4.27 The force P acts on a 15kg crate initially at rest on a horizontal surface (see Fig. 4.39). The coefficients of friction for the crate and the surface are s=0.35 and k=0.20. (a) For what range of values of P will the crate remain at rest? (b) If P=30N, what force does the surface exert on the crate? (c) If P=55N, what is the acceleration of the crate? Figure 4.39 Problem 4.27