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.

Along wire of diameter 1 mm is submerged in an oil bath of temperature ∞ 31°c. the wire has an electrical resistance per unit length of 0.01 ω/m. if a current of 103 a flows through the wire and the convection coefficient is 500 w/m2·k, what is the steady-state temperature of the wire? from the time the current is applied, how long does it take for the wire to reach a temperature that is within 1°c of the steady-state value? the properties of the wire are 8000 kg/m3, 500 j/kg·k, and 20 w/m·k.