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Kinetic friction can be defined as “the frictional force resists the motion of an object and is independent of velocity.” Kinetic Friction acts between moving surfaces. Simply, a body that moves on a surface undergoes a force in the opposite direction of its movement.
| Table of Content |
Key Terms: Kinetic Friction, Friction, Force, Velocity, Normal Reaction Force, Static Friction, Kinetic Friction
What is Kinetic Friction?
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Kinetic Friction can be explained as:
| “The force that acts between moving surfaces. Any body which moves on a surface undergoes a force in the direction opposite to its movement.” |
The magnitude of force usually depends on the kinetic coefficient of friction between the acting materials. Friction is the force that holds back a slippery object. Kinetic friction is part of everything and interferes with the movement of two or more objects.
Kinetic Friction
The force acts in the opposite direction to that in which an object wants to slide. If a car has to stop, we brake and that's where the friction comes in. When walking, when you want to stop suddenly, the friction is still to thank. But when you have to stop in the middle of a puddle, things get complicated because the friction is less and that can not help so much.
Laws of Kinetic Friction
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There are four laws of kinetic friction:
- First Law of Kinetic Friction: The kinetic friction force (Fk) is directly proportional to the normal reaction (N) between two surfaces in contact. Where k = constant is called the kinetic coefficient of friction.
- Second Law of Kinetic Friction: The dynamic frictional force is independent of the shape and apparent area of the contact surfaces.
- Third Law of Kinetic Friction: It depends on the nature and material of the contact surface.
- Fourth Law of Kinetic Friction: It is independent of the speed of the object in contact as long as the relative speed between the object and the surface is not too great.
Solved Examples Related to Kinetic FrictionQues. Determine the maximum speed which can be attained without skidding by a car on a circular, unbanked road of radius R and coefficient of static friction μ? [NCERT 1990] Ans. The given condition can only be fulfilled by the required centripetal force offered by the frictional force that acts between the road and the tyre. Thus, the equation of maximum speed can also be shown as, \({{mv^2} \over R} =\mu mg\) \(\therefore\) Hence, v = \( { \sqrt{\mu Rg}}\) Ques. A constant force of 10 N has been seen to apply to a 2 kg crate placed on a rough surface. The crate experiences friction against a force which moves it over the surface. Thus, considering the coefficient of friction is μk= 0.24, determine the magnitude of friction force that opposes the motion. [2 marks] Ans. As per the question given, the crate usually does not shift vertically or lift off the surface, thus the forces in this direction need to be balanced with one another. As per the diagram below, the two forces are seen to act on the crate, which is an upward normal force (FN), alongside a downward weight force W = mg
Frictional Force Diagram Thus, FN = mg ⇒ 2 \(\times\) 10 = 20 N Now, by using the Kinetic Force Formula, fk = \(\mu_k\) FN in order to determine its magnitude, \(\therefore\) fk = \(\mu_k\) FN = 0.24 \(\times\) 20 = 4.8 N |
Formula of Kinetic Friction
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Kinetic Friction Force is equal to μk times the normal force on a body. It is depicted in Newton (N), Kinetic Friction Force = (kinetic friction coefficient)(normal force)
| Fk = μkη |
- Fk is the force of kinetic friction
- μk is the coefficient of kinetic friction
- η is the normal force (Greek letter “eta”)
Derivation of the Kinetic Friction Formula
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Consider a block of weight mg placed on a horizontal surface. When a body presses against a surface, the surface deforms even though it appears rigid. The deformed surface pushes the body with a normal force R which is perpendicular to the surface. This is called the Normal Reaction Force. It balances mg, that is to say, R = mg. Now we consider that a force P is applied to a block.
Normal Force
The body remains at rest because another force F comes into play in the horizontal direction and opposes the applied force P, determining a net force of zero on the body. This force F acting along the surface of the body in contact with the surface of the table is called the Frictional Force.
Frictional Force
So as long as the body does not move F = P. This means that if we increase P, the friction F also increases, always remaining equal to P. When we increase the applied force slightly beyond the limit friction, the actual move begins. This does not mean that the friction is gone. It just means that the force has exceeded the friction limit.
This frictional force at this stage is called Kinetic Friction or Dynamic Friction. Kinetic friction or dynamic friction is the opposing force that comes into play when a body is moving on the surface of another body.
