Friction is a fundamental force that affects our everyday lives. When we slide a book across a table or walk on a slippery surface, friction comes into play. There are various laws which govern the phenomenon of friction such as Amonton’s and Columnb’s Laws. In this article, we will discuss all the laws which help us understand the concept of Friction.

What is Friction?

The force that opposes the relative motion or tendency of two surfaces in contact is known as friction.

There are several types of friction that result from the interactions between the microscopic irregularities of the surfaces i.e.,

• Static Friction
• Kinetic Friction
• Rolling Friction
• Fluid Friction
• Dry Friction

Note: The roughness, composition, and force forcing the surfaces together determine how much friction there will be.

Read More about Types of Friction.

Formula for Friction

As we know, friction is a force that opposes moving one surface in contact with another or the tendency for something to move. The formula for calculating the force of friction is:

F = μN

Where,

• F is the friction force,
• μ is the coefficient of friction, and
• N is the normal force.

What are Laws of Frictions?

The laws of friction explain how friction acts in relation to normal force and motion. There are five main laws of friction, which include:

• The First Law of Friction: Friction is proportional and perpendicular to the normal force acting on an object.
• The Second Law of Friction: The force of friction is independent of the apparent area of contact between the surfaces.
• The Third Law of Friction: The force of friction is independent of the sliding velocity between the surfaces.
• The Fourth Law of Friction: The force of friction is independent of the direction of sliding.
• The Fifth Law of Friction: The force of friction is directly proportional to the normal force acting on an object.

Amonton’s Law of Friction

Amonton’s Law of Friction are named after the French physicist Guillaume Amontons, who discovered these laws. There are two laws given by Amontons i.e.

• Amonton’s First Law
• Amonton’s Second Law

Let’s discuss these laws in detail as follows.

Amonton’s First Law

The Amontons’ First Law, sometimes known as the Amontons’ Friction Law, is one of the basic theories governing frictional forces. It states that the friction force is directly impacted by the applied normal force. In terms of math, it can be expressed as:

F ∝ N

Where:

• F = The resistive force, or friction, and
• N = is the amount of force an average person would apply to the floor or a push-up board during a push-up.

Amonton’s Second Law

Amonton also put up another important theory regarding friction, which is known as the Angle of Friction or the Second Law of Amontons. The frictional force, according to this equation, is directly determined by the normal force between the bodies rather than by the apparent area of contact between surfaces. The expression in mathematics is as follows:

F ∝ tan(θ)

Where,

• F It is friction that opposes your moving forward, and
• θ is the angle of intersection of the surfaces.

Some Other Laws

Other than these laws, there are some more laws which help us govern the concept of friction. These are:

• Coulomb’s Law of Friction
• Law of Static Friction
• Law of Kinetic Friction

Coulomb’s Law of Friction

Coulomb’s Law of Friction, formulated by Charles-Augustin de Coulomb, describes the relationship between the frictional force and the normal force acting between two surfaces in contact. It is the extension of Amontons’ First Law as it it introduces the constant of proportionality i.e., μ. Mathematically, coulomb’s law of friction is given as follows:

F_f = μ . N

Where,

• F_f is the force of friction,
• μ is the coefficient of friction, and
• N is the normal force.

Law of Static Friction

The maximum static friction force ([Tex]F_{f_s}[/Tex]) that can exist between two stationary surfaces is directly proportional to the normal force (F_N​) acting perpendicular to the surfaces. The coefficient of static friction (μ_s​) represents the proportionality constant. Mathematically, [Tex]F_{f_s}[/Tex] ≤ μ_s​⋅F_N.

The static friction force opposes the impending motion or attempted motion between the surfaces. It acts tangentially to the surfaces in contact.

Law of Kinetic Friction

Kinetic friction force ([Tex]F_{f_k}[/Tex]​​) is directly proportional to the normal force (F_N​), which is the force perpendicular to the surfaces in contact. The coefficient of kinetic friction (μ_k​) represents the proportionality constant. Thus, [Tex]F_{f_k}[/Tex] =μ_k​⋅F_N.

The kinetic friction laws describe that the friction force acting against the motion of an object is dependent upon factors such as the roughness of the surface and the magnitude of the normal force.

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