Calculate the friction force on an object on a flat surface, based on its mass and the coefficient of friction.
On a flat, horizontal surface, the normal force (N) equals the weight of the object: N = m × g. The friction force (F) is then proportional to the normal force:
The coefficient of friction μ depends on which two materials are in contact, not on the contact area. Some typical approximate values: rubber on dry asphalt ≈ 0.7–0.9, steel on steel ≈ 0.5–0.8, ice on ice ≈ 0.03–0.1, wood on wood ≈ 0.2–0.5. Static friction (before anything starts moving) is usually a bit higher than kinetic/dynamic friction (while something is sliding).
No, in the simplified model used here, friction force only depends on the normal force and the coefficient of friction — not on how large an area is in contact. This is a good approximation for most solid materials.
Static friction acts on an object at rest and must be overcome before motion starts — it's usually a bit higher. Kinetic (dynamic) friction acts on an object already sliding, and is usually somewhat lower than the static value.
Coefficients of friction vary with surface condition (moisture, wear, temperature) and are best found in tables for your specific materials, or through your own measurements. The values in this article are only rough typical examples.