1. The problem statement, all variables and given/known data You are lowering two boxes, one on top of the other, down the ramp shown in the figure by pulling on a rope parallel to the surface of the ramp. Both boxes move together at a constant speed of 19.0cm/s . The coefficient of kinetic friction between the ramp and the lower box is 0.435, and the coefficient of static friction between the two boxes is 0.756. There is a rope attached to the lower box and you are standing higher up the ramp holding the rope tugging on the box. The length of the ramp is 4.75m and height is 2.5m The top box is 32kg the bottom one is 48 kg. This is Mastering Physics Homework. Questions asked: What force do you need to exert to accomplish this? <------please answer this one 1st What is the magnitude of the friction force on the upper box? What is the direction of the friction force on the upper box? 2. Relevant equations F=ma Ffriction=mu*Fnormal 3. The attempt at a solution SO I calculated the angle of the ramp from the horizontal to be 27.76 degrees. I do: Net Force X = -80*9.8*sin(angle) + Fpull + Fn*.756 = 0 (1) Net Force Y = Fn - 80*9.8cos(angle) = 0 (2) I solved through the 2nd equation to find Fn to be 693.77 N I solve for Fpull to be -365.16 N and I plugged in 365 but it is wrong, and the negative sign usually is not the problem for Mastering Physics, but I know if I get a negative sign I'm doing something wrong. What am I doing wrong?