Friction Force opposes the direction of motion for an object sliding on a surface. It acts parallel to a surface.
Friction can also act on a stationary object on a slanted surface to prevent it from sliding down the incline.
FF = μFN
Where:
FF = Friction Force (Newtons)
μ = Greek letter “mu” = friction coefficient (dimensionless)
FN = Normal Force (Newtons)
Normal Force is the force of a surface pushing back against a force, perpendicular (normal) to the surface.
According to this equation, friction depends on two things:
1) FN – How hard the object is pressing against the surface (normal force is the reaction against this force)
2) μ – the particular combination of surfaces and how rough they are. The value of μ (friction coefficient) is different for each combination for surfaces.
Flat objects sliding on ice can have friction coefficients in the range of 0.02 to 0.05. On the other end, rough rubber tread on carpet could have a friction coefficient in the range of 0.90 to 1.30.
Many friction coefficients for every day situations are somewhere in the middle of this range. For example, rubber on dry asphalt is around 0.70. A smooth wood block on a smooth table is around 0.20.
Notice that for simple situations, surface area does not affect the friction force. There is no area term in the equation. If you concentrate a weight’s force into a small area, that does increase the pressure (which equals force divided by area), but the overall amount of force is still the same amount, and the friction force ends up the same.
However, many real life situations are more complicated, and for those situations surface area can make a difference. For example, if surfaces can interlock in complex ways (hook and loop fastener), surface area will matter. Car tires are also more complex since they have pressurized air inside, and the tires can flex, so surface area can have an impact in that case as well.