Construct a free-body diagram like that shown in the picture. The black box is denoted as the elevator. The system in question is the person's body depicted as a triangle. There are two opposite forces acting on the body: force brought about by the elevator's motion and the gravitational force. The sum of these vector quantities equal to the net force.
Net Force = ma - mg
Net Force = m(a - g)
where
m is the mass of the body
a is the acceleration of the elevator
g is the acceleration due to gravity equal to 9.81 m/s²