Causes of Friction
by Ron Kurtus (revised 29 September 2016)
Friction is a force that resists the relative motion between two objects or materials. The causes of this resistive force are molecular adhesion, surface roughness, and deformations.
Adhesion is the molecular force resulting when two materials are brought into close contact with each other. Trying to slide objects against each other requires breaking these adhesive bonds. For years, scientists thought that friction was caused only by surface roughness, but recent studies have shown that it is actually a result of adhesive forces between the materials.
But surface roughness is a factor when the materials are rough enough to cause serious abrasion. This is called the sandpaper effect.
When one or both of the materials is relatively soft, much of the resistance to movement is caused by deformations of the objects or by a plowing effect.
Questions you may have include:
- How does adhesion cause friction?
- How does surface roughness cause friction?
- How do deformations cause friction?
This lesson will answer those questions. Useful tool: Units Conversion
When two objects are brought into contact, many atoms or molecules from one object are in such close proximity to those in the other object that molecular or electromagnetic forces attract the molecules of the two materials together. This force is called adhesion. Trying to slide one object across the other requires breaking these adhesive bonds. Adhesion is the essence of friction.
You've seen a water drop adhere to a window pane. The force of friction prevents this liquid from sliding down the solid material. But most cases of friction you see concern a solid object sliding or moving against another solid.
Sliding objects against each other requires breaking these millions of contact points where the adhesion force takes effect, only to result in millions of new contact points of adhesion.
Some solid materials may have a composition that greatly increases their adhesion and makes them even "sticky" to the touch. This stickiness greatly increases the fiction. Rubber and adhesive tape are examples of sticky materials that have this type of friction.
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.
All solid materials have some degree of surface roughness. 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.
Close-up view of surface roughness
At one time it was thought that the surface roughness of materials was the cause for friction. In reality, it only has a small effect on friction for most materials.
If the surfaces of two hard solids are extremely rough, the high points or asperities can interfere with sliding and cause friction because of the abrasion or wear that can take place when you slide one object against the other. This is the "sandpaper effect" where particles of the materials are dislodged from their surfaces. In such a case, the friction is caused by surface roughness, although the adhesion effect still plays a part in the abrasion.
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.
The causes of the resistive force of friction are molecular adhesion, surface roughness, and the plowing effect.
Adhesion is the molecular force resulting when two materials are brought into close contact with each other. Surface roughness is a factor in friction when the materials are rough enough to cause serious abrasion. When one or more of the materials is relatively soft, much of the resistance to movement is caused by deformations or a plowing effect.
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Resources and references
Friction Resources - Extensive list
Friction Concepts - HyperPhysics
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Causes of Friction