Kinetic Friction: Definition, Formula, Applications, and Examples

Friction is the opposing or resistive force which acts between the surfaces in contact. Kinetic friction is the friction that acts between surfaces in motion, whereas static friction is the friction present between stationary surfaces of the stationary bodies. Friction is the force which helps in a lot of aspects of our daily life from walking to turning a car on plain roads. Kinetic Friction can also be further classified according to the type of motion in a plane into sliding friction and rolling friction.

Read further to know more about kinetic friction, its formula, applications and how it differs from static friction with some important FAQs.

Kinetic Friction

Kinetic Friction can be stated as the resistive force that is present between surfaces that are in relative motion relative to each other. The kinetic friction relative to one body acts in the direction which is opposite to the motion of that body. Its value depends on the kinetic friction coefficient present between the two surfaces.

Kinetic friction is present in all daily life applications, in keeping a car on the road, and in stopping the car when the brakes are applied. Kinetic friction, like any other friction, can be stated as a necessary evil because although it helps in a lot of things, nearly 20% of energy is required to overcome it.

Kinetic Friction Formula

Kinetic Friction is the product of the coefficient of friction between the surfaces mentioned by the Greek letter ‘’ and the normal reaction force. Since it is a force just like all other forces the unit of Kinetic Friction is Newton’s whose symbol is ‘N’. The equation of Kinetic Friction can be thus stated as,

Where,

is the Kinetic Friction

is the coefficient of Kinetic Friction

is the normal reaction force acting perpendicular to the body.

Difference between Kinetic and Static Friction

The differences between kinetic friction and static friction have been discussed below:

Static Friction	Kinetic Friction
Static friction is defined as the friction between surfaces of bodies that are in contact with each other but at rest respective to each other.	Kinetic Friction can be stated as the resistive force that is present between surfaces that are in motion relative to each other
The magnitude of static friction is more due to its coefficient being more than the kinetic coefficient.	The magnitude of the kinetic friction is less due to its coefficient being smaller than that of the static coefficient.
Static friction helps in keeping objects in place during their period of rest. It helps in keeping the objects on the table.	Kinetic friction is a necessary evil. Although it helps in keeping vehicles on the road and in their turning, other things a lot of energy is required to mitigate its effects.
The formula for static friction is	The formula for kinetic friction is

Applications of Kinetic Friction

Kinetic friction is applied in the following ways:

• Kinetic friction helps us in walking. During the walking motion, the friction helps the feet in pushing the ground back and as its reactionary force, our body is propelled forward.
• Kinetic friction helps us in turning a vehicle on an unbanked road by providing the necessary centripetal force required for turning.
• Kinetic friction between two bodies helps us in creating fire if the two bodies in play have sufficient coefficient of friction between them.
• Engine parts are well lubricated to reduce damage due to kinetic friction that happens during their working.

Solved Examples of Kinetic Friction

Refer to the solved examples of kinetic friction below:

Example 1.  A box is being pulled over a rough surface with constant velocity by a 100 N force. The mass of the box is 75 kg. Find the coefficient of friction.

 

Solution: Since the box is pulled at a constant velocity so there are no accelerating forces and thus the normal reaction force is equal to the weight of the body acting upward.

Thus,

Normal reaction force or = = 75 x 10 = 750 N

So, = 100/750 = 0.13

Example 2. The kinetic friction coefficient between a box and a rough surface is 0.22. How much force is required to pull it at a constant velocity? The weight of the box is 110 kg.

Solution: Since the constant and relative velocity of the box is required, thus it would be only the force to negate the friction and the normal reaction force is the same as the weight of the box. The magnitude of the force can be calculated by the kinetic friction formula which is

.

So force needed is 110×0.22 = 24.2 N

Example 3. A box of 6 kg has a constant acceleration of  on a rough surface when being pulled by a force of 35N. Find its kinetic friction coefficient.

Solution: Since the box is experiencing constant acceleration it is under a net force. The value of net force is equal to the external force minus the frictional force in this situation.

 

Since we know that

Thus,

= 0.52

Hope this article was insightful in imparting a new piece of knowledge. You can check out other Physics articles for a better understanding of concepts. To learn more about such interesting concepts and their real-life uses keep an eye out on the Testbook website or on the Testbook app. Download the Testbook App now!

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