1. The problem statement, all variables and given/known data We recently performed an experiment to determine the static and kinetic coefficients of friction by moving a box along a board. In some instances we used a pulley and weights to find the static and kinetic coefficient of friction and in the last part we inclined the board and gave a slight push until the box slid down at a constant speed. At the end of the experiment, he's asked us three questions. I've got answers to two of the three (could someone look at those and give critique, or suggestions for addition?) The first question is the one I'm having a little trouble with. 3. The attempt at a solution Why is it necessary for the string to be level in parts one and two? I'm guessing if the pull (string) is horizontal, the normal force would then equal the weight of the box. But I don't know what happens exactly if the string isn't level... Why should the coefficient of kinetic friction be less than the static one? It takes more force to overcome friction when the object is at rest than it does to keep the object moving. Also, when an object is at rest the two surfaces have a chance to fully interlock on the microscopic level. When the object is moving, only macroscopic irregularities can interconnect. What are other sources of error? One source of error is that we may have been accelerating the box, rather than applying a force only sufficient to overcome friction. Despite observing what seemed to be constant motion, the box may have actually been accelerating. Another source of error is the lack of small incremental weights. We had to use larger weights and in most instances we think we were only accurate +/-10g. This introduces a pretty significant margin of error. We also ignore the contribution of friction by the pulley and the string.