Analogy between electric and gravitational forces

The electric(Coulomb's Law) and gravitational forces have similar forms:

Note that gravitational forces are always attractive (F is always negative), while the electric force is attractive for opposite and repulsive for like charges. Also notice that the electric force is MUCH stronger:

k = 8.99x10⁹ Nm²/C²      G = 6.67x10^-11 Nm²/kg²

Examples       Coulomb's law index