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Frictional Force - Formula, Examples, Types, FAQs

Frictional Force - Formula, Examples, Types, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 04:29 PM IST

Friction is a force which opposes relative movement between two objects coming into contact with each other. Such as it is caused by the roughness of the surface in contact and it functions in a direction opposite to the direction of motion. People interact with friction every day from walking, driving and writing to other undertakings. In this article, we will discuss Friction, the laws of friction and a brief overview of the types of friction

This Story also Contains
  1. What is Friction?
  2. Laws of Friction
  3. Causes of friction
  4. Types of Friction
  5. What is Frictional Force?
  6. Frictional Force Examples
  7. Work done by Frictional Force
Frictional Force - Formula, Examples, Types, FAQs
Frictional Force - Formula, Examples, Types, FAQs

What is Friction?

When a body is placed on the surface of another body, then due to the irregularities in their surfaces, the contact between them occurs at a large number of points. at each point of contact, a small area of the two surfaces comes in contact with each other. At these points of contact, the atoms and molecules of the two surfaces come very close to each other and they attract strongly. When we try to move a body places over the other body, the bonds of attraction oppose the motion. This opposing force is recognized as the force of friction.

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Definition of Friction

"Friction is a force that opposes a motion of one surface sliding or moving over another surface. It is caused by the roughness of the surface and always acts in opposite direction to the movement."

Laws of Friction

  • The friction of the moving object is proportional to the normal force( numerically equal to the pressing force).

fN

  • 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(as for solid apparent area is not equal to the actual area of contact).
  • It acts tangentially along with the contact.
  • The direction of friction is always opposite to the direction of relative motion.
  • It can be also defined as the component of contact force which is parallel to the surfaces in contact.
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Friction is a dissipative force that is in most of the cases due to the frictional force there is the liberation of heat.

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Causes of friction

Suppose a body is moving above a surface from east to west then the direction of the frictional force will act in the opposite direction that is from west to east.

The frictional force will be less if the surface of interaction is small and smooth, for providing more friction the interacting surface should be rough.

The breaking of a vehicle moving on the road occurs by taking advantage of the frictional force between the tyres and the road. You can see that the tyres get worn out after running for a while this is due to the friction acting between the tyre and the road. The rougher the tyres are it will be much easy for the vehicle to stop and difficult to gain speed and move smoothly. We can see the math regarding the friction against the motion of vehicles on the road in the coming sections.

Types of Friction

There are different types of frictional forces depending upon the surfaces in contact they are. Let's discuss each type in brief:

Fluid friction

It is the friction that occurs between the layers of the fluids moving relative to the ground, it depends upon the type of fluid and is determined by using a term called viscosity the more viscous fluid have more friction and hence moves less slowly compared to a less viscous fluid.

Dry friction

If the motion between two hard solid surfaces is considered it is called dry friction.it is again divided as static friction and sliding friction based on the state as the objects whether it is moving or not, which will be discussed in detail in the next session.

Lubricated friction

It is the case where a lubricant is present in between the two solid surfaces

Internal friction

It is such a type of frictional force that can be seen in between the elements of a solid material; it can be molecular or atomic in nature.

Sliding friction

Sliding friction is the opposing force that occurs when one body attempts to slide across another's surface.

Static friction

Static friction is the friction between two solids that are not in relative motion with each other. The force that stops a vehicle wheel from sliding as it rolls on the ground is an example of static friction.

Dynamic friction

The friction that acts when a body is actually sliding over the surface of another body. Dynamic friction has another name called kinetic friction.

What is Frictional Force?

Frictional force is experienced due to the friction, it acts parallel to the surface in contact and opposes the relative motion.

S.I. Unit of Frictional force is Newton as it is also a kind of force.

Frictional force Formula

F=μ×N

Where,

where μ is the coefficient of friction and N is the normal force.

How to Calculate Frictional Force?

Now let us consider a situation where we can see how to calculate the frictional force.

