What are the two main factors on which the force of friction depends?

Answer 1

The two factors that come into play when dealing with friction between a body and a surface:

*The weight of the body, which is a function of the mass and the gravitational force said body experiences. Think about it. If you drag sandpaper across a surface, but you lift it up a bit as you drag it, it slides easier than if you let it slide naturally or push down on it. Lifting it up decreases the apparent wait the surface feels, and vice versa. Technically, it's the normal force, which is equal to the magnitude but opposite the direction, of the applied force, which is a composite of the weight and any push/pull force exerted, not the weight alone.

*The frictional coefficient of the two surfaces (K), which varies for each surface based on the microscopic "unevenness" of the two surfaces and whether or not they are stationary or in motion. E.g. Glass is fairly smooth. Meanwhile, sandpaper has many bumps. The more bumps, the more often the surfaces catch each other and the higher the frictional coefficient K between the two. These coefficients have been measured in the lab and there is a list of frictional coefficients for various interacting surfaces that can be found in textbooks and on the internet.

There are two types of frictional forces: static and kinetic.

The static force is the force applied to the object while it stays stationary, up to a maximum force beyond which the two bodies overcome it and the applied surface moves.

The kinetic force is the force the friction is applying to the two surfaces/bodies as the applied surface moves across the base surface, in the opposite direction of the direction of travel, known as displacement.

The two equations describing the maximum static and the kinetic frictional forces are nearly identical in the form:

F = u(Fn)

F is the maximum static or the kinetic force, depending on the context, u is the static or kinetic coefficients, as denoted by an "s" or "k" subscript, respectively, and Fn is the normal force.

Answer 2

The force of friction depends on the nature of the surfaces in contact and the force pressing the surfaces together. Rough surfaces and higher pressure between the surfaces typically result in greater frictional forces.