If the surface energy of a magnesium oxide - nickel oxide (mgo-nio) solid solution is 1.13 j/m2 and its elastic modulus is 196 gpa, what is the maximum length of an internal elliptical flaw, in mm, if the ceramic part must survive an applied tensile stress of 39 mpa normal to this elliptical flaw ?

be careful !

make sure you reference the flaw geometry figures that explain how the length of a flaw type (internal / external) correlates to the value of a.

answer format: x. xx

unit: mm

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 the air is well mixed within the room, and an equal mass flow of air at room temperature (changing with time) is withdrawn through a return duct.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.