Suppose there is a block of wood that weighs 5 kg resting on a table to be pushed from the rest. Considering the coefficient of friction as 0.5 then calculate the frictional force.

Solution:

The Normal reaction , N=5Kg×9.8 N/kg=49 N/Kg
Given, the coefficient of friction equal to, μ=0.5
Then , the frictional force, F=49 N/Kg×0.5=24.5 N

We can define the coefficient of friction as the frictional force acting against a unit normal body

Frictional Force Examples

  1. Rubbing of hands
  2. The motion of a vehicle on the road
  3. Dragging a box
  4. Pushing a chair through the ground
  5. Breaking system of cars
  6. Viscous force on an object moving inside a fluid

Work done by Frictional Force

Case 1: Assume a person is walking on the ground forward then we have to find the work done by the frictional force on the person’s foot,

Work done by frictional force

Equation of work is force times displacement
W=F×S
Here the person lifts his foot on walking and there is no displacement with respect to the ground when considering the relative motion so the work done by the friction, in this case, is zero
Ie, S = 0
Hence W = 0

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Case 2: Now assume the case of a box sliding over a rough surface then there is a frictional force acting opposite to the motion of the box

frictional force example

Here if we consider it as static friction, then the displacement is zero and hence the work done is also zero(work done by a force is dot product of force and displacement)
Now if the box is actually moving then the case here is of sliding friction,
Here, work, W=F×S×Cosθ
W=Ft×S×Cos (1800) since the force and the displacement are in the opposite direction

W=Ft×S×(1)=(Ft×S)


Where Ft is the frictional force and S is the displacement of the box relative to the relative motion between the box and the surface.

Frequently Asked Questions (FAQs)

1. Define Frictional Force.

It is the resistance that occurs opposite to the motion due to the relative motion between two surfaces.

2. What is the meaning of friction that has stopped the body from moving?

It means that the body do not have enough force to act above the frictional force provided by the surface and the body is at rest.

3. What is the Unit of Force?

Unit of force is Newton (N). A Newton is a force required to give a mass of 1 kilogram (1 kg) an acceleration of 1 meter per second squared (1 m/s²). SI unit of mass is kilogram (kg) and acceleration is meter per second squared (m/sec²) hence it is written as kg m/sec² which is denoted by Newton.

4. What is Line of Action:

The application of force can be represented by a line of action. It is the geometric representation of the force. Line of action is referred to as the characteristic line that is associated with each force.

