Physics 10 Spring 2013 Final Exam: You are a Turtle. Name: (c) Randall Munroe, www.xkcd.com also (c) Randall Munroe, www.xkcd.com
Part I: Short-Answer Questions. Answer all the questions in this section. Even more than in previous exams, these questions are designed to test your reasoning process rather than your memory for facts. Please feel free to explain your logic on any problem, whether it asks you to or not. The quality of your reasoning is at least as important as the correctness of the answer. Question #1: You are a turtle. You are sitting on a hilltop, enjoying the warm sunlight on your shell. You notice that the color of the sunlight is very similar to the color of the incandescent light bulbs that humans use. This is because: a) The light bulb filament has roughly the same mass as the Sun. b) The light bulb filament is made of the same type of atoms as the Sun. c) The light bulb filament is roughly the same temperature as the Sun's core. d) The light bulb filament is roughly the same temperature as the Sun's surface. e) The light bulb filament is much colder than as the Sun's surface. Question #2: That evening, you are still sitting on the hill, watching the sunset. (You are a turtle; you are in no rush.) The sunlight reaching you now has a distinct red color, and is also dimmer than it was at noon. This means that: a) The light waves reaching you at sunset have a lower amplitude than those reaching you at noon. b) The light waves reaching you at sunset have a lower frequency than those reaching you at noon. c) A typical photon of the light reaching you at sunset has less energy than they a typical photon at noon. d) Your shell is receiving less thermal energy per second than it did at noon. e) All of the above. f) None of the above. Question #3: You begin wondering about the reasons that the light seems so different at sunset than at noon. Which of the following seems like the best explanation? (Note: we never gave the answer to this question in lecture. However, you do know enough to reason out that one of these answers makes a lot more sense than the others!) a) At sunset, the sunlight reaching you is more strongly filtered by the atmosphere than it is at noon. b) At sunset, the Sun's surface is cooler than it is at noon. c) At sunset, the Sun's surface is hotter than it is at noon. d) At sunset, the Sun is further away from the Earth than it is at noon. e) At sunset, the Earth's gravity is weaker than it is at noon.
Question #4: For at least one of the answers you did not choose on Question #3, please briefly explain how you know that answer is unlikely to be true. Question #5: Early this morning, you were standing at the bottom of the hill. Right now you are on the hilltop. This means that you definitely have more of which type of energy than you had this morning? Question #6: There are many ways you might have reached the top of the hill: you could have walked, been carried by an eagle, taken a helicopter... Decide on a method by which you reached the hilltop, and then explain where the energy you mentioned in Question # came from. Try to trace this energy back through as many steps as you can, until either you conclude that it came from the reactions powering the Sun, or you give a convincing case that it did not come from the Sun. Question #7: You now decide to slide down the snow-covered hillside. As you descend, what is happening to the energy you described in Question #6?
Question #8: Having reached the bottom of the hill, you are now sliding across a flat, ice-covered lake. You are moving east, and are gradually slowing down. If you were to add up all of the forces pushing on you at the moment, which direction would the net force point? Briefly explain your reasoning. Question #9: As a very old and wise turtle, you remember a human named Aristotle who you used to discuss physics with, thousands of years ago. What answer would Aristotle have given to Question #7? Briefly explain why. Question #10: Another human who you used to discuss physics with, more recently, was Albert Einstein. (You humbly believe he got most of his best ideas from you.) Which of the following was not one of Einstein's discoveries? a) The energy of a photon depends on its frequency. b) Time passes at different rates with respect to different observers. c) Two events which are simultaneous as measured by one observer, are not simultaneous as measured by another observer. d) Distant galaxies are moving away from us; in other words, the universe is expanding. e) Light behaves both as a particle and as a wave.
Question #11: You take some time to reflect back over your many centuries of observing humans, and the interesting discoveries they have made. Place the following pieces of knowledge in order, from the earliest discovery to the most recent discovery: a) The universe is expanding. b) The Sun is the center of the solar system. c) The Earth is a sphere. d) Energy cannot be created or destroyed. e) The Higgs Boson exists, and can be created in particle accelerators. Earliest discovery <------------------------------------> Most recent discovery Question #12: In addition to following human progress in science, you have always had an abiding interest in the way they transform this knowledge into technological progress. Label each of the following accomplishments with an A, B, or C, according to the following scheme: A: Humans are able to do this today. B: Humans are not yet able to do this, but might be able to someday. (Should their silly species survive long enough.) C: According to our current understanding of physics, this will never be possible. Build a vehicle that travels faster than sound. Build a vehicle that travels faster than light. Build a vehicle that travels very close to the speed of light. Accelerate subatomic particles to very close to the speed of light. Create small amounts of antimatter. Create small amounts of energy. Visit the moon. Visit a solar system outside our own. Question #13: The existence of human beings (and more importantly, of turtles) is only possible because previous generations of stars had already completed their lifetimes before our solar system formed. As Carl Sagan used to say, We are all stardust. Explain briefly why this is true.
