The coefficient of friction refers to the ratio between the force applied to push an object placed on a surface and the force that resists this motion. The coefficient of kinetic friction is the value when the object is moving, while the coefficient of static friction is the value when the object is at rest.
Many products are designed with safety in mind by utilizing the coefficient of kinetic friction, such as how a car stops when the brake pedal is pressed.
In this article, we will explain the coefficient of kinetic friction, its differences from the coefficient of static friction, applications, principles, and measurement methods.
What is the Coefficient of Kinetic Friction?
When pushing an object placed on a surface, the coefficient of friction (μ) is defined as the ratio of the applied force (F) to the normal force acting perpendicular to the contact surface, and the following relationship holds true.
| μ＝F/N |
The frictional force is a reactionary force that opposes the applied force and acts in the opposite direction. Specifically, the coefficient of kinetic friction refers to the coefficient of friction when the object is in motion due to the applied force.
The coefficient of kinetic friction remains constant regardless of the speed at which the object is moving. This is known as Coulomb’s law.
Differences from the Coefficient of Static Friction
The frictional force when an object is in motion is called kinetic friction, whereas the frictional force that must be overcome to start the object moving is called static friction.
Both the kinetic friction and the static friction, when divided by the applied force, yield the coefficient of kinetic friction and static friction, respectively. The following relationship holds between the coefficient of static friction and the coefficient of kinetic friction.
| Coefficient of Static Friction (μ1) > Coefficient of Kinetic Friction (μ2) |
When gradually increasing the force applied to an object, the force required to initiate motion is balanced by the static friction. In other words, until the object starts to move, the static friction force acts in the opposite direction and is equal to the applied force.
The static frictional force just before an object starts to move is called the maximum static frictional force. On the other hand, the frictional force while the object is in motion is called kinetic frictional force.
The applied force and the kinetic frictional force have the same value and remain constant while the object is in motion.
Applications of the Coefficient of Kinetic Friction
A representative example of practical items that utilize the coefficient of kinetic friction is the brake system found in automobiles. When you press the brake pedal while driving, kinetic friction occurs between the brake pads and the disc rotors, which are rotating with the tires, causing the tires to slow down.
If the coefficient of kinetic friction between the brake pads and the disc rotors is too high, the brakes will engage too abruptly. Conversely, if it is too low, the braking performance will be poor.
Additionally, the coefficient of kinetic friction is important for the soles of shoes, which prevent people from slipping on the ground. It is a crucial physical property when selecting comfortable and safe materials and surface patterns.
Principles of Kinetic Friction
The kinetic friction force is a reactive force opposing the applied force. The cause of this reactive force is primarily influenced by three forces between the object and the contacting surface (contact surface). The first is van der Waals forces between molecules or atoms of the object and the contact surface. The second is interactions in solid-solid contact surfaces such as metallic bonding, ion bonding, hydrogen bonding, and covalent bonding (the dominant interaction depends on the materials of the object and the contact surface). The third is the environment where the object and the contact surface meet.
The first and second forces are largely determined by the materials of the object and the contact surface. However, the third factor, the environment where the object and the contact surface meet, can vary significantly depending on the physical shape, cleanliness, and roughness of the contact surface.
The reason why the coefficient of kinetic friction shows a smaller value than the coefficient of static friction is explained by the fact that when the object is in motion, the contact area between the object and the contact surface decreases (source).
Source: R. Holm, Electric Contacts, Gebers, Stockholm, 1946
Measurement Methods for the Coefficient of Kinetic Friction
The typical methods for measuring the coefficient of kinetic friction and the coefficient of static friction involve measuring load power and measuring vibration damping. The load power method involves placing an object on a horizontal surface and pulling it with a driving motor to measure the force required.
The vibration damping method measures the damping caused by friction when an object is placed on a horizontal surface. Additionally, the method calculates the angle at which an object placed on an inclined plane begins to slide; this is known as the incline method.
 Measurement of Friction Coefficient (Incline Method) |
Conclusion
This article has explained the coefficient of kinetic friction, which is observed when an object is in motion. The coefficient of kinetic friction is defined as the ratio of the applied force pushing an object placed on a surface to the resisting force trying to keep it stationary.
Moreover, it is typically smaller than the coefficient of static friction observed when the object is at rest. The coefficient of kinetic friction is utilized in designs such as automobile brakes and shoe soles that prevent slipping on the ground.
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