Answer
$F = 3.8 N$
Work Step by Step
We have the following information
Down quark charge : $-1/3 e$
$r = 2.6 \times 10^{-15} m $
To find the force, we use Coulomb's Law
$F = \frac{q_1q_2}{4 \pi \epsilon_o r^2}$
$F = \frac{(-1/3e)(-1/3 e)}{4 \pi \epsilon_o r^2} $
$F = \frac{1/9 (1.6 \times 10^{-19} C)^2}{4 \pi (8.85 \times 10^{-12} F/m) (2.6 \times 10^{-15} m)^2} $
$F = 3.8 N$
