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Homework Statement
A linear elastic spring is [tex]0.15m[/tex] long. When the upper end is held in the hand and a [tex]0.5kg[/tex] mass is suspended from the lower end, its length becomes [tex]0.22m[/tex]. What is the spring constant?
Homework Equations
[tex]E_{E}=\frac{1}{2}kx^{2}[/tex]
[tex]\vec{F_{spring}}=kx[/tex]
[tex]\vec{F_{G}}=mg[/tex]
[tex]E_{G}=mgh[/tex]
The Attempt at a Solution
First method:
Since the block is at rest, [tex]\vec{F}_{net}=0[/tex]
[tex]\vec{F}_{G}-\vec{F}_{spring}=0[/tex]
[tex](0.5)(9.8)=k(0.07)[/tex]
[tex]k=70N/m[/tex]Second method:
If we set the gravitational reference to where the block stops, it will be [tex]0.07m[/tex] up originally (with no elastic energy since it is not stretching the spring yet). Then when it is at the bottom, it will be [tex]0[/tex], and so all that energy will be elastic energy.
[tex]E_{G}=E_{E}[/tex]
[tex](0.5)(9.8)(0.07)=\frac{1}{2}k(0.07)^{2}[/tex]
[tex]k=140N/m[/tex]
I know the first method is right, because with [tex]k=70N/m[/tex], [tex]\vec{F}=kx[/tex] will be [tex]=4.9N[/tex], which is must be to cancel out the gravitational force and have it at rest. But shouldn't the second method also yield the same answer?