Friction is the force in nature that resists the relative motion of solid surfaces, fluid layers, and material elements which are in contact. In other words, friction prevents objects from sliding against each other. It is generated by the imperfections on the two surfaces in contact. When two surfaces in contact try to slide against each other, the friction between the two surfaces converts kinetic energy trying to displace the objects into thermal energy or heat. Just rub your palms together and feel them heat up! This force have both benefits and drawbacks. Friction is necessary to walk without slipping; at the same time, roughly 20 per cent of the engine power of automobiles is consumed in overcoming frictional forces in the vehicle’s moving parts. We discuss everything about Friction and its application in detail on this page. Read on to know more.

What is Friction?

Friction

Friction is the resistance offered by a surface when an object moves or tries to move about it. It acts at the point of contact of two surfaces. It is a contact force because it requires physical interaction between the surfaces which are in relative motion.

Definition: Friction is a contact force that always opposes the relative motion of two objects which are in contact. It acts in the direction opposite to the direction of motion of the object.

For example, when we push a box kept on the ground to the right side, then the force of resistance on the box will act towards the left side, and it will act at the point of contact.

Friction Definition

Causes

At the microscopic level, every surface has irregularities. Irregularities are the minute hills and dells (small projections) on the surface of an object. These irregularities on the surface make it rough and oppose the motion of a surface over another surface.

This happens because irregularities of one surface interlock with the irregularities of another surface. When we press the two surfaces in contact hard, the interlocking between two surfaces increases, and this results in an increase in the force of friction.

When an object is resting on a surface, it gets enough time for its irregularities to interlock with the irregularities of the other surface. We need to apply more force to make the object move in this case. Once the body starts moving, its irregularities will not get enough time for interlocking, so the friction experienced by the moving object is less than the what it experiences at the start.

No surface is completely smooth. There is no such thing as a perfectly smooth surface. All surfaces have some roughness, which provides resistance to the object’s motion.

Types of Friction

There are four types of friction and they are:

1. Static friction
2. Sliding friction
3. Rolling friction and
4. Fluid friction

Static Friction \(\left({{f_s}}  \right)\)

Static friction is when motion develops in an object. For example, when we push a table kept on the ground, it does not move. It keeps the object at rest. It opposes the tendency of motion of an object.

If the applied force exceeds a certain upper limit, the object moves. The static friction is also known as the self-adjusting force because it increases instantaneously and becomes equal to the applied force.

The upper limit or maximum value of the static friction is called the static limiting friction, which is denoted by (\left({{F_s}} \right)). This limiting value does not depend on the area of contact.

\({f_s} = \, \le {F_s}\)
The static limiting friction is proportional to the normal reaction between the two surfaces in contact.

\({F_s} \propto R\)
\( \Rightarrow {F_s} = {\mu _s}R\)

Where \(R\) is the normal reaction and \({\mu _s}\) is the coefficient of static friction. The coefficient of friction is dimensionless whose value depends on the surface conditions and the property of the materials of two objects in contact.

Sliding or Kinetic Friction  \(\left({{f_k}} \right)\)

The force of friction that opposes the relative motion of two objects in contact is called static or kinetic friction. For example, a box sliding down an inclined plane. Here the value of sliding value is less than the static one.

\({f_k} = {\mu _k}R = {\mu _k}mg\)

Where \(R\) is the normal reaction, \({\mu _k}\) is the coefficient of sliding friction, \(m\) is the mass of the object and \(g\) is the acceleration due to gravity. From the above relation, we can conclude that friction offered by the moving surfaces in contact increases when the weight of the object is increased.

Practice Exam Questions

Rolling Friction \(\left({{f_r}} \right)\)

Rolling friction is when an object rolls over the surface of another object. For example, a boy pushing a trolley. Here, the value of the force is less than the static and sliding values. This is because the surface area of contact is the least in the case of the rolling friction.
Rolling friction is directly proportional to the weight of the rolling object. It increases as the radius of the rolling object decreases.

\({f_r} = {\mu _r}W\)

Fluid Friction

Fluid friction acts within the layers of the liquid or a liquid can offer it to the object moving inside it. This is also known as viscous drag. It depends on the nature of fluid and the shape of the object moving inside a liquid.

More viscous fluids offer more resistance to the motion of the object. For example, we will experience this force in fluids while stirring honey with the help of a straw than when we stir the water with the same straw.

Factors Affecting Friction

The following are the factors that affect this resistive force:

1. Irregularities on surfaces in contact: The irregularities on surfaces in relative motion are the main factor affecting friction force. A surface with more irregularities or roughness will offer more resistance to the object’s motion over it. Since no surface is entirelydevoid of this resistive force, all the surfaces, even the polished ones, will offer some friction. Also, these interlocking of irregularities helps make a perfect grip onto a surface.
2. Weight of the object: Heavier objects experience more friction than lighter objects because the force is directly proportional to the normal reaction.
3. Force of adhesion between surfaces in contact: When surfaces of two objects come in contact, the intermolecular force of attraction between the two objects provides a strong adhesion force. This resists the relative motion of the surfaces to some extent.

Applications

Activities like walking, holding objects in hand, driving on the road, pushing or pulling objects on the ground involve friction.

