Questions for Chapter 4

1. What do we mean by acceleration? What is the acceleration of gravity?

2. What is momentum? How can momentum be affected by a force? What do we mean when we say that momentum will be changed only by a net force?

3. What is free-fall, and why does it make you weightless?

4. The densities of Jupiter (1.33) the Sun (1.41) and Saturn's moons Dione (1.44) and Rhea (1.33) are all about the same in spite of having HUGE differences in gravity holding them together. Numerically how can this be? What is the underlying physical cause that gravity does not make the most massive object the densest?

5. State each of Newton's three laws of motion. For each law, give an example of its application.

6. What are the laws of conservation of momentum, conservation of angular momentum, and conservation of energy? For each, give an example of how it is important in astronomy.

7. Imagine you are out in space and push away from you an object having a mass identical to your own. What happens? Explain.

8. Define kinetic energy, radiative energy, and potential energy, for each type of energy, give at least two examples of objects that either have it or use it.

9. Define temperature and thermal energy. How are they related? How are they different?

10. What do we mean by mass-energy? Is it a form of kinetic, radiative, or potential energy?

11. Summarize the universal law of gravitation in words.

12. Consider an object shot upward from the earth with less than escape velocity. Describe its motion.

13. Why do objects fall at the same rate?

14. How could I double the Earth’s escape velocity? [more than one way, but the universal gravitational constant remains the same.]

15. What is the difference between a bound and an unbound orbit? What orbital shapes are possible?

16. State, then explain, Kepler's three laws?

17. How did Newton's understanding of gravity extend Kepler's Laws?

18. What quantities do we need to know if we want to measure an object's mass with Newton’s version of Kepler's third law? Explain.

19. Explain why orbits cannot change spontaneously. How can a gravitational encounter cause an orbit to change? How can an object achieve escape velocity?

20. Explain how the Moon creates tides on Earth. Why do we have two high and low tides each day?

21. How do the tides vary with the phase of the Moon? Why?

22. How would you make ocean tides stronger or weaker? More or less frequent? [Be careful to state what you would change and what effect it would have on the frequency of tides.]

23. Describe the tides (if any) if we had no moon.

24. What is tidal friction? What effects does it have on Earth? How does it explain the Moon's synchronous rotation?

25. Would you fall at the same rate on the Moon as on Earth?

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer.

26. If you bought a pound of chocolate on the Moon, using a pound scale from a store on Earth, you'd get a lot more chocolate than if you bought a pound on Earth.

27. Suppose you could enter a vacuum chamber (on Earth), that is, a chamber with no air in it. Inside this chamber, if you dropped a hammer and a feather from the same height at the same time, both would hit the bottom at the same time.

28. When an astronaut goes on a space walk outside the Space Station, she will quickly float away from the station unless she has a tether holding her to the station or constantly fires thrusters on her space suit.

29. I used Newton's version of Kepler's third law to calculate Saturn's mass from orbital characteristics of its moon Titan.

30. If we could somehow replace the Sun with a giant rock that has precisely the same mass, Earth's orbit would not change.

31. The fact that the Moon rotates once in precisely the time it takes to orbit Earth once is such an astonishing coincidence that scientists probably never will be able to explain it.

32. If an asteroid passed by Earth at just the right distance, Earth's gravity would capture it and make it our second moon.

33. When I drive my car at 30 miles per hour, it has more kinetic energy than it does at 10miles per hour.

34. Someday soon, scientists are likely to build an engine that produces more energy than it consumes.