Causes of Friction

Friction is a force resisting motion of an object when in contact with another. This resistive force is caused by the surface roughness of the contact area of the materials, molecular attraction or adhesion between materials, and deformations in the materials. The cause of friction may be any or all of these items and this applies to sliding, rolling and fluid frictions.

Questions you may have about friction include:

• How does surface roughness cause friction?

• How do deformations cause friction?

• How does molecular attraction cause friction?

Surface roughness

Most friction results because the surfaces of materials being rubbed together are not completely smooth. If you looked at what seems to be a smooth surface under a high-powered microscope, you would see bumps, hills and valleys that could interfere with sliding motion. Of course, the rougher the surface, the more the friction.

Close-up view of surface roughness

Treads add to friction

Treads or grooves on one or both sliding surfaces can increase the friction, especially if the treads have sharp edges and are not parallel with the line of motion. The most common use if treads are seen in automobile and bicycle tires, as used in rolling friction. You also may see them on pads intended to keep surfaces from sliding.

Sharp edges of treads add to sliding friction

The number and types of grooves or treads is an added factor to the friction equation.

Molecular adhesion

Another factor in friction can be caused by molecular adhesion or attraction. Ultra-smooth materials and "sticky" materials fall in this category.

Ultra-smooth

If both surfaces are ultra-smooth and flat, the friction from surface roughness becomes negligible, but then friction from molecular attraction comes into play. This can often become greater than friction if the surfaces where relatively rough.

Sticky materials

Rubber is an example of a material that can have friction caused by molecular attraction. Discounting resistance due to deformations with rubber, it is its stickiness factor that causes it to grip so well and have so much friction.

Fluids

Fluids often exhibit molecular adhesion, increasing the friction. This adhesion force is often seen in the capillary effect. This is where water will be pulled up a glass tube by the forces of molecular adhesion. That same force can slow down fluid motion.

One example is how a coin will easily slide down a ramp. But if you wet the coin, it will stay in place. That is because of the molecular friction of the fluid on the hard surfaces.

The motion of two fluids or two sections of a fluid against each other is also slowed down by the molecular attraction factor. This type of fluid friction is usually not considered as friction and is studied under the complex field of fluid dynamics.

Deformations

Soft materials will deform when under pressure. This also increased the resistance to motion. For example, when you stand on a rug, you sink in slightly, which causes resistance when you try to drag your feet along the rug's surface. Another example is how rubber tires flatten out at the area on contact with the road.

When materials deform, you must "plow" through to move, thus creating a resistive force.

Pushing object on soft surface

When the deformation becomes large, such that one object sinks into the other, streamlining can affect the friction, similar to what happens in fluid friction.

In conclusion

Friction is a force resisting motion of an object when in contact with another. It is caused by the surface roughness of the contact area of the materials, deformations or the molecular attraction. The standard friction equation holds for hard objects being held back by surface roughness.