a). Gravitational force exerted by the Earth = (mass) x (grav acceleration)
-- 8.1 kg rock: (8.1 kg) x (9.81 m/s²) = 79.46 Newtons
-- 8.4 x 10⁻⁴ kg pebble: (8.4 x 10⁻⁴ kg) x (9.81 m/s²) = 0.00824 Newton
b). Acceleration following release.
Near the Earth's surface, and neglecting air resistance, all objects have the same acceleration when released. If air resistance can be neglected, then a feather, the 0.00084 kg pebble, the neckline of an evening gown, a school-bus, and the battleship USS Midway, if dropped together, will fall together and hit the ground at the same time.