Sir Isaac Newton
Beaver and Steve met Sir Isaac Newton as they once walked through a rift in the space/time continuum in Comic 14. They were just about to witness him 'invent' gravity, but the apple that Steve had plucked, turned out to be the one that hit Newton and this created an alternate timeline where Newton invented lunch instead.
After briefly returning to the present day and noticing how life had become more difficult without 'some sort of force of mutual attraction between objects with mass', they decided they had to go back in time and restore history by hitting Newton with a different apple, launched from a catapult.
The force that they propelled the apple with (which incidentally was equal to its mass multiplied by its acceleration) was, however, too great and ultimately lethal to Sir Newton, who perished as a result in Comic 15.