An 80 kg man stands in an elevator. What force does the floor of the elevator exert on the man ifa) the elevator is stationary? b) the elevator accelerates upward at 2 m/s2? c) the elevator rises with constant velocity 4 m/s? d) while rising, the elevator decelerates at 1.5 m/s2? e) the elevator descends with constant velocity 7 m/s?