Hey guys, I have my physics final tomorrow and I have this simple question from a practice exam. I have gotten the correct answer by using the momentum principal, but when I try to check my work using the energy principal I get twice the correct answer. So I would appreciate it if you guys could help me figure out what I am mixing up in the energy principal. Thanks a lot.(adsbygoogle = window.adsbygoogle || []).push({});

1. The problem statement, all variables and given/known data

A 5 kg block is attached to a spring with stifness = 1500 N/m and relaxed length 0.2 m. You hang the spring and mass from the ceiling. When the block is hanging motionless from the spring, what is the stretch?

3. The attempt at a solution

Using the momentum principal:

Pf-Pi=0=Fnet

Fnet=Fspring-Fgrav

S=mg/Ks=.03266. (The correct answer)

Using the energy principal:

System: Block + Spring + Earth

Initial State: Block is released from rest, spring is relaxed.

Final State: Block is at rest, hanging from the spring.

*I assume that the final height of the block is 0, and the initial height is = to the stretch.* (I think this is where I am going wrong?)

Ef=Ei + Wsurroundings

Kf + Uf,spring + Uf,grav = Ki + Ui,spring + Ui,grav + 0

0 + (1/2)(Ks)(s^2) + mgh = 0 + 0 + mgh

*I assume that the final height of the block is 0, and the initial height is = to the stretch.* (I think this is where I am going wrong?)

(1/2)(1500)(s^2) = mg(s)

s=2mg/1500=.065333

Thanks again for the help

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# Homework Help: Simple Spring problem

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