There are far easier ways to measure the acceleration due to gravity using Arduino. You can buy a photointerrupter like http://www.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&keywords=425-1935-5-ND&x=0&y=0&cur=USD, build the simple circuit shown in the data sheet. Then get a piece of plexiglas maybe 2" x 10" and use electrical tape to make uniformly spaced opaque strips that are perpendicular to the long axis. Take the signal from the photointerrupter and input that into one of the interrupt pins for timing purposes.
Dropping the plexiglass strip through photointerrupter, lets call it a photogate, from the times displayed by the Arduino on the computer screen or LCD you can determine the acceleration due to gravity.
You may find that the plexiglass hits the photogate and this will increase the uncertainty of the measurement and you can either accept that or break the photogate and mount the two pieces on another frame to make a larger fixture so the plexiglass will hit it less often.
I have left out some steps as there is a derivation needed utilizing some kinematics to be worked out but it is not too bad.