The person is standing on the floor of the elevator, and he isn't
floating up or smashing through the floor.
So the person's acceleration is exactly the same as the elevator's
acceleration.
Now, Newton's second law of motion says:
F = M · a
(the net force on the person) = (the person's mass) x (his acceleration)
Net force = (80 kg) x (0.5 m/s²)
= 40 newtons .