Applications of Kinetic Friction
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Some real examples of kinetic friction are given in the following points.
- Friction plays a pivotal role in everyday events such as the friction of two objects. The resulting movement turns into heat and thus in some cases causes a fire.
- It is also responsible for wear and tear, which is why we need oil to lubricate machine parts because it reduces friction.
- When two objects are rubbed against each other, the frictional force is converted into thermal energy, in some cases causing a fire
- Kinetic friction is responsible for the wear and tear of machine parts, so it is important to lubricate machine parts with oil.
| Important Concept Related Topics | ||
|---|---|---|
| Centripetal and Centrifugal Force | Derivation of Centripetal Acceleration | Impending Motion |
| Static Friction | Aristotle Fallacy | Coefficient of Static Friction |
Types of Friction
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There are two main types of friction which are mentioned in the following points.
- Static Friction
- Kinetic Friction
Static Friction
Static friction can be defined as a frictional force applied to an object that causes it to remain at rest until the force of static friction is overcome. Static frictional force is often represented as a self-regulating force.
Kinetic Friction
Kinetic friction if often expressed as a retarding force experienced by a body while it is in motion. Kinetic friction is the frictional force that resists the motion of an object and is independent of velocity.
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Difference Between Static Friction and Kinetic Friction
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There are several differences between Static Friction and Kinetic Friction. Some of them include:
| Static Friction | Kinetic Friction |
|---|---|
| Static friction can be defined as the friction found between two or more objects that do not move relative to one another. | Kinetic friction can be expressed as the friction found between two or more objects that move relative to one another. |
| The amplitude of the static friction is greater because of the greater value of its coefficient. | The amount of dynamic friction is relatively lesser because of the low value of its coefficient. |
| The equation denoting static friction is, Fs = μsη | The equation denoting dynamic friction is, Fk = μkη |
Previous Year Questions
- From the top of a tower a body A is projected vertically up … [JKCET 2007]
- Assuming earth to be an inertial frame … [JKCET 2009]
- A body is sliding on a smooth inclined plane requires … [JIPMER 1999]
- An engine of power 58.8kW pulls a train of mass 2×105 kg with a velocity… [KEAMEE 2009]
- The minimum force required to move a body up an inclined plane ...[AP EAPCET 2004]
- Water falls from a height of 60m60m at the rate of 15kg/s to operate a turbine ...[NEET 2021]
- Starting from rest, the time taken by a body sliding down on a ..[BITSAT 2008]
- A bar AB of weight W rests like a ladder with upper end A ...[NEET 2018]
- A body of mass 5 kg is suspended by a spring balance on an inclined … [BITSAT 2006]
- A body of mass M hits normally a rigid wall with … [BITSAT 2018]
- A man of weight 80 kg is standing in an elevator … [DUET 2003]
- A light inextensible string that goes over a smooth fixed … [BITSAT 2013]
- A ball is released from the top of a tower … [JKCET 2007]
- There are two identical springs each of spring constant … [UPSEEE 2017]
- A 10gm bullet moving directly upward at 1000 m/s strikes … [UPSEEE 2017]
Things to Remember
- Kinetic friction is part of everything and interferes with the movement of two or more objects.
- First law: The kinetic friction force (Fk) is directly proportional to the normal reaction (N) between two surfaces in contact. Where k = constant is called the kinetic coefficient of friction.
- Second law: The dynamic frictional force is independent of the shape and apparent area of the contact surfaces.
- Third law: It depends on the nature and material of the contact surface.
- Fourth law: It is independent of the speed of the object in contact as long as the relative speed between the object and the surface is not too great.
- The formula of the kinetic friction is Fk = μkη.
- Friction plays a pivotal role in everyday events such as the friction of two objects. The resulting movement turns into heat and thus in some cases causes a fire.
Sample Questions
Ques. What is meant by kinetic friction? (2 marks)
Ans. Kinetic friction can be expressed as a force that acts between any moving surfaces. A body moving on the surface experiences a force in the direction opposite to its movement. The magnitude of the force will depend on the kinetic coefficient of friction between the two materials. Friction is the force that holds back a slippery object.
