Friction is the resisting force that opposes the movement of one object or layer over a surface. The frictional force acts on a surface that is in contact with another surface, and it impedes the motion of the objects. This implies that friction is an opposing force that resists the motion of the object. It is responsible for bringing an object to rest from a state of motion or reducing its velocity over time. For e.g. when we slide down a trolley across a horizontal floor with all our force, it moves for some time, but it stops after a point. It happens due to the force of friction. Friction can be broadly divided into two categories – static and kinetic. Static friction exists when an object is at rest. On the other hand, kinetic friction comes into play between moving surfaces, i.e., when one object is moving on the surface of another object. In this article, we will be focusing on kinetic friction.

Kinetic friction vs Static friction

Kinetic friction can be defined as the friction acting between the moving surfaces. In simple words, the force of friction acting upon a box when it is sliding on a floor would be considered kinetic friction. Static friction is defined as the force of friction acting upon an object which is at rest relative to another object. Once we apply enough force to overcome the static friction and the box starts moving, the friction at play would be kinetic friction.

Kinetic friction is much weaker and much less pronounced as compared to static friction. It happens because forces of adhesion are associated with static friction, i.e., the surfaces of two objects are closely bound to each other. Once the object starts moving, there are fewer points of adhesion between two surfaces due to which the magnitude of kinetic friction is much lower.

Friction depends on the roughness of the surface. Even when a surface seems smooth, there would be friction associated with it because no surface in the world can be 100% smooth. The magnified version of the surface would reveal several bumps and pits that contribute to the roughness and hence, the friction. The more the roughness, the higher the friction. Sometimes, the surfaces are deliberately made rough to increase friction.

Kinetic Friction Formula

The formula of kinetic friction is similar to the formula of static friction. It is equal to the product of the coefficient of kinetic friction and normal perpendicular force between the moving surfaces. In mathematical terms, the kinetic friction formula can be expressed as follows:

Fk = 𝜇kN, where Fk is the kinetic friction, 𝜇k is the coefficient of kinetic friction, and N is the normal perpendicular force acting between the moving surfaces. The more the perpendicular force, the higher the kinetic frictional force.

Applications of Kinetic Friction

There are several applications of kinetic friction. However, not all of them are to our advantage. Major applications are listed below:

1. Igniting a fire: Rubbing two rough surfaces against each other results in kinetic friction. The kinetic frictional force is converted to thermal energy, which initiates fire. Hence, fire can be ignited by virtue of a frictional force.
2. Oiling of machine parts: Machine parts are kept well-oiled so as to minimise the kinetic frictional forces between the moving surfaces. The friction acting in the machine parts can actually cause wear and tear and also generate huge amounts of unnecessary heat. Oiling is done to smoothen the surface and prevent the generation of heat -the smoother the surface, the lower the friction.
3. Roughness in tires: Car and motor vehicle tires are deliberately made rough so as to increase the kinetic friction between the tire and the road. It helps to ensure that brakes are applied on time, and major accidents are prevented. In the case of smooth tires, the application of brakes becomes a huge issue.
4. Writing on surfaces: The reason why it is possible to write on a paper is because of the existence of the force of friction between the two surfaces. 
5. Walking: This is one of the most important applications of kinetic friction. It enables us to walk. Our legs exert force in the backward direction, and since friction is an opposing force, it pushes us in the forward direction. The more the friction, the easier it is for us to walk. This is why it is better to walk in shoes that have strong spikes as it increases the roughness and hence, increases the frictional force.

Conclusion

Kinetic friction, also known as sliding friction, acts upon two moving surfaces. It is a necessary evil as it causes several problems (like wear and tear of machine parts), but it also helps carry out several essential activities like walking or writing on surfaces. Kinetic friction is directly proportional to the normal force acting between the moving surfaces, which is also dependent on the mass of the object. This implies that the force of kinetic friction would be more pronounced on a heavier object than on a lighter object. Kinetic friction is weaker compared to static friction, which is why it is always easier to keep an object moving rather than getting it into motion from a state of rest.