Calculate Elevator Accel. - FN=mg+ma

[mpd105]

Homework Statement

A person stands on a bathroom scale in a motionless elevator. When the elevator begins to move, the scale briefly reads only 0.71 of the person's regular weight. Calculate the magnitude of the acceleration of the elevator.

Homework Equations

FN=mg+ma

The Attempt at a Solution

With the way the problem is worded, I'm assuming that the elevator is going down, which could mean that the equation should be FN=mg-ma? Other than assuming that I really have no idea how to start this problem.

 

[Spinnor]

You are in an elevator at rest standing on a scale. What are the forces that act on you?

Sum F = N - mg = o

Now the elevator accelerates downward with acceleration a, what are the forces that act on you?

Sum F = N' - mg = ma

But N' = .71mg