Static Friction

What is Static Friction? - Definition and Examples

A force resists the motion of any solids, and layers of liquids. This force arises when two materials are sliding over one another. Friction is present all around us. For example, when we are walking then our feet are in contact with the ground. When we walk, then the backward movement of our feet exerts a force on the ground as the other foot moves forward. When this force is exerted on the ground, then an equal and opposite force is exerted by the ground on our feet. This is in accordance with one of the laws of motion. When we want to stop while running on the steep ground, then we are able to do that due to the existence of friction. One thing to keep in mind when we talk about friction is that it will always act in the direction opposite to that of the relative motion, hence the assistance in slowing down the speed and eventually, stopping. 

Friction can be classified into several categories, namely: dry friction, lubricated friction, fluid friction, internal friction, and skin friction.  

fluid friction: the friction which exists between the different layers or films of a liquid is known as fluid friction. This type of friction usually arises among the layers of a viscous fluid or between two different viscous fluids.  

Lubricated friction: we know that a type of lubricant, usually oil, is used in many machines in order to reduce the wear and tear of the machine parts. The frictional force which exists between this lubricant and the two surfaces of the any solid is known as lubricated friction. 

Skin friction: When a liquid move across the surface of any solid then there exists a frictional force between the liquid and the solid surface. This type of friction is known as skin friction. It is also commonly called as a “drag”.  

Internal friction: frictional force not only exists between the external surface, but it is also present among the elements that build up a substance. For example, the elements making up a solid. Friction arises between the elements when the configuration of a substance or a solid in this case is changed from its previous configuration. In other words, it arises when the body undergoes what is called, deformation.  

Dry friction: dry friction is the frictional force which comes into play when two solid surfaces come into contact. This type of friction can be broadly classified into two categories. The two categories are kinetic friction and static friction.  

Kinetic friction: the dry friction between two moving surfaces, which are sliding over one another or rubbing against each other is known as kinetic friction. Kinetic friction is also commonly known as sliding friction or dynamic friction.  
Now let us look at another category of dry friction, which is static friction. 

Static friction is the frictional force between the surfaces of two substances when they are not in motion with respect to each other. Therefore, in this case, instead of the bodies being in relative motion, the bodies are stationary with respect to each other. But as soon as the body starts to move, eventually, the kinetic frictional force comes into play. The intensity with which the surfaces of the two bodies stick to each other or grip each other is represented by a numerical value known as the coefficient of friction. The force which assists an object to stay at the surface of another object instead of falling down is known as the normal force. Basically, the normal force is applied by the surface on which the object is kept on the object in order to provide it support. If we consider the normal force of a force which is uneven, then the normal force will act at an angle and we will have to consider its component. When the surface is even or flat then the normal force will be taken equal to the force of gravity. 

The coefficient of the static friction is denoted by the symbol μs. This coefficient of static friction is generally found to be greater than the value of the coefficient of the kinetic friction. The static friction, as we have seen, arises due to the roughness or the irregularities which are present on the surface of an object, these small irregularities are known as asperities. If we want an object to start moving, then we need to overcome this static friction which is usually done by applying an external force which leads the body to start sliding or moving. Now, before the motion of the body begins or before the sliding of the object begins, there is a maximum value which the friction can attain. This maximum value which the friction can attain is found by multiplying the coefficient of the static friction and the normal force which is exerted by the surface in contact with the body. Therefore, Fmax= μs N, where N is the value of the normal force applied by the surface on the body in Newtons. As long as sliding does not take place, the frictional force can take any value starting from zero to max. If we apply a force which is smaller in value than this static frictional force, then a force which comes into action which will be equal and opposite to this applied force. If the force applied is greater than this static friction, then the body will start to slide or move. As soon as the movement of the body occurs, or when the body starts to slide, then the static friction ceases to have an impact and the kinetic frictional force comes into action.

Now, let us look at the various examples of the static frictional force. Suppose a car is rolling down a steep slope. Then when the breaks are applied, the car moves a bit further down the steep road but then eventually comes to a halt. At this position, the static frictional force is the force which is stopping the slipping of the wheels of the car from occurring.