18. Crates of mass 80.0 kg are loaded onto a truck, a vertical lift of 1.3 m. To make the work easier, a 2.6 m long ramp is used and the crates are pushed along the ramp from the ground onto the truck. The coefficient of friction between the crates and the ramp is 0.56. 2.6 m 80.0 kg Hx-0.56 1.3 m (a) Determine the minimum force required to push a crate along the ramp. Include an appropriate free-body diagram.(11.8 N) (b) If a crate is let go from rest at the top of the ramp and begins to slide, how long will it take to reach the bottom of the ramp? Include a new free-body diagram. (5.9 s) (c) What minimum value for the coefficient of static friction will prevent the crate from beginning to slide if it is let go at the top of the ramp? (0.58)