# Homework Help: Quick dynamic/spring question

1. Jul 24, 2009

### hachi_roku

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

2. Relevant equations

3. 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 im not understanding why its 1.2 minus 9x. im trying to understand this visually with arrows but its not quite working...pls help!

2. Jul 24, 2009

### 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?

3. Jul 25, 2009

### hachi_roku

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