1. Acts between pencil and paper and enables us to write.
2. Acts between the ground and our feet and helps us to walk without slipping.
3. Acts between the matchstick and the chemical-coated side of the matchbox helps to ignite it.
4. Acts between the ground and tires of vehicles supports the movement of vehicles on the ground.

Disadvantages

It is very clear from the definition of friction that it has a disadvantage of resisting the relative motion of the two objects in contact. Some other disadvantages of the force of friction are as follows:

1. To overcome this force, energy is wasted.
2. It produces energy dissipation in the form of heat between the rubbing parts of machinery.
3. It causes wear and tear of parts that rub against each other.

Methods to Increase and Decrease Friction

1. Methods to increase: Sometimes we need to increase friction to perform certain activities like walking. We can increase it by making the contact surfaces rough. This can be done by providing grooves on the surfaces. For example, grooves are made on the tires of vehicles and the soles of shoes to prevent slippage. We can also increase it by increasing the mass of the object.
2. Methods to decrease: In order to reduce the energy loss and wear and tear of the rubbing parts of the two objects in contact, it is desirable to reduce friction. We can reduce it by lubricating the contact surfaces.

For example, we can apply grease or oil between the surfaces which are in relative motion. We can polish the contact surfaces. Fluid friction can be reduced by providing a streamlined shape to the object moving through the fluid. For example, ships have streamlined shapes to overcome the resistance by seawater and aeroplanes have streamlined shapes to reduce air drag. It can also be reduced by using ball bearings. Ball bearings convert sliding friction into rolling friction.

Solved Examples

Q.1. A \({\rm{5}}\,{\rm{kg,}}\) box on a horizontal table is pushed by a horizontal force of \({\rm{15}}\,{\rm{N}}.\) If the coefficient of friction is \(0.4\), will the box move?
Ans: Given, the mass of the box, \({\rm{m}}\,{\mkern 1mu} {\rm{ = }}\,\,{\rm{5}}\,\,{\rm{kg}}\)
The coefficient of friction, \({\rm{\mu = 0}}{\rm{.4}}\)
The weight of the box, \({\rm{W = m \times g = 5 \times 9}}{\rm{.8}}{\mkern 1mu} \,{\rm{N = 49}}{\mkern 1mu} \,{\rm{N}}\)
The normal reaction on the box, \({\rm{R = 49}}\,\,{\rm{N}}\)
The force exerted of the box, \({\rm{F = \mu R = 0}}{\rm{.4 \times 49 = 19}}{\rm{.6}}{\mkern 1mu} \,{\rm{N}}\)
Since this force is greater than the applied horizontal force, the box will not move.

Q.2. Determine the maximum acceleration of the train in which a box lying on its floor will remain stationary, given that the coefficient of static friction between the box and the floor of the train is \(0.15\).
Ans: Given, the coefficient of static friction, \({\mu _s} = 0.15\)
Let \(m\) be the mass of the box, \(a\) be the maximum acceleration and \(R\) be the normal reaction.
The acceleration of the box is due to static friction.
\(ma = {\mu _s}R = {\mu _s}mg\)
\(a = {\mu _s}g\)
\(a = 0.15 \times 9.8\,{\mkern 1mu} {\rm{m}}\,{{\rm{s}}^{ – 2}} = 1.47{\mkern 1mu} \,{\rm{m}}\,{{\rm{s}}^{ – 2}}\)

Summary

This article concludes that the force of friction is a necessary evil that results from the interlocking of the irregularities of the rubbing surfaces. It always resists the relative motion between surfaces of two objects in contact. This force is also desirable for proper movement. We can never eliminate this resistive force completely, but we can reduce it to a greater extent if needed.

Related Topics

1. Motion 2. Force 3. Acceleration

Frequently Asked Questions (FAQs)

The most frequently asked questions about the topic are answered here:

Q1. Is friction good or bad?
Ans: Friction can be beneficial as well as detrimental at times. It helps in making a proper grip between contact surfaces. For example, it acts between our shoe and ground and helps in walking. However, it results in wear and tear when surfaces of two objects rub continuously against each other for a long time. Such as in the tyres of vehicles.

Q2. What are the laws of friction?
Ans: The following are the laws of friction:
(a) It always acts along the contact surfaces of two bodies. It always opposes the tendency of relative motion between the surfaces.
(b) It does not depend on the area of contact.
(c) It depends on the nature of surfaces in contact.
(d) It \(\left( F \right)\) is directly proportional to the normal reaction \(\left( R \right)\) between the two bodies in contact.
\(F = \mu R\)
Where \(\mu \) is the coefficient of friction.

Q3. What are the negative effects of friction?
Ans: The negative effects of friction are as follows:
(a) It opposes the motion of an object.
(b) It causes wear and tear.
(c) It produces unwanted waste of energy.

Q4. What are the \(4\) types of friction?
Ans: The \(4\) types are static, kinetic, rolling and fluid frictions.

Q5. What type of friction is the weakest?
Ans: Rolling friction is the weakest because, during rolling, the contact surface area is the least. It comes to play when an object rolls over the surface of another object.

We hope that this article covering the topic of Friction has helped you in understanding the concept. However, if you have any queries, ping us through the comment box below and we will get back to you as soon as possible.