Static Friction

There are several theories regarding the causes of static friction, and like most friction-related concepts, each one proves valid under some conditions, but fails under other circumstances. For real-world applications (especially those related to industrial machinery and motion). Control the two most widely-accepted theories behind static friction have to do with the microscopic roughness of surfaces.

Regardless of how “perfectly” a surface is machined, finished, and cleaned, it will inevitably have asperities – essentially “roughness,” consisting of peaks and valleys, much like a mountain range. (Technically the “peaks” are the asperities.) When two surfaces are in contact, it may appear that they have a large, well-defined area of contact, but in reality, contact occurs only at certain places – that is, where the asperities of both surfaces interfere.

The sum of these small areas of contact between the asperities is referred to as the “real” or “effective” area of contact. Because these individual areas of contact are very small, the pressure (pressure = force ÷ area) between the surfaces at these points is very high. This extreme pressure allows adhesion to occur between the surfaces, via a process known as cold welding, which occurs at the molecular level. Before the surfaces can move relative to each other, the bonds that cause this adhesion must be broken.

In addition, the roughness of the surfaces means that at some locations, the asperities of one surface will settle into the valleys of the other surface – in other words, the surfaces will interlock.

These interlocked areas must be broken or plastically deformed before the surfaces can move. In other words, abrasion must occur. So, in most applications, static friction is caused by both adhesion and abrasion of the contacting surfaces.

Laws Of Static Friction

There are two laws of static friction:

1. First law: The maximum force of static friction is not dependent on the area of contact.
2. Second law: The maximum force of static friction is comparative to the normal force i.e., if the normal force increases, the maximum external force that the object can endure without moving, also increases.

Derivation for the formula of static friction

Let us consider a block of weight mg lying on a horizontal surface as shown in the figure. When a body press against a surface, the surface deforms even if it appears to be rigid. The deformed surface pushes the body with a normal force R that is perpendicular to the surface. This is called normal reaction force. It balances mg that is 

R = mg

Now let us consider that a force P is applied on the block. Clearly the body remains at rest because some other force F comes into play in the horizontal direction and opposes the applied force P resulting in net force zero on the body. This force F acting along the surface of the body in contact with the surface of the table is called frictional force.

So as long as the body doesn’t move F = P. This means that if we increase P, friction F also increases, remaining equal to P always.

This frictional force which comes into play until the actual motion has started is known as static friction.

Coefficient Of Static Friction

Static Friction is friction which is experienced when an object is placed on a surface. And, Kinetic friction is due to the movement of an object on a surface. Friction is well characterized by the coefficient of friction and is explained as the ratio between the frictional force and the normal force. This helps the object to lie on a surface. The coefficient of static friction is a scalar quantity and denoted as μ_s.

The formula for the coefficient of static friction is expressed as

[Tex]\mu_{s} = \frac{F}{N}[/Tex]

Where

μ_s​ = coefficient of static friction

F = static frictional force

N = normal force

Friction is a force resisting relative motion and it occurs at the interface between the bodies, but also within the bodies, like in case of fluids. The concept of friction coefficient was first formulated by Leonardo da Vinci. The magnitude of the coefficient of friction is determined by the properties of the surfaces, surroundings, surface features, presence of the lubricant, etc.

Laws of Friction

There are five laws of friction and they are:

• The friction of the moving object is proportional and perpendicular to the normal force.

• The friction experienced by the object is dependent on the nature of the surface it is in contact with.

• Friction is independent of the area of contact as long as there is an area of contact.

• Kinetic friction is independent of velocity.

• The coefficient of static friction is greater than the coefficient of kinetic friction.

When we see any object, we can see the smooth surface but when the same object is viewed under a microscope, it can be seen that even the smooth appearing object has rough edges. Tiny hills and grooves can be seen through the microscope, and they are known as irregularities of the surface. So, when one object is moved over the other, these irregularities on the surface get entangled giving rise to friction. More the roughness, more will the irregularities and greater will be the force applied.