How can you increase friction between two surfaces?

• A. Increase velocity and lubrication.
• B. Increase the normal force or rougher surfaces/materials.
• C. Decrease contact area.
• D. Use a smoother material.

Answer: (B)

Friction in dry contact depends mainly on how hard the surfaces are pressed together and how rough the surfaces are. When you increase the normal force—that is, press them together more—there are more microscopic contact points that resist sliding, so the friction force grows. Rougher surfaces or materials that interlock more at the microscopic level raise the resistance to motion, increasing friction as well.

Lubrication lowers the friction coefficient, making it easier for surfaces to slide, so it’s not a way to increase friction. Increasing speed doesn’t reliably increase friction in the common model, where kinetic friction is roughly constant with speed. Decreasing the contact area doesn’t generally change friction in the standard dry-friction picture, so that approach isn’t effective for increasing friction.

So, pressing surfaces harder or using rougher materials are the effective ways to boost friction.