1. Which of the following correctly lists our cosmic address from small to large?

Transcription

1 1. Which of the following correctly lists our cosmic address from small to large? (a) Earth, solar system, Milky Way Galaxy, Local Group, Local Super Cluster, universe (b) Earth, solar system, Milky Way Galaxy, Local Super Cluster, Local Group, universe (c) universe, Local Super Cluster, Local Group, Milky Way Galaxy, solar system, Earth (d) universe, Local Group, Local Super Cluster, Milky Way Galaxy, solar system, Earth (e) Earth, Local Group, Local Super Cluster, Milky Way Galaxy, solar system, universe (a) is the right answer. 2. How long is a light-second? (a) m (b) m (c) m (d) m (e) m (a) is the right answer. The speed of light is c = m/s. So one light-second is c (1 sec) = m. 3. If you are looking at a star that is 4 light-years away from Earth, you are looking at: (a) The star as it is right now (b) The star as it was 4 years ago (c) The star as it will be 4 years in the future (d) The star as it was years ago (e) The star as it will be years in the future (b). The light has to travel for 4 years to get to Earth. 4. Which one of the following is the correct description or the right usage of the Astronomical Unit (AU)? (a) 1 AU is the average distance of the most distant planet in the solar system from our sun. (b) When the parallax angle of an astronomical object is a small angle p, then the distance to that object is given by 1 AU/p. (c) 1 AU equals 1.3 light-second. (d) 1 AU equals km. (e) None of the above (b). It s the distance of Earth from the sun. 500 light-seconds or km. Distance measurement using parallax relies on our knowing 1 AU accurately. 1

2 5. What is the current estimate of the age of the universe? (a) years (b) years (c) years (d) years (e) years (e). 14 billion years. 6. Which one of the following is a false description of an astronomical object? (a) A star is a hot ball of gas powered by nuclear reaction at its core. (b) A galaxy is an island of stars in space containing stars held by gravity. (c) The observable universe may not be the whole universe. (d) The star systems can form a cluster, but galaxies do not form clusters. (e) A star system can have more than one star in the system. (d). For instance, the Local Super Cluster is a cluster of galaxies including our galaxy. 7. Where or when is the star stuff formed? (a) Only during the first 300,000 years of the universe (b) Only inside relatively light stars like our sun (c) Only inside stars much heavier than our sun (d) Only when a supernovae explosion occurs (e) Inside any bright star (e). Star stuff means heavier elements of which we are made. It is made inside bright stars. See the summary of Chapter 1. NOTE ADDED: Although (a-d) are clearly wrong since star stuff cannot be generated only in those times or places, I can see that one can conceivably be confused by the qualifier any in (e). Hence, I am going to give everyone full mark for this one. 2

3 8. If the distance to galaxy A from us is the farther than the the distance to galaxy B from us, which of the following is right? (a) If the two galaxies are within our Local Group, they can move in any directions, but the speed of A must be greater than the speed of B. (b) If the two galaxies are within our Local Group, they can move in any directions, but the speed of B must be greater than the speed of A. (c) If the two galaxies are within our Local Group, they can only move away from us but their speed can be essentially random. (d) If the two galaxies are outside of our Local Group, they can move in any directions, but the speed of B must be greater than the speed of A. (e) If the two galaxies are outside our Local Group, they only move away from us and the speed of A is greater than the speed of B. (e). Universe expands. Farther galaxies receed faster. This is the raisin problem in the homework. 9. Which one of the following falsely describes a part in scientific method? (a) Making a hypothesis (b) Performing experiments to test the hypothesis (c) Analyzing the experimental outcome to determine the validity of the hypothesis (d) Deciding that the hypothesis will apply to all physical situations when it is confirmed (e) Making a new hypothesis based on the experimental outcome (d). One can never be sure that one s hypothesis will apply to all physical situations. One cannot decide what a law of nature should be. 10. Which of the following correctly describes Kepler s first law? (a) There are comets that goes around the sun only once. (b) The moon is pulled by Earth just as Earth is pulled by the moon. (c) The sun sits at one of the two foci in Earth s elliptic orbit. (d) A moving object will keep its velocity unless there is a force acting on it. (e) All forces are attractive. (c). Kepler s first law states a planet s orbit is an ellipse with the sun at one focus. Earth is a planet. 3

