There are many types of friction like sliding friction, rolling friction, static friction, etc. The coefficient of friction involved in kinetic friction is called the coefficient of kinetic friction. Frictional force can be divided into two categories based on the state of motion- static friction and kinetic friction. Kinetic friction is the friction involved between two sliding objects when they are in relative motion. In this article, we discuss the formula for the coefficient of kinetic friction and see a few examples of how to calculate the coefficient of kinetic friction. For more details, you read our blog post on the coefficient of friction.
The equation for the coefficient of kinetic friction
Fk = μk * N [frictional force equation where N is the normal force]
μk = Fk / N [ The coefficient of kinetic friction is denoted by a mu symbol with a subscript k.(μk)]
From the above equation coefficient of kinetic friction can be defined as the ratio between kinetic friction and normal force. So, when the normal force is of unit value then the coefficient of kinetic friction is equal to the kinetic friction itself. The coefficient of kinetic friction is unique for a pair of materials.
The coefficient of kinetic friction is applicable only when the object is in motion. When the object is at rest, the coefficient of static friction is used. The coefficient of kinetic friction is generally less than the value of the coefficient of static friction.
You can visit this link for examples of values of coefficient of kinetic friction for different pairs of materials.
Solved examples on calculation for kinetic friction
1.A wooden box is being slid against a glass surface. The coefficient of friction between the wooden box and the glass surface is 0.5. The normal force acting between the box and the glass surface is 50 N. Calculate the kinetic friction between the surfaces.
We know the formula for kinetic friction force as Fk = μk * N
Hence the value of kinetic friction will be = 0.5 * 50 = 25 N
2. A book is resting on a table. The weight of the book is 20 N. The coefficient of friction between the table and the book is 0.3. Calculate the coefficient of kinetic friction?
In this case, the book is in a stationary condition hence we cannot calculate the kinetic friction. There is no kinetic friction and the coefficient of friction provided above is static friction as the book is in a state of rest.
3. A car is moving down the slope with an angle of inclination of 60 degrees. the weight of the car is 1500 N. Calculate the coefficient of kinetic friction if the kinetic friction is 750 N.
Since an incline of 60° is involved here we will have to resolve the forces, by resolving the weight we get the normal force as mg cos θ. So the equation for kinetic friction gets modified as
Fk = μk * mg cos θ
750 = μk*1500 * cos 60
μk = 1
