In the context of kinetic friction between two solid surfaces, which method will most directly reduce the kinetic frictional force $f_k$ for a given normal force $N$?

• A. Apply a lubricant between the surfaces to reduce the coefficient of kinetic friction ${\mu }_k$.
• B. Increase the normal force $N$ by pressing the surfaces together more strongly.
• C. Increase the contact area while keeping the materials and normal force the same.
• D. Increase the roughness of both surfaces to increase mechanical interlocking.

Answer: (A)

Verified step by step guidance

1. Recall the formula for kinetic frictional force: \(f_{k} = \mu_{k} N\), where \(f_{k}\) is the kinetic frictional force, \(\mu_{k}\) is the coefficient of kinetic friction, and \(N\) is the normal force.
2. Understand that for a given normal force \(N\), the kinetic frictional force \(f_{k}\) depends directly on the coefficient of kinetic friction \(\mu_{k}\).
3. Analyze how each method affects \(\mu_{k}\) or \(N\): applying a lubricant reduces \(\mu_{k}\), increasing normal force \(N\) increases \(f_{k}\), increasing contact area does not affect \(f_{k}\) significantly in kinetic friction, and increasing roughness tends to increase \(\mu_{k}\).
4. Conclude that the most direct way to reduce \(f_{k}\) for a fixed \(N\) is to reduce \(\mu_{k}\) by applying a lubricant between the surfaces.
5. Therefore, applying a lubricant lowers the coefficient of kinetic friction, which directly reduces the kinetic frictional force without changing the normal force.