Equilibrium? What does this mean?

[flyingpig]

Homework Statement

Two small beads having positive charges 3q and q are fixed at the opposite ends of a horizontal insulating rod, extending from the origin to the point D, a third small charged bead is free to slide on the rod at a distance x away from the origin (but shorter than D). Can the equilibrium be stable?

Now I broke it down to

0 = 3d² - 6xd +2x²

Solving x = 0.951

The book said that it can be stable if it is positive. But wouldn't a negative charge also worked since

$$k\frac{3q*-q_{x}}{x^2} = k\frac{q*-q_{x}}{(d-x)^2}$$

Negative and both sides and hence they cancel?

 

[gneill]

Consider the equilibrium situations for a marble at rest balanced at the top of a hill versus a marble at rest in a valley between hills. Are both stable equilibriums?

 

[flyingpig]

I forgot t mention d = 1.50m...