5. Why do some surfaces feel "sticky" even though they're dry?
Some materials, like rubber, can form temporary molecular bonds with surfaces they contact, creating adhesion. This microscopic "stickiness" contributes to friction, even without visible moisture. It's why rubber-soled shoes often provide good grip on various surfaces.
6. How does the texture of a surface affect friction?
Surface texture influences friction by changing the amount of actual contact between surfaces. Rough surfaces generally have higher friction due to more interlocking of surface irregularities. However, extremely smooth surfaces can sometimes exhibit high friction due to increased molecular interactions.
7. Why does friction usually generate heat?
Friction generates heat because it converts kinetic energy into thermal energy. As surfaces interact at the microscopic level, the energy of motion is transformed into vibrations of atoms and molecules, which we perceive as heat.
8. How does friction contribute to wear and tear in mechanical systems?
Friction causes wear by gradually eroding surfaces in contact. As microscopic irregularities rub against each other, tiny particles are dislodged, leading to material loss over time. This process can change component dimensions and properties, potentially leading to system failure if not addressed.
9. Why do we use sandpaper, and how does it relate to friction?
Sandpaper utilizes high friction to smooth surfaces by abrasion. The rough particles on sandpaper create significant friction when rubbed against a surface, removing small amounts of material. This process effectively increases friction to achieve a smoother result.
10. How does friction help us walk?
Friction between our shoes and the ground allows us to push off and move forward without slipping. Without friction, walking would be like trying to move on ice - our feet would slide instead of gripping the surface.
11. How does the normal force affect friction?
The normal force, which is perpendicular to the surface, directly affects the magnitude of friction. As the normal force increases (e.g., more weight pressing down), the frictional force increases proportionally, as shown in the formula F = μN.
12. Why do race car drivers want their tires to heat up?
Race car drivers prefer warm tires because heat softens the rubber, increasing its coefficient of friction with the track. This results in better grip, allowing for faster cornering and improved overall performance.
13. What is the difference between friction and drag?
While both oppose motion, friction typically refers to forces between solid surfaces in contact, while drag refers to forces exerted by fluids (liquids or gases) on objects moving through them. Drag depends more on an object's speed and shape than on its surface characteristics.
14. What is the relationship between friction and traction?
Traction refers to the grip between a moving object and the surface it moves on, essentially describing beneficial friction. Good traction, like a tire gripping the road, is crucial for controlled movement, while poor traction can lead to slipping or sliding.
15. How does friction affect energy conservation?
Friction often leads to energy dissipation, converting kinetic energy into heat. This process is irreversible in practical terms, making friction a key factor in the apparent loss of energy in mechanical systems. It's why perpetual motion machines are impossible in reality.
16. How does friction affect the efficiency of machines?
Friction reduces machine efficiency by converting some of the input energy into heat instead of useful work. This is why lubrication and design improvements to reduce friction are crucial in engineering, as they can significantly increase a machine's overall efficiency.
17. How does friction impact the design of sports equipment?
Friction is crucial in sports equipment design. For example, the texture of a basketball or the grip on a tennis racket is optimized for control. In contrast, swimsuits and skis are designed to minimize friction for better speed. Understanding and manipulating friction is key to enhancing athletic performance.
18. How does friction impact the design of conveyor belts?
Conveyor belt design must balance friction. Sufficient friction is needed between the belt and the items it carries to prevent slipping, and between the belt and its drive mechanism for proper operation. However, excessive friction with support rollers or other components can lead to energy waste and wear.
19. Why do we sometimes use friction to start fires?
Friction can be used to start fires because it generates heat. When two surfaces are rubbed together vigorously, the friction-induced heat can raise the temperature of nearby combustible materials to their ignition point. This principle is used in traditional fire-starting methods like rubbing sticks together.
20. What's the difference between static and kinetic friction?
Static friction prevents an object from starting to move when at rest, while kinetic friction acts on an object that's already in motion. Static friction is typically greater than kinetic friction, which is why it's often harder to start pushing an object than to keep it moving.
21. What is rolling friction, and how does it differ from sliding friction?
Rolling friction occurs when an object rolls over a surface, like a wheel on a road. It's generally much lower than sliding friction because the contact point is constantly changing, reducing the time for molecular bonds to form between surfaces.
22. What is fluid friction, and how does it differ from solid friction?
