Figure 4-19: An Orbit Analogy (a) To make a ball on a string move at high speed around a small circle, you have to exert a substantial pull on the string. (b) If you lengthen the string and make the same ball move at low speed around a large circle, much less pull is required. (c) Similarly, a planet that orbits close to the Sun moves at high speed and requires a substantial gravitational force from the Sun, while (d) a planet in a large orbit moves at low speed and requires less gravitational force to stay in orbit.