antigravity script

I had written earlier about lookign for a script to make my catapult fire, and have had the idea since to maybe put a triggerable antigravity pad (or somethign like that) under the arm, so that it would give a quick anti-grav burst to send the arm upwards.

if any one couild show me how to buold something like this that would be awesome please keep in mind I am a total scripting noob lol.