Fluid friction occurs when a solid object moves through a fluid (liquid or gas) or when fluid layers move relative to each other. Unlike solid friction, fluid friction increases with the object's speed and depends more on the object's shape and the fluid's properties than on surface textures.
23. How does air resistance relate to friction?
Air resistance, or drag, is a type of fluid friction that opposes an object's motion through air. Like surface friction, it increases with speed and depends on the object's shape and surface characteristics. However, it doesn't require direct contact between solid surfaces.
24. How does friction affect the motion of objects in fluids?
In fluids, friction manifests as drag or fluid resistance. It opposes the motion of objects through the fluid, increasing with speed and depending on the object's shape and the fluid's properties. This is why streamlined shapes are used to reduce drag in vehicles and aircraft.
25. What is the concept of "frictionless" surfaces in physics problems?
In physics, "frictionless" surfaces are idealized concepts used to simplify problems and focus on other forces. While not physically realizable, these theoretical scenarios help students understand fundamental principles without the complicating effects of friction.
26. What is the relationship between friction and wear in mechanical joints?
Friction in mechanical joints leads to wear over time as surfaces in contact erode each other. The rate of wear is generally proportional to the frictional force. Proper lubrication and material selection can reduce friction and consequently slow down wear, extending the life of the joint.
27. Why is understanding friction important in the design of prosthetic limbs?
Friction is crucial in prosthetic design for both comfort and functionality. The interface between the prosthetic and the user's body must have enough friction to prevent slipping, but not so much as to cause discomfort or skin damage. Additionally, joints in prosthetics must balance friction for smooth, controlled movement.
28. How does friction contribute to the phenomenon of static electricity?
Friction plays a role in generating static electricity through the triboelectric effect. When certain materials are rubbed together, electrons can transfer from one material to the other due to the friction between their surfaces, resulting in a buildup of static charge.
29. What is the importance of friction in the process of writing with a pencil?
Friction is essential for writing with a pencil. As the pencil moves across paper, friction causes tiny graphite particles to break off and adhere to the paper fibers. Without this frictional interaction, the graphite would not transfer to the paper, and no mark would be made.
30. What is frictional force?
Frictional force is the resistance that one surface or object encounters when moving over another. It acts opposite to the direction of motion and is caused by the interactions between the microscopic irregularities on the surfaces in contact.
31. How does the coefficient of friction relate to frictional force?
The coefficient of friction is a measure of how much two surfaces resist sliding across each other. It's directly proportional to the frictional force - a higher coefficient results in greater friction. The relationship is expressed in the formula: F = μN, where F is the frictional force, μ is the coefficient of friction, and N is the normal force.
32. Why doesn't the frictional force depend on the surface area of contact?
Counterintuitively, frictional force doesn't depend on the surface area of contact because as the area increases, the pressure decreases proportionally. The total number of microscopic contact points remains roughly the same, resulting in no net change in friction.
33. What role does friction play in braking systems?
Braking systems rely on friction to slow or stop moving vehicles. When brake pads press against a rotating disc or drum, the resulting friction converts the vehicle's kinetic energy into heat, reducing its speed. The effectiveness of brakes depends largely on the coefficient of friction between the brake components.
34. How does friction affect the movement of tectonic plates?
Friction between tectonic plates resists their relative motion, causing stress to build up over time. When this stress exceeds the frictional force, it results in sudden movements, causing earthquakes. The study of friction between plates is crucial in understanding seismic activity.
35. What role does friction play in heat generation?
Friction converts kinetic energy into thermal energy. When two surfaces rub against each other, the energy of motion is partially transformed into heat due to the interactions between the microscopic irregularities on the surfaces.
36. Can frictional force ever be zero?
In practical situations, frictional force is never truly zero. Even in seemingly frictionless environments like space, there's always some resistance. However, in idealized physics problems, we sometimes assume zero friction to simplify calculations.
37. Why do car tires need to be replaced periodically?
Car tires wear out because friction gradually erodes the rubber. As the tread depth decreases, the tire's ability to grip the road (especially in wet conditions) diminishes, reducing safety. Regular replacement ensures optimal friction for braking and cornering.
38. How do lubricants reduce friction?
Lubricants, like oil or grease, create a thin film between moving surfaces. This film separates the surfaces, reducing direct contact between their irregularities. As a result, the surfaces can slide more easily over each other, decreasing friction.
39. How does friction affect the motion of objects on an inclined plane?
