An indestructible bullet, 2cm long, is fired straight through a board which is 10 cm thick. The bullet strikes the board with a speed of 420m/s and emerges with a speed of 280m/s. What is the average acceleration of the bullet through the board? What is the total time the bullet is in contact with the board? How many thicknesses of board ( calculated to the nearest tenth cm ) would it take to completely stop the bullet?