For smaller buildings, engineers design elevators to travel slowly because it is more economical. An elevator driven by an electric motor carries a group of people, of total mass 900 kg, to the upper floor of the building. The mass of 900 kg accelerates uniformly from rest to a speed of 4 ms-1, in a time of 3 s, and continues at this speed until it is brought to rest by applying brakes. (i) Calculate the acceleration of the total mass and the force required to produce this acceleration.