(theta) = Uk I tried to solve it, but couldn't figure it out... This is my work Since it is dragged at constant speed, the acceleration is 0. So a = Ftotal/m = 0 Ftotal = F*cos(theta) - (mg - F*sin(theta))Uk = 0 F*cos(theta) / (mg - F*sin(theta)) = Uk... which is wrong... Can anyone help me?