There are two types of dry friction:

1. Static friction – this friction applies when an object is stationary. It is calculated using:

   

   μ_s is the coefficient of static friction and N is the normal force acting on the friction contact surface. The “≤” sign means that it can go up to a maximum value of F_f,max = μ_sN.

   It is presented this way because the static friction force only serves to counter the action force to prevent motion, and in some cases a force of less than μ_sN is adequate. However when the action force exceeds μ_sN then we have kinetic friction instead.

2. Kinetic friction – this is the frictional resistive force that acts on the object as long as it is in motion and in contact with the friction surface. It is calculated using:

   

   μ_k is the coefficient of kinetic friction and N is the same normal force. Typically μ_k is less than μ_s.

Are there only two simple equations in this topic? Is this too good to be true? Yes and no… =(

The key to this topic is constructing the correct FBD with all forces identified, and determining whether the frictional force is F_f = μ_sN or F_f < μ_sN, or whether it switches to kinetic friction (F_f = μ_kN).

Let’s look at an example now.