Static and Kinetic Friction

Introduction

The term “friction” refers to a force that acts on an object in touch with another object or surface. We must have already heard of this term either in school or at home since it involves everyday activities and objects we use in our daily lives. 

Friction is basically of two types: static frictional force and kinetic frictional force. 

The Frictional Force

The frictional force is a kind of force that acts between two surfaces and prevents sliding movement between them. Not only does the frictional force prevent it, but it also allows you to accelerate forward while running. 

There are many examples of frictional force in our day-to-day lives. The most common example can be of running, i.e., when you keep your foot on the ground, it grips the ground and pushes it backward, accelerating you in the forward direction. If friction was not there, you would have propelled yourself forward while running.

Types of Friction

There exist two types of frictional forces called static friction and kinetic friction. 

If you are standing on a steep hill and someone pushes you from the back, you’ll start sliding. However, here, though the two surfaces will be sliding past each other, there will still remain a kind of frictional force. We call this type of force kinetic friction. 

This force of kinetic nature always opposes the sliding motion of an object and reduces its speed where the object just slides across each other. For example, if a person slides into second base in a basketball game, the kinetic frictional force helps to slow down the pace. If there was no such force as kinetic friction, then the basketball player would go on sliding.

The second type of frictional force is known as the force of static friction. In this kind of frictional force, there is a resistance that is applied to an object which is at rest until any other force is exerted on it. Therefore, to compare, static friction is the one that resists an object from going into motion while it is at rest, while in the case of kinetic friction, the frictional force resists the motion of an object. 

There also exist two other types of frictional forces: rolling and sliding friction. Rolling is a kind of frictional force that acts on the objects when they roll over another surface. The rolling frictional force is weaker than that of static and kinetic friction forces, but it helps in explaining why ground transportations uses wheels. That is, you can see ball bearings used in the wheels of vehicles that reduce the force of friction between the inner and outer cylinders.

Fluid friction is a type of frictional force which acts on an object that moves through a fluid. This fluid can be a substance that can flow and take the shape of whichever vessel it is contained in. These fluids can be liquids or gases. Examples of fluid friction can be a swimmer who cuts through the water’s edges though the water provides resistance to him/her.

Kinetic Friction and its Formula

As you know, kinetic friction is an opposing frictional force that prevents motion. This frictional force is only for objects that are in motion and it is also known as sliding frictional force. This is so because kinetic frictional forces oppose the sliding motion. 

There are some coefficients of the kinetic frictional force. We can best define kinetic friction with the help of an equation. Kinetic friction depends upon its coefficient for its magnitude of the normal force that is exerted by the surface on the object. Thus, for kinetic friction, the equation can be written as:

                                    Fk= kFn

Here,

Fk stands for kinetic frictional force,

k stands for coefficient of kinetic friction, and

Fn stands for the normal force which is the same as the object’s weight when no other external forces are acting, i.e, Fn= mg, where m is the mass of the object and g is the gravitational force. 

Applications of Kinetic Friction

• It is used in the ball bearings on wheels.

Ball bearings are used in the wheels of vehicles to prevent sliding motion. If ball bearings are not used in the vehicles, then the frictional force between the parts of the wheel will be equal to that of sliding. And thus, the friction can be considerably more and can cause a significant loss of energy. By using ball bearings, the sliding friction gets changed into rolling friction which is very small compared to that of sliding friction. 

Other uses:

• It is used in thermionic emission.
• It is applied in thermal ionisation.
• It is used in chemical kinetics.
• It is applied in Einstein’s laws of radiation.
• It plays an important role in our daily lives such as in rubbing objects, walking, running, etc.
• The kinetic frictional force is the main reason behind the wear and tear of machine parts. Thus, one should lubricate machine parts with oil.

Laws of Kinetic Friction

There are four laws of kinetic friction. They are as follows:

• The first and most crucial law is that the kinetic frictional force is directly proportional to the normal reaction force between the surfaces which are in contact. In this law, the coefficient of kinetic friction is constant.
• The second law states that kinetic frictional force is independent of any shape or area of the surfaces which are in contact.
• The third law states that kinetic frictional force depends upon the nature of the material of the surfaces in contact.
• The fourth and the last law states that the force of kinetic friction is irrespective of the velocity of objects in contact where the relative velocity of the body in contact between the object and the surface is not very large.

Difference Between Static Friction and Kinetic Friction

The static frictional force acts on a resting body whereas kinetic friction acts on a body that is in motion. 

The magnitude of the static frictional force is greater than that of kinetic frictional force due to its greater value of the coefficient. The equation for static friction is as:

                        Fs= sN

Whereas the equation for kinetic friction is written as:

                        Fk= k N

Advantages of Frictional Force

• Friction helps us in walking, running, cycling, and similar activities.
• Frictional force helps us to wear anything.
• It helps us in making fire.
• It helps us in the energy transfer mechanism.
• It helps us to grip something or hold onto objects.
• The brakes of cars work on the theory of friction.
• Due to the presence of frictional force, the asteroids coming towards the earth get burnt due to friction through the atmosphere before entering the earth.

Disadvantages of Frictional Force

• Friction produces unnecessary amounts of heat which result in energy wastage.
• The frictional force acts in a direction opposite to that of the motion and hence slows down the motion of the objects.
• The frictional forces between tree branches cause forest fires.

Conclusion 

In this article, we learned about the frictional forces and their types. There are two types of friction in nature which are kinetic friction and static friction. We have studied both types and learnt the difference between these two. Also the application of friction is explained in this article.