4 11. Which conservation law directly explains Kepler s second law? (a) Energy conservation (b) Power conservation (c) Angular momentum conservation (d) Linear momentum conservation (e) Force conservation (c). When a planet is closer to the sun, it moves faster, just like the spinning figure skater spins faster when the arms are drawn closer. NOTE ADDED: Energy conservation tells you that when a planet is closer to the sun, it must move faster since kinetic energy has to increase. However, energy conservation does not tell you how the orbit should change. In particular, energy conservation is not enough to get the equal area sweep in equal time law of Kepler. This is due to angular momentum conservation. Thus strictly speaking only (c) is the right answer. However, I realize that this distinction may be too much for an introductory course like ours. I will allow both (a) and (c) to be right. 12. Suppose a new planet is found at 16 AU from our sun. What will its period be? (a) 32 years (b) 64 years (c) 128 years (d) 256 years (e) 512 years (b). Use p 2 = a 3. p = 16 3 = 64. 4

5 13. Which one of the following about Newton s three laws of motion is true? (a) If the speed of the object is close to the speed of light, Newton s first law does not apply. False. The law of inertia applies to all speed. (b) When the speed of an object stays the same, there cannot be a net force acting on this object. False. The object can change direction. That cannot happen without a force. (c) When the velocity of an object stays the same, there cannot be a net force acting on this object. True. This is Newton s first law. (d) The laws apply only to objects on Earth. They do not apply to astronomical objects. False. The laws are universal. (e) The laws apply only to a moving object. They do not apply to a stationary object. False. 14. According to Newton s second law, (a) The acceleration caused by a force is inversely proportional to the mass of the object. True. F = ma means a = F/m. (b) The acceleration caused by the same amount of force depends not only on the mass of the object but also on the shape of the object. False. There is no shape information in F = ma. (c) Friction force does not count as a real force. False. Friction is a real force. (d) Every mass attracts every other mass. This is Newton s law of gravity, not the second law. (e) The magnitude of the gravitational force between two objects is proportional to the product of the two masses. This is Newton s law of gravity, not the second law. 15. What is the force that is directly responsible for propelling a rocket when its engine is fully on? (a) The gravitational force of large objects in space, such as galaxies, collectively acting on the rocket (b) The chemical composition of the exhaust gas (c) The reaction force exerted by the exhaust gas (d) Earth s gravity (e) Earth s angular momentum (c). This is Newton s third law. Action and Reaction. 5

6 16. What is a parsec? (a) The same as 1 light-year. (b) The same as 500 light-second. (c) The size of the observable universe. (d) The size of our solar system. (e) The distance of an object that has 1/3600 degree parallax angle (e). This is the definition. 17. Which one of the following people championed the geocentric view of the solar system? (a) Ptolemy (b) Copernicus (c) Galileo (d) Kepler (e) Newton (a). All others are advocates of the heliocentric (sun-centered) view. 18. The radius of the sun is known to be R = m. The density of the sun is known to be about kg/m 3, that is, 1 m 3 volume of the sun contains about 1, 400 kg of material. Assuming the sun is a sphere (volume = 4πR 3 /3), which of the following is a good estimate of the mass of the sun? (a) kg (b) kg (c) kg (d) kg (e) kg (e). Volume is V = 4πR 3 /3 = m 3. Multiply this by the density to get M = m kg/m 3 = kg (1) 6

7 19. Visible colors correspond to the wavelength range of nm nm. Which one of the following is the equivalent frequency range? (Hz = 1/s) (a) Hz Hz (b) Hz Hz (c) Hz Hz (d) Hz Hz (e) Hz Hz (e). Use λf = c. f = m/s/( m) = Hz and f = m/s/( m) = Hz 20. When the frequency of a light wave is f, what is the energy of the corresponding energy of the photon? (h is the Planck constant.) (a) E = hf (b) E = hf 2 (c) E = hf 3 (d) E = h/f (e) E = h/f 2 (a). 7