a 3 kg rock sits on a 0.8 meter ledge. If it is pushed off, how fast will it be going at the bottom?

As long as it sits on the shelf, its potential energy relative to the floor is . . .   Potential energy =      (mass) x (gravity) x (height) =                                        (3 kg) x (9.8 m/s²) x (0.8m) = 23.52 joules .If it falls from the shelf and lands on the floor, then it has exactly that same amount of energy when it hits the floor, only now the 23.52 joules has changed to kinetic energy.   Kinetic energy =                                          (1/2) x (mass) x (speed)²                                                 23.52 joules = (1/2) x (3 kg) x (speed)²Divide each side by  1.5 kg :     23.52 m²/s² = speed²Take the square root of each side:    speed = √(23.52 m²/s²) =  4.85 m/s  (rounded)