Halliday, Resnick & Walker — Coulomb's Law: Problem 32

32 In Fig. 21-22a, particle 1 (of charge $q_1$) and particle 2 (of charge $q_2$) are fixed in place on an x axis, 8.00 cm apart. Particle 3 (of charge $q_3 = +6.00 \times 10^{-19} \text{ C}$) is to be placed on the line between particles 1 and 2 so that they produce a net electrostatic force $\vec{F}_{3,\text{net}}$ on it. Figure 21-22b gives the x component of that force versus the coordinate $x$ at which particle 3 is placed. The scale of the x axis is set by $x_s = 8.0 \text{ cm}$. What are (a) the sign of charge $q_1$ and (b) the ratio $q_2/q_1$?