1Midterm 2 PRS QuestionsPRS questions from the lectures afterMidterm 1 but before Midterm 2PRS Question. You want to launch arocket into space, and you want tomaximize its kinetic energy.Suppose that you can increase therocket’s mass or its launch velocity byabout the same amount, but you can’tincrease both. Which would youchoose?1. I’d make the rocket moremassive2. I’d make the rocket fasterIf we race two equal-mass ballsdown these two tracks, which ballwill win the race?1. The ball on the flat track2. The ball on the “V-shaped” track3. Neither! Both balls will reach thefinish line at the same timeAnother delightfulinterlude from thegood folks atPRSWhat 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 potentialenergy (GPE) and zero kinetic energy (KE) because v = 0• As gravity pull the ball toward the ground, the GPE isconverted to kinetic energy, and the ball rolls faster• At the bottom of the “V”, it attains its minimum GPE andmaximum KE. This is where it rolls the fastest• As it climbs back up the track, the KE is converted back intoGPE and the ball slows downBecause the v-shaped track converts more GPE into KE, theball on that track moves much faster than the ball on the flattrack for much of the race, and consequently it wins in the end.2A Sun-like star puts out 9 x 1026 Joules everysecond. The speed of light is 3 x 108 m/s.How much mass does the star convert toenergy every second?1. 10 kg2. 100 kg3. 104 kg4. 108 kg5. 1010 kgE = mcE = mc22Hurray! PRS QuestionTwo types of nuclear reactionsthat release mass-energyFission FusionLess mass is present after than before the reaction...E = mcE = mc22Human nuclearpower plants usefission (e.g., ofuranium or plutonium)Cosmic power plants use fusion (e.g., two hydrogen atoms fusedto form one helium atom)A nice cosmic powerplant: our Sun!Cosmic power plant• mass-energy = E/c2• c2 = (3 x 108 m/s)2 = 9 x 1016 m2/s2• E = 9 x 1026 Joules• Therefore, mass = 1010 kgE = mcE = mc223Atom 1: 92 protons, 143 neutrons, 92 electronsAtom 2: 92 protons, 143 neutrons, 91 electronsAtom 3: 91 protons, 143 neutrons, 91 electronsAtom 4: 92 protons, 144 neutrons, 92 electronsWhich of these atoms are isotopes?1. 1 and 22. 2 and 33. 3 and 14. 1 and 45. 3 and 4• A particular element always has thesame number of protons• atomic number = # protons• atomic mass number = #protons +#neutrons (aka “atomic weight”).• Isotopes are the same element butwith different atomic weightsEnergyAtom energy levels are often depicted in a simpleway as shown below. PRS Question! Atoms oftenspontaneously emit radiative energy (light). Which ofthe electron transitions shown below (A, B, C, D, or E)would cause the atom to emit light?1. A2. B3. C4. D5. E6. A and B7. A and C8. A, B, C9. A, B, C, D10. All of themnot allowedIonization. Energyadded, not removed.Excitation. Energy added, not removed.Opposite charges attract, but similar charges repel eachother. The nucleus of helium contains two positiveprotons and two neutral neutrons. Why don’t thesimilarly-charged protons in this nucleus push each otheraway?1. There is a strong chemical bond inside thenucleus.2. The attraction of the negatively-charged electronsholds the protons together.3. There is a force that is even stronger than thecharge force that holds the nucleus together, butit only acts on short distance scales, i.e., whenparticles are quite close together.Which of the following is least likely to beaccelerating?1. A car driving down a winding, narrowmountain road.2. A racecar driver.3. A car driving through Texas on theinterstate.4. A bumper car at a county fair.5. A planet orbiting a star.4Newton’s Universal Law of Gravitation Between every two objects there is an attractiveforce, the magnitude of which is directlyproportional to the mass of each object and inverselyproportional to the square of the distance betweenthe centers of the objects.(G = the “gravitational constant”, a constant of nature)Two equal masses separated by distance d exert a force F oneach other due to gravity. How large would the gravitationalattraction be if the distance between them was doubled?1. 1/4 F2. 1/2 F3. The force would be the same.4. 2 F5. 4 FIf the Sun suddenly collapsed into a black hole withthe same mass but 10 times smaller diameter, howwould 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 rocketblasts off, it pushes off theground in order to launch itselfinto the air.1. TRUE2. FALSE5(1)(2)(3)The NASAPegasus rocketYou are a shuttle astronaut returning to the shuttle after attemptingto fix the International Space Station with a hammer. As you arejetting back to your shuttle, your lifeline breaks, your jets run outof fuel, your radio goes dead, and you miss the shuttle. To getback safely, you should:1) use a swimming motion with your arms and legs2) throw the hammer at the shuttle to get someone’s attention3) throw the hammer away from the shuttle4) make a hammering motion in the direction of the shuttle5) make a hammering motion away from the shuttlePRS QuestionEscape VelocityIf the mass of the Earthwere increased by afactor of 10, what wouldthe new escape velocity be?1. 1/4 its current value2. 1/3 its current value3. the same as its current value4. 3 times the current value5. 4 times the current valueEscape VelocitySuppose that you couldchange a planet’s properties.To make it easier to launch spacecraft off this planet, youwould:1. Decrease the radius of the planet2. Decrease the mass of the planet3. Both of the above4. Can’t answer -- need more information6PRS question.True or False: To steer abicycle, you just move thehandlebars.1. TRUE2. FALSEActually, to steer a bicycle you must lean to move your bodyweight and manipulate the angular momentum in the spinningbicycle wheels.Two angles - right ascensionand declination - indicate thelocation of an object in the sky.• One can imaginemoving a telescope onthe ground to thedesired angles.• But how is the HubbleSpace Telescope movedto look at an object?PRS question.Which part of Newton’s physics of motioncould be used to point the Hubble SpaceTelescope at an interesting object?1. Newton’s 1st Law2. Newton’s