Midterm 2 PRS Questions

Transcription

1 Midterm 2 PRS Questions PRS questions from the lectures after Midterm 1 but before Midterm 2 PRS Question. You want to launch a rocket into space, and you want to maximize its kinetic energy. Suppose that you can increase the rocket s mass or its launch velocity by about the same amount, but you can t increase both. Which would you choose? 1. I d make the rocket more massive 2. I d make the rocket faster If we race two equal-mass balls down these two tracks, which ball will win the race? Another delightful interlude from the good folks at PRS 1. The ball on the flat track 2. The ball on the V-shaped track 3. Neither! Both balls will reach the finish line at the same time What happens if I place a ball at the top of the V in the lower track and then let it go? The ball starts at rest with maximum gravitational potential energy (GPE) and zero kinetic energy (KE) because v = 0 As gravity pull the ball toward the ground, the GPE is converted to kinetic energy, and the ball rolls faster At the bottom of the V, it attains its minimum GPE and maximum KE. This is where it rolls the fastest As it climbs back up the track, the KE is converted back into GPE and the ball slows down Because the v-shaped track converts more GPE into KE, the ball on that track moves much faster than the ball on the flat track for much of the race, and consequently it wins in the end. 1

2 E = mc 2 A Sun-like star puts out 9 x Joules every second. The speed of light is 3 x 10 8 m/s. How much mass does the star convert to energy every second? Hurray! PRS Question kg kg kg kg kg Two types of nuclear reactions that release mass-energy Fission Fusion Less mass is present after than before the reaction... Human nuclear power plants use fission (e.g., of uranium or plutonium) E = mc 2 Cosmic power plants use fusion (e.g., two hydrogen atoms fused to form one helium atom) A nice cosmic power plant: our Sun! Cosmic power plant mass-energy = E/c 2 c 2 = (3 x 10 8 m/s) 2 = 9 x m 2 /s 2 E = 9 x Joules Therefore, mass = kg E = mc 2 2

3 Atom 1: 92 protons, 143 neutrons, 92 electrons Atom 2: 92 protons, 143 neutrons, 91 electrons Atom 3: 91 protons, 143 neutrons, 91 electrons Atom 4: 92 protons, 144 neutrons, 92 electrons Which of these atoms are isotopes? 1. 1 and and and and and 4 A particular element always has the same number of protons atomic number = # protons atomic mass number = #protons + #neutrons (aka atomic weight ). Isotopes are the same element but with different atomic weights Atom energy levels are often depicted in a simple way as shown below. PRS Question! Atoms often spontaneously emit radiative energy (light). Which of the electron transitions shown below (A, B, C, D, or E) would cause the atom to emit light? Ionization. Energy added, not removed. 1. A 2. B 3. C 4. D 5. E 6. A and B 7. A and C 8. A, B, C not allowed 9. A, B, C, D 10. All of them Energy Excitation. Energy added, not removed. Opposite charges attract, but similar charges repel each other. The nucleus of helium contains two positive protons and two neutral neutrons. Why don t the similarly-charged protons in this nucleus push each other away? 1. There is a strong chemical bond inside the nucleus. 2. The attraction of the negatively-charged electrons holds the protons together. 3. There is a force that is even stronger than the charge force that holds the nucleus together, but it only acts on short distance scales, i.e., when particles are quite close together. Which of the following is least likely to be accelerating? 1. A car driving down a winding, narrow mountain road. 2. A racecar driver. 3. A car driving through Texas on the interstate. 4. A bumper car at a county fair. 5. A planet orbiting a star. 3

4 Newton s Universal Law of Gravitation Between every two objects there is an attractive force, the magnitude of which is directly proportional to the mass of each object and inversely proportional to the square of the distance between the centers of the objects. (G = the gravitational constant, a constant of nature) Two equal masses separated by distance d exert a force F on each other due to gravity. How large would the gravitational attraction be if the distance between them was doubled? 1. 1/4 F 2. 1/2 F 3. The force would be the same F 5. 4 F If the Sun suddenly collapsed into a black hole with the same mass but 10 times smaller diameter, how would the Earth s orbit change? 1. It would become 10 times smaller. 2. It would become 100 times smaller. 3. The Earth would plunge into the black hole. 4. The Earth would spiral into the black hole. 5. Nothing would change. PRS question. True or False: When a rocket blasts off, it pushes off the ground in order to launch itself into the air. 1. TRUE 2. FALSE 4

