Understanding the Force on a Spring: Why is it 1.2 - 9x?

[hachi_roku]

Homework Statement

a .7 lb block rests on top of a .5 lb block supported by but not attached to a spring of constant 9 lb/ft. the upper block is suddenly removed. determine the maximum velocity reached by the .5 lb block.

Homework Equations

The Attempt at a Solution

i have already solved the probelm. my question is, why is the expression for the force on the spring F = 1.2 - kx =>> 1.2 - 9x.

i get the 1.2 part but I am not understanding why its 1.2 minus 9x. I am trying to understand this visually with arrows but its not quite working...pls help!

 

[rock.freak667]

So when both blocks are on the spring, what is the extention? (What is the initial energy of the spring?)

When the 0.5lb block is on the spring alone, what is the extension? (What the energy it has due to the 0.5lb block alone?)

What is the change in elastic energy of the spring converted to?

 

[hachi_roku]

it doesn't say in the question...the solution manual calls it x.