Answer :
The rock's potential energy just as it rolls off the edge of the cliff is
(mass) (gravity) (height) = (10kg) (9.8 m/s²) (10 m) = 980 kg-m²/s²
That's exactly the kinetic energy it will have when it hits the ground.
Kinetic Energy = (1/2) (mass) (speed)²
980 kg-m²/s² = (1/2) (10kg) (speed)²
Divide each side by 5kg :
Speed² = (980 kg-m²/s²) / (5 kg) = (196 m/s)²
Speed = square root of (196 m²/s²) = 14 m/s
By the way . . . the mass doesn't matter. Without air resistance
(which is always ignored in problems like this), It doesn't matter
if it's a rock, a feather, a bowling ball, or a bus. If it falls from
10 m, then it lands at 14 m/s . That's how gravity works.
