What is friction?

Friction is the resistance to motion of one object moving relative to another. It is not a fundamental force, like gravity or electromagnetism. Instead, scientists believe it is the result of the electromagnetic attraction between charged particles in two touching surfaces.

In liquids, friction is the resistance between moving layers of a fluid, which is also known as viscosity. In general, more viscous fluids are thicker, so honey has more fluid friction than water.

The atoms inside a solid material can experience friction as well. For instance, if a solid block of metal gets compressed, all the atoms inside the material move, creating internal friction.

In nature, there are no completely frictionless environments: even in deep space, tiny particles of matter may interact, causing friction.

Causes of Friction

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.

Molecular adhesion

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.

Sticky materials

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

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.

Surface roughness

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.

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.

ADVANTAGES OF FRICTION

Friction plays a vital role in our daily life. Without friction we are handicap.

1. It is becomes difficult to walk on a slippery road due to low friction. When we move on ice, it     becomes difficult to walk due to low friction of ice.
2. We can not fix nail in the wood or wall if there is no friction. It is friction which holds the nail.
3. A horse can not pull a cart unless friction furnishes him a secure Foothold.

DISADVANTAGES OF FRICTION

Despite the fact that the friction is very important in our daily life, it also has some disadvantages like:

1. The main disadvantage of friction is that it produces heat in various parts of machines. In this way     some useful energy is wasted as heat energy.
2. Due to friction we have to exert more power in machines.
3. It opposes the motion.
4. Due to friction, noise is also produced in machines.
5. Due to friction, engines of automobiles consume more fuel which is a money loss.

METHODS OF REDUCING FRICTION

There are a number of methods to reduce friction in which some are discussed here.

USE OF LUBRICANTS: The parts of machines which are moving over one another must be properly lubricated by using oils and lubricants of suitable viscosity.

USE OF GREASE: Proper greasing between the sliding parts of machine reduces the friction.

USE OF BALL BEARING: In machines where possible, sliding friction can be replaced by rolling friction by using ball bearings.

DESIGN MODIFICATION: Friction can be reduced by changing the design of fast moving objects. The front of vehicles and airplanes made oblong to minimize friction.