A thin, nonconducting rod bent into a semicircle of radius \(R\) carries a total charge \(Q\) distributed uniformly along its length. A point charge \(q\) of the same sign as \(Q\) is held fixed at the center of curvature of the semicircle. Which of the following expressions is equal to the magnitude of the electrostatic force exerted on the point charge by the semicircular rod?