In the context of kinetic friction between two sliding solid surfaces, which method is most likely to reduce the kinetic friction force?

• A. Roughen both surfaces by sanding them
• B. Apply a lubricant (e.g., oil) between the surfaces
• C. Increase the contact area while keeping the normal force the same
• D. Increase the normal force pressing the surfaces together

Answer: (B)

Verified step by step guidance

1. Understand that kinetic friction force \(F_k\) is given by the formula \(F_k = \mu_k N\), where \(\mu_k\) is the coefficient of kinetic friction and \(N\) is the normal force pressing the two surfaces together.
2. Recognize that roughening both surfaces by sanding them typically increases the coefficient of kinetic friction \(\mu_k\), which would increase the friction force rather than reduce it.
3. Note that increasing the contact area does not affect the kinetic friction force significantly because friction depends primarily on the normal force and the coefficient of friction, not on the contact area.
4. Understand that increasing the normal force \(N\) will increase the kinetic friction force since \(F_k\) is directly proportional to \(N\).
5. Applying a lubricant between the surfaces reduces the coefficient of kinetic friction \(\mu_k\) by creating a thin layer that allows the surfaces to slide more easily, thus reducing the kinetic friction force.