Somewhere in your book or your notes, you must have met the formula for the
gravitational attraction between two bodies. If you can go back and find it, you
only need to plug your numbers into that formula, and out will pop the answer.
Formula: Force = G (mA x mB) / (distance)²
If everything is in SI units, then G = 6.67 x10⁻¹¹ newton-meter² / kilogram²
You said that
mA = 8.1 kg
mB = 6.5 kg
distance = 0.5 m .
Force = (6.67 x 10⁻¹¹ nt-m²/kg²) (8.1kg x 6.5kg / (0.5m)² =
(6.67 x 10⁻¹¹ nt-m²/kg²) ( 52.65 kg² ) / (0.25 m²) =
1.4047 x 10⁻⁸ newtons .
That's roughly 5.052 x 10⁻⁸ ounce . (5% of one micro-ounce)