Aperson with no experience in fluid mechanics wants to estimate the friction factor for 1-in.-diameter galvanized iron pipe at a reynolds number of 12000. the person stumble across the simple equation of f = 64/re and use this to calculate the friction factor. consider the problem with this approach and estimate percentage of their error (fa-f)/fa, where fa – actual friction factor.

To save energy, the air supply to a 2000 ft office has been shut off overnight and the room temperature has dropped to 40°f. in the morning, the thermostat is reset to 70°f and warm air at 120 f begins to flov in at 200ft'/min. the air is well mixed within the room, and an equal mass flow of air at room temperature (changing with time) is withdrawn through a return duct. the air pressure is nearly 1 atm everywhere. ignoring heat transfer with the surroundings and kinetic and potential energy effects, estimate how long it takes for the room temperature to reach 70°f. plot the room temperature as a function of time.