Newton's formula for the force of gravity:
F = G · M₁ · M₂ / D²
Here's what the letters mean:
F . . . the strength of the gravitational force between two objects
G . . . the universal gravitational constant for SI units: 6.67 x 10⁻¹¹ N·m²/kg²
M₁ . . . the mass of one object
M₂ . . . the mass of the other object
D . . . the distance between the centers of the objects
If the radius of each grapefruit is 0.08 meter and their skins are touching,
then their centers are 0.16 meter apart.
The force between them is
F = G · M₁ · M₂ / D²
= (6.67 x 10⁻¹¹ N·m² / kg²) · (0.45 kg) · (0.45 kg) / (0.16 m)²
= (6.67 x 10⁻¹¹ · 0.45 · 0.45) / (0.0256) N·m²·kg²/kg²·m²
= ( 1.35 x 10⁻¹¹) / (0.0256) Newton
= 5.28 x 10⁻¹⁰ Newton (rounded)
(about 0.0000000019 ounce)