Chapter 6, Problem 84P

In Fig. 6-58, force $\stackrel{\to }{F}$ is applied to a crate of mass m on a floor where the coefficient of static friction between crate and floor is µ_s. Angle θ is initially 0° but is gradually increased so that the force vec tor rotates clockwise in the figure. During the rotation, the magnitude F of the force is continuously adjusted so that the crate is always on the verge of sliding. For µ_s = 0.70, (a) plot the ratio F/mg versus θ and (b) determine the angle θ_inf at which the ratio approaches an infinite value. (c) Does lubricating the floor increase or decrease θ_inf, or is the value unchanged? (d) What is θ_inf for µ_s = 0.60?

Figure 6-58 Problem 84.