5 PRS Question (1) You are a shuttle astronaut returning to the shuttle after attempting to fix the International Space Station with a hammer. As you are jetting back to your shuttle, your lifeline breaks, your jets run out of fuel, your radio goes dead, and you miss the shuttle. To get back safely, you should: (2) The NASA Pegasus rocket 1) 2) 3) 4) 5) use a swimming motion with your arms and legs throw the hammer at the shuttle to get someone s attention throw the hammer away from the shuttle make a hammering motion in the direction of the shuttle make a hammering motion away from the shuttle (3) Escape Velocity If the mass of the Earth were increased by a factor of 10, what would the new escape velocity be? /4 its current value 1/3 its current value the same as its current value 3 times the current value 4 times the current value Escape Velocity Suppose that you could change a planet s properties. To make it easier to launch spacecraft off this planet, you would: Decrease the radius of the planet Decrease the mass of the planet Both of the above Can t answer -- need more information 5

6 PRS question. True or False: To steer a bicycle, you just move the handlebars. 1. TRUE 2. FALSE Actually, to steer a bicycle you must lean to move your body weight and manipulate the angular momentum in the spinning bicycle wheels. Two angles - right ascension and declination - indicate the location of an object in the sky. One can imagine moving a telescope on the ground to the desired angles. But how is the Hubble Space Telescope moved to look at an object? PRS question. Which part of Newton s physics of motion could be used to point the Hubble Space Telescope at an interesting object? Three balls in space near a planet This could be accomplished various ways, but a frequently used method is to employ: 1. Newton s 1st Law 2. Newton s 2nd Law 3. Newton s 3rd Law 4. Conservation of linear momentum 5. Conservation of angular momentum 6

7 Three balls in space near a planet Suppose an astronaut places three equal mass balls next to each other out in space above some planet as shown on the previous screen. The balls are initially at rest. After some time, the balls will 1. still be the same (i.e., nothing will have changed). 2. all be falling toward the planet at the same speed. 3. all be falling toward the planet with ball 3 moving fastest. 4. all be falling toward the planet with ball 1 moving fastest. Three balls in space near a planet F=ma The same effect causes tides. 7

8 Survey question -- tides without oceans Tides are readily apparent in the oceans because liquid water flows more readily than rock. Can tidal effects be observed easily on planets that have no water? 1. No, tidal effects are too subtle if there aren t oceans. 2. Yes. Io is constantly stetched by tidal force in different directions as it orbits Jupiter. This generates friction inside Io, which heats its interior! Tidal friction Jupiter s moon Io is the most volcanically-active object in the solar system Every seven days, Io, Jupiter, Europa, and Ganymede line up as shown at right. The sum of the gravitational tugs at this time pulls Io into an elliptical orbit. This alignment effect is an example of an orbital resonance. 8

9 Four fundamental forces. Astronomy?? PRS question. Which of the forces drives the ejection of water geysers out of Enceladus? Waves in water What happens when a wave moves through a pond from the left side to the right side? 1. The water molecules move from the left side to the right side. 2. The wave moves from left to right, but the water particles move from right to left 3. The wave moves from left to right and the water molecules move up and down but don t move sideways. The force of gravity Electromagnetic force The strong nuclear force The weak nuclear force PRS Question: A movie theater works on the principle of 1. Specular reflection 2. Diffuse reflection 9

10 Specular reflection vs. diffuse reflection (aka scattering) In specular reflection, the angle of incidence = angle of reflection In diffuse reflection, photons are reflected in semi-random directions Reflection vs. Scattering Reflection: angle of incidence = angle of reflection Scattering: angle of scattering is random 10