where θ 0 is the angle at which the distance is minimized (Figure 6). The distance r begins at infinity (when θ 1 – θ 0 = −90°), and decreases gradually until θ 1 – θ 0 = 0°, when the distance reaches a minimum, then gradually increases again to infinity at θ 1 – θ 0 = 90°. The minimum distance b is the impact parameter, which is defined as the length of the perpendicular from the fixed center to the line of motion. The same radial motion is possible when an inverse-cube central force is added.