This is known as a ratio of limiting frictional force or F. The members will be in contact and force of friction will be placed with N or normal reaction in a ration.
F / N =µ
F = µN
Here you must consider two situations,
First, if the friction is static then it’ll be written as 1 – l or 1 – ls.
Second, if the friction is kinetic then it will be Ilk.