The Blasted Elevator Questions; General Physics- Please help

[dext]

hey everyone,
sheesh, our professor has us do our homework online, and I SWEAR my answer is right, but apparently it isnt. Here is the question:

You leave the doctors office after your annual checkup and recall that you weighed 684N. You then step onto an elevator with a scale. Find the magnitude of the elevator's acceleration if the scale reads 724N.

How do i solve this problem?
Thanks.

 

[dext]

Essentially, what I did was divide her weight in the drs office by 9.8, then I entered that mass into Newtons 2nd law equation. I divided 724 by mass--- Doesn't that give me the acceleration?

 

[leright]

You're supposed to show your work when asking questions. We're not doing your homework for you.

However, find the mass, m = 684/9.81 = 69.7 kg. Find the difference between your weight at rest and your weight on the accelerating elevator. 724-684 = 40 N. Now, find the acceleration. a = 40/69.7 = 0.574 m/s^2.

 

[leright]

Essentially, what I did was divide her weight in the drs office by 9.8, then I entered that mass into Newtons 2nd law equation. I divided 724 by mass--- Doesn't that give me the acceleration?

The net force is not 724N. 724N is the sum of the normal force exerted by the elevator and your weight.