Physics
posted by Tim on .
A carefully designed experiment can measure the gravitational force between masses of 9 kg. Given that the density of iron is 7860 kg/m3, what is the gravitational force between two 9.00kg iron spheres that are touching in N?

V=4πR³/3
Density ρ=m/V = 3•m/4•π•R³ = >
R=cuberoot(3•m/4•π• ρ) = cuberoot(3•9•9/4•π•7860) =0.065 m.
the gravitational constant
G =6.67•10^11 N•m²/kg²,
F =G•m1•m2/R² = =6.67•10^11•9²/0.065²=1.28•10^6 N 
The third line
R=cuberoot(3•m/4•π• ρ) = cuberoot(3•9/4•π•7860) =0.065 m. 
I'm supposed to get 3.21e07 N.

F =G•m1•m2/(2R)² = 6.67•10^11•9²/(2•0.065)²=1.28•10^6/4 =0.32•10^6 N