On an inclined plane, friction opposes the component of gravity parallel to the surface. It can prevent objects from sliding down if static friction is strong enough, or slow their descent if kinetic friction is present. The steeper the incline, the more likely friction will be overcome by gravity.
40. How does friction affect the spread of fires?
Friction plays a role in fire spread by generating heat as burning materials move or fall. Additionally, the roughness of surfaces (a factor in friction) can affect how easily fires spread across them. Smoother surfaces generally allow for faster fire spread than rough ones.
41. Why do different pairs of materials have different coefficients of friction?
The coefficient of friction varies between material pairs due to differences in surface properties at the microscopic level. Factors include the materials' hardness, surface roughness, chemical reactivity, and the strength of intermolecular forces between the surfaces.
42. How does friction impact the efficiency of electric motors?
Friction in electric motors, particularly in bearings and between moving parts, converts some electrical energy into heat instead of motion. This reduces the motor's efficiency. Minimizing friction through proper lubrication and design is crucial for maximizing motor performance and lifespan.
43. How does friction affect the motion of objects in granular materials like sand?
In granular materials, friction occurs between individual grains as well as between the grains and a moving object. This complex interaction can lead to unique behaviors, such as the ability of dense granular materials to momentarily support weight like a solid when subjected to sudden forces.
44. How does friction affect the behavior of fluids in pipes?
Friction between a fluid and the pipe walls, as well as between layers of the fluid itself, creates resistance to flow. This friction leads to pressure drops along the pipe and can cause turbulence. Understanding this is crucial in designing efficient piping systems for various applications.
45. Why is friction important in the process of kneading dough?
Friction during kneading helps develop gluten in dough by causing protein molecules to align and form networks. The mechanical action, which involves friction between the dough and the kneading surface or hands, is crucial for achieving the desired texture in many baked goods.
46. How does friction impact the design and function of zippers?
Zippers rely on a balance of friction. The interlocking teeth must have enough friction to stay closed when desired, but not so much that the zipper is difficult to open or close. The slider helps overcome the friction between teeth when changing the zipper's state.
47. What role does friction play in the formation of sedimentary rocks?
Friction is involved in the erosion, transport, and deposition processes that form sedimentary rocks. It affects how particles are worn down, how they move in water or air, and how they settle and compact. The size and shape of sediment particles are influenced by the frictional forces they encounter.
48. How does friction affect the movement of glaciers?
Friction between a glacier and the underlying rock or sediment resists the glacier's movement. This friction can cause the bottom layers of ice to melt under pressure, creating a thin water layer that allows the glacier to slide more easily. Understanding this friction is crucial in glaciology and climate studies.
49. Why is friction important in the design of climbing gear?
Climbing gear relies heavily on controlled friction. Ropes and harnesses must have enough friction to grip and hold securely, while allowing controlled movement. Climbing shoes are designed to maximize friction with rock surfaces. Proper understanding and utilization of friction is literally life-saving in this context.
50. How does friction contribute to the aging of pavements and roads?
Friction between vehicle tires and road surfaces gradually wears away the pavement. This process is accelerated by factors like heavy traffic, weather conditions, and the use of studded tires. Over time, this frictional wear can lead to rutting, cracking, and other forms of road deterioration.
51. What is the role of friction in the function of brake pads?
Brake pads rely on friction to slow or stop a vehicle. When the brake is applied, the pads press against the brake rotor or drum, creating friction that converts the vehicle's kinetic energy into heat. The material composition of brake pads is carefully designed to provide optimal friction and wear characteristics.
52. How does friction affect the efficiency of wind turbines?
Friction in wind turbines occurs in bearings, gearboxes, and between the blades and the air. While some friction is necessary for the blades to capture wind energy, excessive friction in mechanical components reduces overall efficiency. Minimizing internal friction is a key consideration in wind turbine design and maintenance.
53. Why is understanding friction important in the field of tribology?
Tribology, the study of friction, wear, and lubrication, is crucial for improving the efficiency and longevity of mechanical systems. Understanding friction at various scales helps in developing better lubricants, designing more durable materials, and creating more efficient machines across numerous industries.
54. How does friction impact the effectiveness of insulation materials?
Friction plays a role in the effectiveness of insulation by affecting how air or other fluids move within the insulating material. Materials with high internal friction (like fiberglass) trap air more effectively, reducing convection and improving insulation properties. Understanding this helps in designing more efficient insulation for various applications.

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