How Should the Spring Constant k Affect the Efficiency of a Catapult?

[pcgmks]

Independent variable and spring constant k

Hi all,
If someone could spare a minute to help answer my question I would appreciate it greatly,

I am trying to design an experiment to investigate the efficiency of a catapult,
But need help in what i should use as the independent variable-
and how i should use the spring constant k ( get a spring with a known constant k or change the constant each time)

I am using a Catapult- a standard weight- and will to measure distance of projectile.
find the kinetic energy- divide it by the Potential energy to get % efficiency,

2. Homework Equations

Elastic Potential Energy Ee=0.5kx^2 (hookes law)
Kinetic Energy Ek= 0.5mv^2 (v=d/t)

To find the Energy efficiency = Ek/Ee x100

3. The Attempt at a Solution

Need to clarify what i need to change each time to get a range of data,
Would the independent variable - being the length of the catapult arm work (is this the same as how far the spring or rubber band is stretched -the x in elastic pot. formula -) ?
or do I change something to do with the Elastic energy properties like the constant k?

the the dependent variable is how far the projectile is launched. and was thinking that i collect how far the weight is projected each time- from the same angle of 45*?

if anyone could help soon, it would be much appreciated,
thanks

 

[pcgmks]

Would it be easier if i had a spring with a known spring constant k and change the arm length each time,
( i know that a larger arm will make it go further but has it got to do with torque or is it irrelevant to this experiment),