Five Laws of Friction

Friction is a force that resists rolling or sliding motion of objects over one another/or a surface: for example, developed traction due to friction that helps us to walk without slipping. However, automobiles use around 20 per cent of the engine power to overcome frictional forces between the moving parts. 

The main reason behind the cause of Friction between two metals is the force of attraction called adhesion. It occurs between the microscopically irregular contact regions of the surfaces. Frictional force mainly arises when the welded junctions shear off each other. Even when the irregularities of the harder surface plough across the softer Surface, Friction occurs. Let’s move ahead and explore the five laws of friction. 

History of Amonton’s Five Laws Of Friction

Guillaume Amonton was a physicist and an inventor of instruments. He is a renowned name in the fields of physics and chemistry. He is a pioneer theorist who did a detailed study on the concept of friction and presented the five laws of friction that the engineering community has widely accepted. 

Five Laws of Friction

Amonton’s Five Laws of Friction are as follows:

First Law

• “For any object under motion, the friction is proportional and perpendicular to normal load.”

• Fstatic α N

• Limiting force is basically static frictional force at its highest.

• According to the first law of friction, the frictional force increases with the increasing normal force.

• In the case of static friction, an object’s weight plays an important role in the normal reaction.

Second Law

“The surface characteristics an object comes in contact with determines the friction between them.” 

The nature of the contacting surfaces affects the coefficient of friction. For example, surfaces such as rubber on pavements have higher friction coefficient compared to ice on steel.

In the friction formula, the constant of proportionality is known as the coefficient of friction.

Fstatic α N

Fstatic = µfrictionN

Where µ = coefficient of friction

The coefficient of friction is dependent on the nature of the surface, which varies based on factors such as:

• Property of material.

• How rough or smooth the contact surface is.

• Contact surface’s dryness or wetness.

Third Law

“For any two objects moving across each other, the area of contact surface does not influence the Frictional force between them.”

The pressure of the similar mass objects counters the increasing contact region of the two entities. Thus, regardless of the object’s contact area, the friction force stays constant. 

For example, consider turning a box such that a minimal area of the box stays in contact with the floor. Changing the box’s orientation will not change the normal force required to move the box. The box’s weight remains constant, regardless of its orientation or how large or small the surface area is in contact. Thus, the normal force stays constant even though the contact area changes.

Fourth Law

“The kinetic friction between two dry objects is independent of the relative surface velocity.” Therefore, kinetic friction is independent of velocity.

The following factors affect Kinetic Friction.

• Normal force

• The friction coefficient for kinetic motion

Thus, there is no mention of the contact region’s relative speed.

Fifth Law

“The coefficient of static friction is more than the coefficient of kinetic friction.”

Consider two bodies in contact with each other. The laws of friction explain that if there is any motion between the two bodies in touch with each other, then there exists kinetic friction. It is given by the formula: 

fk = µk N

where,

fk = Kinetic Friction

µk = Coefficient of Kinetic Friction

N = Normal Force

Kinetic friction acts opposite to the relative velocity of the body. For example, if a body X moves over a body Y towards the right, the Kinetic Frictional Force acts on X towards the left.

However, when two bodies possess no motion relative to one another, the friction between them is static friction. Therefore, it is always less than or equal to the kinetic friction.

It is necessary to understand that the value of static friction is not constant like kinetic friction. For instance, suppose the maximal static frictional force for an object on a surface is 10 N. If we apply the standard force of 5 N, the object remains constant. Therefore, the static friction value remains at 5 N. However, when the applied force exceeds 10 N, the body begins to move, transforming the static friction into kinetic friction.

Until the Normal Reaction Force is constant, the static and kinetic frictional forces remain independent of the contact area.

The static friction obtains the highest value when the body is about to move. The friction at this point is known as Limiting Friction. It is usually a little higher than kinetic friction and is calculated as follows:

fmax = µs N

where,

fmax = Limiting friction

µs = Coefficient of Static Friction

N = Normal reaction force

Conclusion

Amonton’s five laws of friction represent the nature of reaction a body exerts when moved against the surface of another body. The frictional force applied by a body against another body is independent of velocity but is dependent on the nature of the surface. However, these laws of friction fail to explain friction force at the Nano level.