1. Normal Force (N): The normal force is the perpendicular force pressing one surface against the other. The greater the normal force, the greater the friction force.

2. Coefficient of Friction (μ): The coefficient of friction is a material property that quantifies the resistance to motion between two surfaces. It depends on the materials and surface characteristics and can be static (μ_s) for stationary objects or kinetic (μ_k) for objects in motion.

3. Surface Roughness: The roughness or smoothness of the surfaces also affects friction. Rougher surfaces have more microscopic irregularities, creating higher resistance to motion and thus greater friction.

4. Lubrication: The presence of lubricants, such as oils or greases, between two surfaces reduces friction by forming a protective layer that allows surfaces to slide more easily.

5. Temperature: In some cases, temperature can affect the coefficient of friction. For instance, the friction between rubber and pavement decreases as the temperature rises.

6. Humidity: For porous materials, such as wood, the moisture content can influence friction, with higher humidity generally leading to reduced friction.

7. Surface Contamination: The presence of dirt, dust, or other particles between surfaces can increase friction by acting as obstacles that hinder smooth movement.

These factors collectively determine the magnitude of the friction force, often expressed as F_friction = μN, where μ is the coefficient of friction and N is the normal force. Understanding and controlling these factors is essential in various engineering and everyday applications, from designing braking systems to optimizing machinery efficiency.