The quantity \(\mu_s\) in Equation 5-1b is called the \(\textbf{coefficient of static friction}\). Its value depends on the properties of both surfaces in contact; for example, \(\mu_s\) has different values for steel on steel, for steel on glass, and for glass on glass. Because both \(f_s\) and \(n\) have units of force, Equation 5-1b tells us that \(\mu_s\) must be dimensionless.