An 80-kg man rides in an elevator from the ground to an upper floor. The elevator goes from rest to \(10 \mathrm{~m} / \mathrm{s}\) in 5 seconds, and then continues at the same constant speed for 10 seconds, before slowing down to a stop in \(5 \mathrm{sec}-\) onds. How much push from the elevator floor does the man feel during 

(a) the initial acceleration, 

(b) the constant-speed phase, and 

(c) the final deceleration?