Final answer:
The correct answer is option (d), indicating that the frictional force during sliding can be equal to or less than the product of the coefficient of kinetic friction and the normal force, depending on the specific situation and forces involved.
Step-by-step explanation:
When two objects slide past one another, the force of friction between them can be understood in terms of static and kinetic friction. Static friction is present when objects are not in motion relative to one another, and its magnitude is given by fs ≤ μ_s N, where μ_s is the coefficient of static friction and N is the normal force. In the state of motion, we deal with kinetic friction, which is less than static friction and is described by fx k = μ_k N, with μ_k being the coefficient of kinetic friction.
The correct answer to the question is (d) The frictional force is determined by other forces on the objects so it can be either equal to or less than μ_k N. The frictional force experienced by objects sliding past one another is not fixed: it can be either less than or exactly equal to the maximum value given by the coefficient of kinetic friction times the normal force (μ_k N), depending on the particular circumstances and forces acting at that moment.