Which factor generally increases friction between two surfaces in contact?

• A. Higher speed
• B. Lower mass
• C. Smoother surfaces
• D. Rougher surfaces

Answer: (D)

Friction is mainly influenced by how rough the contacting surfaces are. Rougher surfaces have many tiny bumps and irregularities that catch on each other and create more resistance as you try to slide one past the other. Those microscopic contact points also increase the actual area where the surfaces touch, adding more forces that oppose motion. Because of this, rough textures require more force to keep things sliding, so friction goes up.

Higher speed doesn’t inherently increase friction in most everyday cases, since kinetic friction is roughly constant with speed. Lower mass reduces the normal force pushing the surfaces together, which usually lowers friction. Smoother surfaces have fewer interlocking asperities, so they tend to produce less friction.