Spring & Oscillation

1. Mar 29, 2009

azn4life1990

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

Astronauts in space cannot weigh themselves by standing on a bathroom scale. Instead, they determine their mass by oscillating on a large spring. Suppose an astronaut attaches one end of a large spring to her belt and the other end to a hook on the wall of the space capsule. A fellow astronaut then pulls her away from the wall and releases her. The spring's length as a function of time is shown in the figure

http://session.masteringphysics.com/problemAsset/1073872/3/knight_Figure_14_36.jpg
What is her mass if the spring constant is 240N/m
2. Relevant equations

Hookes Law
F = -kx
U = (1/2)*k*x2

3. The attempt at a solution

So the spring was pulled back 1.4 metres and then released where it went to the point 0.6
so 0.6 is the point at equilibrium, so -0.8 is the displacement and that is the x

F = (240 * 0.8)
and then im not sure what to do, im not even sure if im doing this right and help would be appreciated

2. Mar 29, 2009

azn4life1990

im pretty sure my approach is wrong...

F = -260 * -0.8 = 208N

mg = weight

208/9.8 = m

m = 21.224489795918367346938775510204kg

im sure thats wrong is there a better way to approach this problem and why is the way im approaching it wrong?