© Blayne Heckel.
Two nearby, oppositely charged electric charges exert a net force on a third charge, as shown in the top part of the figure. Because the charges have opposite signs, the force from one charge is attractive and the other charge is repulsive. The individual forces (black arrows) largely cancel one another, leaving a net force (red arrow) significantly smaller than the individual forces. The gravitational force, illustrated in the bottom part of the figure, is always attractive: The net force of two nearby masses, "m," acting as a third mass, "M," is essentially twice as large as the individual forces. Because the gravitational force has only one sign—attractive—it cannot be shielded. (Unit: 3)