hai vat nho giong nhau thi khoi luong nhu nhau la: m (kg)
khoang cach giua hai vat la r(m)
luc hap dan F(hd) = G* m² / r² (N)
lực tĩnh diện F = k* I-eI * I-eI / r² (vi moi vat thua 1 electron nen dien tich moi vât la: -e *(Culong)
theo bai ta co F(hd) = F
<=>G* m² / r² = k* I-eI * I-eI / r²
<=>G* m² = k* e²
=> m² =(k* e²) /G = [9*10⁹ * (1.6 *10^-19)² ] / (6.67* 10^-11) ≈3.45*10^-18
=> m = 1.86*10^-9 kg

(lay G = 6.67*10^-11 Nm²/kg²; k = 9*10^9 Nm²/C² )