Content Writer
Kinetic Friction is a force that acts between moving surfaces. An object moving on a surface experiences a force in the opposite direction of its movement. The magnitude of the force depends on the coefficient of kinetic friction between the two materials. The coefficient of Kinetic Friction is denoted by the symbol μk. The force of Kinetic Friction is the product of the coefficient of kinetic friction (μk) and normal force (η). Kinetic Friction Formula is given as Fk = μkFη. It is expressed in Newtons (N).
Read More: NCERT Solutions For Class 11 Physics Laws of Motion
| Table of Content |
Key Terms: Kinetic Friction, Kinetic Friction Formula, Motion, Force, Coefficient of Kinetic Friction, Normal Force, Friction
What is Kinetic Friction?
[Click Here for Sample Questions]
Kinetic Friction is defined as the resistance towards the motion caused by the contact between a surface and the moving object against it. The force of kinetic friction acts in the opposite direction of the moving object. It means that on pushing something forward, friction will push it backward.
- The force of kinetic friction is only applicable to a moving object, else it is known as sliding friction.
- The magnitude of the force would depend upon the coefficient of kinetic friction between the two materials in contact.
- The coefficient of Kinetic Friction is represented by the symbol μk.
- The kinetic friction force is expressed in the units of Newtons (N).
![Kinetic Friction]()
Kinetic Friction
Kinetic Friction Formula
[Click Here for Previous Years’ Questions]
Kinetic Force is the retarding force between two moving planes that are in contact with each other. The kinetic friction force is μk times the normal force on an object.
Kinetic Friction Formula is given as
| Fk = μkFn |
Where
- Fk denotes the Force of Kinetic Friction.
- μk denotes the Coefficient of Kinetic Friction.
- Fn denotes the Normal Force.
Read More: Class 11 Laws of Motion Class Important Questions
If the situation involves a horizontal surface and no other vertical forces are acting, then,
| Fn = mg |
Where
- Fn is the Normal Force.
- m is the mass of the object.
- g is the acceleration due to gravity.
Read More: Value of G
Solved ExamplesExample 1: A child is playing on the ground. What will be the kinetic friction if the friction coefficient is given as 0.5 and the football is kicked with the force of 150 N? Solution: Given parameters are,
Using the Kinetic Friction Formula, Fk = μkFn Fk = 0.5×150 = 75 N Thus, the force of kinetic friction is 75 N. Example 2: A basketball player is playing basketball. Calculate the kinetic friction if the friction coefficient is 0.8 and the ball is thrown with the force of 200N. Solution: According to the question,
Using Kinetic Friction Formula, Fk = μkFn Fk = 0.8 × 200 Fk = 160 N Thus, the kinetic friction is 160 N. |
Also Read:
Previous Years’ Questions (PYQs)
- If the coefficient of static friction is μs and coefficient of kinetic friction is…
- Which is true for rolling friction, static friction, and kinetic friction…
- A block of mass 2 kg rests on a plane inclined at an angle… (JCECE 2005)
- The work done by a body against friction always results in…
- If the coefficient of static friction between the tyres and road… (BCECE 2007)
- Three blocks of mass 4kg, 2 kg, and 1 kg respectively are in contact… (NEET 2015)
- A 600 kg rocket is set for a vertical fring. If the exhaust speed is… (NEET 1990)
- A bullet of mass 10 g moving horizontally with a velocity of… (NEET 2016)
- A brick of mass 2kg slides down an incline of height 5m and angle… (BITSAT 2019)
- A heavy uniform chain lies on horizontal table top. If the coefficient… (NEET 1991)
- Two masses as shown in the figure are suspended from a massless… (NEET 2000)
- A bullet is fired from a gun. The force on the bullet is given by… (NEET 1998)
Things to Remember
- Kinetic Friction is defined as the force that acts between two moving surfaces.
- The force of kinetic friction acts in the opposite direction of the object in motion.
- The coefficient of kinetic friction is (μk).
- Kinetic Friction Force is the product of the coefficient of kinetic friction (μk) and normal force (η).
- Kinetic Friction Formula is given as Fk = μkFn
Sample Questions
Ques. A worker pushes a large cardboard box across the floor. In this case, the coefficient of kinetic friction is 0.520. The mass of the box is 75.0 kg, and the worker is exerting a force of 400 N forward. What will be the kinetic friction force? (3 Marks)
Ans. According to the question,
- Coefficient of Kinetic Friction (μk) = 0.520
- Mass of the box (m) = 75 Kg
The normal force on an object on a flat surface is η = mg. Substituting its value in the Kinetic Friction Formula,
Fk = μk η
Fk = μk mg
Fk = (0.520) x (75.0 kg) x (9.80 m/s2)
Fk = 382.2
Fk = 382.2 N
Thus, the kinetic friction force is 382.2 N.
