5. (a) At what position in its elliptical orbit is the speed of a planet a maximum? (b) At what position is the speed a minimum? 6. You are given the mass and radius of planet X. How would you calculate the freefall acceleration on this planetâs surface? 7. (a) If a hole could be dug to the center of the Earth, would the force on an object of mass m still obey Equation 13.1 there? (b) What do you think the force on m would be at the center of the Earth? 8. Explain why it takes more fuel for a spacecraft to travel from the Earth to the Moon than for the return trip. Estimate the difference. 9. A satellite in low-Earth orbit is not truly traveling through a vacuum. Rather, it moves through very thin air. Does the resulting air friction cause the satellite to slow down? Problems The problems found in this chapter may be assigned online in Enhanced WebAssign 1. straightforward; 2. intermediate; 3. challenging 1. full solution available in the Student Solutions Manual/Study Guide Analysis Model tutorial available in Enhanced WebAssign Guided Problem Master It tutorial available in Enhanced WebAssign Watch It video solution available in Enhanced WebAssign Section 13.1 Newtonâs Law of Universal Gravitation Problem 12 in Chapter 1 can also be assigned with this section.