Elastic Potential- Acceleration with springs

[hsphysics2]

Homework Statement

A 1.6kg mass is suspended from a spring inside a stationary elevator. The spring stretches 12cm. The elevator now accelerates upwards, and the spring extends a further 12cm. What is the elevator's acceleration?

Homework Equations

F_y= ma_y
U_e= 1/2 (kx²)

The Attempt at a Solution

I'm just not sure on the best way to start this question.

 

[AATroop]

I would start with the first equation you provided. $F_y = ma_y$. What's the acceleration ($a_y$) in this problem? Where you can find a force on the system?

 

[hsphysics2]

F_y=ma_y
kx-mg=ma_y
a_y=(kx-mg)/m

but k is not given...

 

[AATroop]

You need to find k first. What's the spring doing BEFORE the elevator starts moving?

 

[hsphysics2]

F_y=ma_y
kx-mg=0
k=(mg)/x
k=1306.666 N/m

F_y=ma_y
kx-mg=ma_y
a_y=(kx-mg)/m
a_y=9.79999 m/s²

 

[AATroop]

I got a different value for k, could you double check it? My answer was a magnitude different.

 

[hsphysics2]

oops, so k=130.666 N/m

 

[AATroop]

Right, and you also know the distance so now would be a good time to find the potential.

 

[Doc Al]

Looks to me like you're done. No need for the potential in this problem.

 

[AATroop]

Ah, yep. I was hasty.