Elevator Acceleration Question

[pdonovan]

Question:
You've always wondered about the acceleration of the elevators in the 101-story-tall Empire State Building. One day, while visiting New York, you take your bathroom scale into the elevator and stand on it. The scale reads 150 lbs as the door closes. The reading varies between 120 lbs and 170 lbs as the elevator travels 101 floors.

a.)What is the maximium acceleration upward?
b.)What is the maximium magnitude of the acceleration downward?


I used the formula Fnet = N - Fg and still cannot get the correct answer.
I currently have ma = N - mg where m = 77.11kg, N = 756N, and g = 9.8.
So, 77.11a = 756 - 77.11(9.8)

Where did I go wrong?
Thank you for any help!

 

[osnarf]

Well, you know what the Fnet is.

If your just standing on the ground not moving, it's just your weight (well, it's zero if you count the ground pushing back up on you).

Bottom line is the scale is going to show the net downward force you exert on it. So you may want to set it up
Fnet = m*a + Fweight

 

[pdonovan]

How can Fnet = ma + Fweight when Fnet = ma?