Answer:
[tex]2.9\cdot 10^{-7}N[/tex]
Explanation:
The electrostatic force between the two balloons is given by:
[tex]F=k\frac{q_1 q_2}{r^2}[/tex]
where
[tex]k=9\cdot 10^9 Nm^2 C^{-2}[/tex] is the Coulomb's constant
[tex]q_1=q_2=-4 nC=-4\cdot 10^{-9}C[/tex] is the charge on each balloon
[tex]r=70 cm=0.7 m[/tex] is the distance between the balloons
Substituting into the formula, we find
[tex]F=(9\cdot 10^9 Nm^2C^{-2})\frac{(-4\cdot 10^{-9}C)^2}{(0.70 m)^2}=2.9\cdot 10^{-7}N[/tex]