Ques. What is Kinetic Friction Formula? (3 Marks)
Ans. The kinetic friction formula is:
Fk = μk η
Where
- Fk: Force of kinetic friction
- μk: Coefficient of kinetic friction
- η: Normal force
Ques. A vehicle is moving at a uniform speed with a normal force of 1000 N. The kinetic friction applied to this car is 500 N. What will be the coefficient of the kinetic friction involved? (3 Marks)
Ans. Given parameters are:
- Normal Force Fn = 1000N
- Kinetic Friction Force Fk = 500N
Using Kinetic Friction Force Formula,
Fk = μkFn
Rearranging it,
μk= Fk/Fn
μk = 500/1000 = 0.5
Thus, the coefficient of kinetic friction is 0.5.
Ques. Give a few examples of Kinetic Friction. (3 Marks)
Ans. Here are a few examples of Kinetic Friction:
- A man running in a race.
- A moving car or a truck.
- A player playing football.
- A rolling ball or football.
- Movement of Snake.
Ques. A child is playing football in a school playground. What will be the kinetic friction if the friction coefficient is 0.6 and the ball is thrown with the force of 300N? (3 Marks)
Ans. According to the question,
- Friction Coefficient μk = 0.6
- Fn = 300 N
Using Kinetic Friction Formula,
Fk = μkFn
Fk = 0.6 × 300
Fk = 180 N
Thus, the kinetic friction, in this case, is 180 N.
Ques. What are the laws of Kinetic Friction? (5 Marks)
Ans. There are four main laws of kinetic friction which are described below:
- First Law: Kinetic friction force (Fk) is directly proportional to the normal force (N) between two materials in contact.
- Second Law: The force of kinetic friction is independent of the shape and the apparent area of the surfaces in interaction.
- Third Law: The magnitude of the force depends upon the nature and material of the surface in contact.
- Fourth Law: Kinetic Force is independent of the velocity of an object in contact given that the relative velocity between the object and the surface is not too large.
Ques. Give any two real-life applications of Kinetic Friction. (3 Marks)
Ans. Here are a few real-life examples of kinetic friction:
- Kinetic Friction is responsible for wear and tear and thus we need oil to lubricate machine parts to reduce friction.
- When two objects are rubbed are in contact with each other or are rubbed against each other, the frictional force is converted into thermal energy, in a few cases giving rise to fire.
Ques. A bus is moving at a uniform speed on the highway. If the friction coefficient is 0.7 and the normal force is 200 N, what will be the kinetic friction force? (3 Marks)
Ans. Given parameters are,
- Coefficient of Kinetic Friction μk = 0.7
- Normal Force Fn = 200 N
Using the Kinetic Friction Formula,
Fk = 0.7×200 = 140 N
Thus, the force of kinetic friction is 140 N.
Ques. What is meant by Kinetic Friction? (3 Marks)
Ans. Kinetic friction is defined as a type of force that acts between moving surfaces. It is opposite to the direction of the object that is an object that is being moved over a surface will experience a force in the opposite direction as its movement. In kinetic friction, the magnitude of the force depends on the coefficient of kinetic friction between the two materials being in contact with each other.
Ques. List the difference between Static Friction and Kinetic Friction. (3 Marks)
Ans. The difference between Static Friction and Kinetic Friction is listed below:
| Static Friction | Kinetic Friction |
|---|---|
| It is a type of friction present between two or more objects which are not moving. | It is a type of friction that is present between two or more objects that are in motion. |
| The magnitude of static friction is greater because the value of its coefficient is more. | The magnitude of the kinetic friction is less due to the low value of the kinetic friction coefficient. |
Ques. What is the coefficient of Kinetic Friction? (3 Marks)
Ans. The coefficient of kinetic friction is defined as a unitless quantity that depends on the surface roughness of the two materials in contact. The coefficient of kinetic friction would be high for a rougher surface, and vice-versa. For instance, the coefficient of kinetic friction for wood on wood is 0.2.
Ques. A baller threw a shot-put ball on the ground. What will be the kinetic friction if the friction coefficient is 0.9 and the shot-put ball was thrown with a force of 400N? (3 Marks)
Ans. Given that,
- Friction Coefficient μk = 0.9
- Fn = 400 N
Using Kinetic Friction Formula,
Fk = μkFn
Fk = 0.9 × 400
Fk = 360 N
Thus, the kinetic friction, in this case, is 360 N.
Check-Out:
| Class 11 & 12 PCMB Study Guides | ||
|---|---|---|
Comments