Hooke's law and kinetic energy?

[jevillan]

Homework Statement

A 0.75 kg block is held at rest against a spring compressing it 27.0 cm. The spring constant is 735 N/m. The block is released and it begins to move toward a loop across a frictionless surface. How much kinetic energy does the block have when the spring is compressed 13.5 cm? What is the radius of the loop if it just makes it around the top of the loop?

Homework Equations

F = kx
Ek = (1/2)mv^2

The Attempt at a Solution

F = kx
F = 735*0.27 = 198.45

So far I've found the force when the spring is compressed 27 cm...not too sure how that value of Force is even relevant to finding the answer :S

 

[gneill]

Investigate the energy stored in the spring and the use of conservation of energy to determine the velocity of the block at various points along its journey.