Scale Hanging From Ceiling of Elevator Question

[hueyosie]

Homework Statement

An object is hung from a spring scale attached to the ceiling of an elevator. The scale reads 65 N when the elevator is standing still. What is the reading of the scale when the elevator is moving upward with a speed of 7.6 m/s and decelerating at 2.4 m/s^2?

Homework Equations

F=ma

The Attempt at a Solution

I know that I should use the F=ma equation but according to the solutions guide, I should use F=ma=Wa/g to get (65 N)(-2.4 m/s^2)/(9.81 m/s^2) = -16 N
then do -16N-(-65N) = 49N. I am not understanding where the Wa/g part came from or why I should subtract -65N from -16N. Any help would be much appreciated! I'm having a lot of trouble with my physics class :(

 

[tal444]

Wa/g is where your are dividing the weight of the scale by acceleration of gravity to get the mass of the scale, then you multiply by the acceleration of the elevator to get the force acting on the scale. As for the second part, think about it. When you are deccelerating in an elevator going up, do you feel heavier or lighter? Does the decceleration "take away" or "add to" your weight?

 

[hueyosie]

Wa/g is where your are dividing the weight of the scale by acceleration of gravity to get the mass of the scale, then you multiply by the acceleration of the elevator to get the force acting on the scale. As for the second part, think about it. When you are deccelerating in an elevator going up, do you feel heavier or lighter? Does the decceleration "take away" or "add to" your weight?

Oh, I definitely understand it now! Thank you sooo much for explaining it to me! :)

 

[tal444]

Np ;)