Ques. What is the relationship between static friction and kinetic friction? (2 marks)
Ans. The force of static friction keeps an object stationary at rest. Once the static frictional force is overcome, the kinetic frictional force is what slows down a moving object.
Ques. What is friction? What are its types? (2 marks)
Ans. Friction is the force resisting relative motion and occurs at the interface between the bodies, and in the case of fluids, it occurs within the bodies. There are two main types of friction which are mentioned in the following points.
- Static Friction
- Kinetic Friction.
Ques. State the laws of kinetic energy. (4 marks)
Ans. There are four laws of kinetic friction:
- First law: The kinetic friction force (Fk) is directly proportional to the normal reaction (N) between two surfaces in contact. Where k = constant is called the kinetic coefficient of friction.
- Second law: The dynamic frictional force is independent of the shape and apparent area of the contact surfaces.
- Third law: It depends on the nature and material of the contact surface.
- Fourth law: It is independent of the speed of the object in contact as long as the relative speed between the object and the surface is not too great.
Ques. A man pushes a cardboard box with a mass of 75.0 kg to the ground. The dynamic friction coefficient is μk = 0.520. The worker exerts a force of 400.0 N forward. What is the intensity of the friction force? In the above question, calculate the net force that moves the box? (3 marks)
Ans. On a flat surface, the normal force of an object can be obtained from the formula η = mg (1)
Substituting the value of in the equation Fk = μkη, we get
Fk = (0.520) (75.0kg ) (9.80m / s2) = 382.2N
The net force acting on a body is the sum of all the forces acting on the body. In this case, the forces acting on the body are the force exerted by man and the dynamic friction acting in the opposite direction. If the forward movement is considered positive, the net force is calculated as follows:
Fnet = Fworker – FK
By replacing the values of the previous equation, we obtain
Fnet = 400 N – 382.2 N = 17.8 N
Ques. Why is the rolling motion subject to friction? (4 marks)
Ans. In theory, a ball makes point contact with the surface, but in reality, the sphere (and/or the surface) deforms under the effect of the load and the contact area becomes elliptical.
In theory, rolling surfaces, such as those found in most linear bearings (except plain bearings) should not be subjected to frictional forces.
But in real-world applications, three factors cause friction on the running surfaces:
- Micro-slip between the surfaces (the surfaces slide relative to one another)
- Inelastic properties (i.e. deformation) of the materials
- Roughness of the surfaces
Ques. What are the 5 examples of kinetic friction? (5 marks)
Ans. The examples of kinetic friction are given below
- A rock rolling off a slope.
- A man running in a 100 m race.
- A moving truck or car.
- A rolling football.
- Movement of a snake.
- Moving on roller skates.
- Mountain Skiing.
- Sliding down a kid's slide.
Ques. Differentiate between static friction and kinetic friction. (4 marks)
Ans. The difference between static and kinetic friction are as follows:
| Static Friction | Kinetic Friction |
|---|---|
| Static friction can be defined as the friction found between two or more objects that do not move relative to one another. | Kinetic friction can be expressed as the friction found between two or more objects that move relative to one another. |
| The amplitude of the static friction is greater because of the greater value of its coefficient. | The amount of dynamic friction is relatively lesser because of the low value of its coefficient. |
| The equation denoting static friction is, Fs = μsη | The equation denoting dynamic friction is, Fk = μkη |
Ques. Kinetic friction is also known as: (1 mark)
(a) Sliding friction
(b) Dynamic friction
(c) Both sliding and dynamic friction
(d) Static friction
Ans. The correct answer is c. Both sliding and dynamic friction.
Ques. Kinetic friction is always: (1 mark)
(a) Greater than static friction
(b) Lesser than static friction
(c) Equal to static friction
(d) Equal to contact force
Ans. The correct answer is b. Lesser than static friction.
Ques. A car of 1000 kg moving with a velocity of 18 km/hr is stopped by the brake force of 1000 N. What is the distance covered by it before coming to rest? (3 marks)
Ans. Given,
m = 1000 kg
u = 18 kmph = 5 m/s
v = 0
F = ma = 1000a
Given F = 1000N
→ 1000 = 1000a
a = 1m/s2
Now, v2 - u2 = 2as
s = v2 - u2 /2a
= -25/2
= 12.5 m (the negative sign is of the direction)
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