In the context of kinetic friction between two sliding solid surfaces, which change will most directly reduce the kinetic frictional force?

• A. Increase the contact area while keeping the normal force the same
• B. Increase the normal force pressing the surfaces together
• C. Replace the surfaces with rougher materials (higher coefficient of kinetic friction)
• D. Apply a lubricant between the surfaces

Answer: (D)

Verified step by step guidance

1. Recall the formula for kinetic frictional force: \(F_k = \mu_k \times N\), where \(F_k\) is the kinetic frictional force, \(\mu_k\) is the coefficient of kinetic friction, and \(N\) is the normal force pressing the surfaces together.
2. Understand that the kinetic frictional force depends directly on the coefficient of kinetic friction and the normal force, but not on the contact area between the surfaces.
3. Analyze each option: Increasing the contact area does not change \(F_k\) because \(\mu_k\) and \(N\) remain the same; increasing the normal force \(N\) increases \(F_k\); using rougher materials increases \(\mu_k\), thus increasing \(F_k\).
4. Recognize that applying a lubricant reduces the coefficient of kinetic friction \(\mu_k\) by creating a thin layer that allows surfaces to slide more easily, thereby directly reducing \(F_k\).
5. Conclude that the most effective way to reduce kinetic frictional force is to decrease the coefficient of kinetic friction, which is achieved by applying a lubricant between the surfaces.