For an attractive force (α < 0), the orbit is an ellipse, a hyperbola or parabola, depending on whether u 1 is positive, negative, or zero, respectively; this corresponds to an eccentricity e less than one, greater than one, or equal to one. For a repulsive force (α > 0), u 1 must be negative, since u 2 is positive by definition and their sum is negative; hence, the orbit is a hyperbola. Naturally, if no force is present (α=0), the orbit is a straight line.