A quiet moment during a test is interrupted as a physics book falls from a table and strikes the floor. The reaction force to the force of the Earth pulling the book
downwards is
O the book hits the floor
O the floor pushes up on the book
O the book exerts an upward pull on the Earth
O the force of gravity on the book

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?