1. The problem statement, all variables and given/known data Which of the following statements about friction is correct? I. Kinetic friction always decreases the mechanical energy of a system. II. The static friction force cannot change the mechanical energy of a system. III. The force of friction always points in the direction opposite to the way that a system moves. None of the statements is correct. Statement III only Statements I & II only Statement II only Statements I & III only Statements II & III only Statement I only. All three statements. 2. Relevant equations W = F * D Mechanical Energy = KE + Ug 3. The attempt at a solution I cannot find the culprit. All appear to be correct and I cannot form any counter-examples. Kinetic friction is a non-conservative force, so it must decrease the mechanical energy of a system where friction is present. Static friction does not do work, so it must not affect the mechanical energy of a system. For the third; I cannot imagine a system in which friction points in the direction of motion. Kinetic friction is caused by this motion.