A car is outfitted with a flat piece of plywood mounted vertically on its front bumper. As seen in Figure P3.91, a block of wood is simply placed in front of the car just as the car begins to accelerate. 

(a) If the coefficient of static friction between the block and the plywood is µ_S - 0.90, what acceleration is needed to keep the block from falling? 

(b) Safety concerns limit the maximum speed of the car to 50 m/s (about 110 mi/h). How long can the car keep the block from falling this way? 

(c) If the mass of the wooden block is doubled, how does the answer to part (a) change?

Figure P3.91