How Do You Calculate the Normal Force on a Person in an Accelerating Elevator?

[jg727]

Homework Statement

A person of mass m is standing in an elevator of mass M. The elevator is moving downward, but has an upward acceleration of a. To an observer fixed on the Earth, the force exerted on the person by the floor of the elevator is

a. (m+M)g b. m(g+a) c. m(g-a) d. M(a-g) e. M(a+g)

Homework Equations

I think I have the Free Body Diagrams correct, but I am confused by finding the normal force on the person by the floor of the elevator.

Any help would be great. Thank you.

 

[technician]

There are only 2 forces acting on the person of mass m.
1)The downwards force of gravity (the weight of the person)
2) The upwards force due to the reaction at the floor.
If the person is accelerating upwards there must be a resultant force upwards.
Can you use 1) and 2) to write down an expression for the resultant upwards force.
Then resultant force = m x acceleration