At the same instant that a 0.5 kg ball is dropped from 25 m above Earth, a second ball, with a mass of 0.25 kg is thrown straight upward from Earth's surface with an initial speed of 15.mls. They move along nearby lines and pass without colliding . At the end o 2.0 s the velocity of the center of mass of the two-ball system is :

On a mild saturday morning while people are working inside, the furnace keeps the temperature inside the building at 23degreesc. at noon the furnace is turned off, and the people go home. the temperature outside is a constant 16degreesc for the rest of the afternoon. if the time constant for the building is 3 hr, when will the temperature inside the building reach 18degreesc? if some windows are left open and the time constant drops to 2 hr, when will the temperature inside reach 18degreesc?