Ican't find this answer anywhere, so i'll ask here: what item did they base the si unit of mass (grams) off of? (in other words, to what object did they say "hey, lets make this equal 1 gram and then go from there"

An electron moves with a constant horizontal velocity of 3.0 × 106 m/s and no initial vertical velocity as it enters a deflector inside a tv tube. the electron strikes the screen after traveling 26 cm horizontally and 19 cm vertically upward with no horizontal acceleration. what is the constant vertical acceleration provided by the deflector? (the effects of gravity can be ignored.)