Question #14: When our Sun reaches the end of its lifetime, it will: a) explode in a Supernova. b) run out of fusion fuel, and gradually cool toward zero temperature. c) run out of fission fuel, and immediately cool to zero temperature. d) become a planet much like the Earth. e) collapse in a final singularity called the Big Crunch. f) none of the above; the sun will exist in its present state forever. Question #15: If the Sun were a much more massive star than it is, which answer would you have chosen for Question #14? Question #16: According to our best current understanding, which of the following statements about the very early universe (shortly after the Big Bang) is not true? a) Galaxies (once they had formed at all) were much closer together than they are now. b) There was less iron in the universe than there is now. c) The total energy in the universe was exactly the same as today. d) The speed of light was faster than it is today. e) There were no turtles. Question #17: You can remember when most scientists subscribed to the Steady State model of the universe. Briefly describe how they would have answered Question #15.
Question #18: Give an approximate sketch of the Milky Way galaxy (the galaxy that we live in.) If you feel that your sketch is inaccurate in some ways, describe briefly how it is inaccurate.
Question #19: Would it be possible to travel to the center of the Milky Way galaxy and back in one human lifetime? Briefly explain why or why not. Question #20: While you have always been fascinated by science, you also realize that it is only one of many styles of thought important to humans. (Or to turtles.) Give one question that could be investigated using the methods of science. (It can be a question that we already know the answer to, or one to which we haven't yet learned the answer.) Now give one question that cannot be answered by scientific methods. Briefly explain why your first question is scientific and your second one is not.
Section II: Long Problems. Please choose any two of the following topics, and write about them in moderate detail on the attached blank sheets. If you need more space, you can grab extra blank papers from the front of the room and staple them to your exam. You may answer more than two problems if you like, but please clearly mark which ones are your main answers, and which are extra credit. As always, Extra Credit problems will be less important than regular problems in deciding your final course grade. Problem #1: In order to investigate the history of the Universe, Edwin Hubble and his colleagues had to answer two questions about distant galaxies: How far away are they?, and How fast are they moving toward us or away from us? For each of these two questions, briefly describe at least one method that could be used to attempt to answer it. Which of the two questions is more difficult to answer, and why? Problem #2: For at least one of the five discoveries listed in Question #11, explain how it was accomplished. Issues that you may want to address include, but are not limited to: What had people believed about the topic before the discovery was made? What observations led people to its discovery? What obstacles did they have to overcome to make sense of these observations? Problem #3: Among the fundamental forces of nature that you have discussed with humans over the years are gravity, electrical force, magnetic force, strong nuclear force, and weak nuclear force. Select at least two of these forces, and discuss the role they play in governing the universe. Among the topics you might consider are: What sort of objects does this force affect? What determines how strong the force is? Which direction does it push? How does it depend on the distance between the objects? And what, if anything, can this force do that none of the other fundamental forces do? Problem #4: One of the most difficult things to understand about Quantum Mechanics is Particle-Wave duality. In other words, the fact that light and matter both seem to behave in some ways as waves, and in other ways as particles. Describe at least one way in which light seems to behave like a wave, and at least one way in which it seems to behave like a particle. Then do the same for an electron: how does it behave like a wave, and how does it behave like a particle?
Problem #5: Describe at least one interesting unanswered question in science, and explain how we might go about investigating the topic in the future. What observations would we need to make to learn the answer? What difficulties prevent us from answering the question now? What possible answers might we find, and what new questions would they lead to? Problem #6: As always, I invite you to invent your own question. Pick any topic we have discussed this semester (and which you have not already written about in the Long-Answer sections of Midterm #1 or #2) and come up with a question you would like to answer. If you choose this option, please talk to me before beginning to write, to make sure that we both agree on a good question. (c) Stephen Player and the ever-awesome Terry Pratchett