Which change would increase the frictional force between two contacting surfaces (assuming the coefficient of friction remains constant)?

Which change would increase the frictional force between two contacting surfaces (assuming the coefficient of friction remains constant)?

• A. Reducing the weight
• B. Increasing the normal force
• C. Using a lubricant
• D. Smoothing surfaces

Answer: (B)

Frictional force scales with how hard the surfaces are pressed together when the coefficient of friction is held constant. That pressing force is the normal force, which on a horizontal surface is basically the weight. So increasing weight pushes the surfaces together harder, raising the normal force and thus increasing friction: F_f = μ N, with μ fixed, means larger N gives larger friction. Reducing weight would lower friction, and changes like adding a lubricant or smoothing surfaces would typically change μ, which isn’t allowed here, so they wouldn’t increase friction under the given condition.

Frictional force scales with how hard the surfaces are pressed together when the coefficient of friction is held constant. That pressing force is the normal force, which on a horizontal surface is basically the weight. So increasing weight pushes the surfaces together harder, raising the normal force and thus increasing friction: F_f = μ N, with μ fixed, means larger N gives larger friction. Reducing weight would lower friction, and changes like adding a lubricant or smoothing surfaces would typically change μ, which isn’t allowed here, so they wouldn’t increase friction under the given condition.