Question

A 8.1-kg rock and a 8.4 × 10-4-kg pebble are held near the surface of the earth. (a) determine the magnitude of the gravitational force exerted on each by the earth. (b) calculate the magnitude of the acceleration of each object when released.

Answer 1

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.