Unformatted text preview: Student's Choice #1
Bad movie physics 3D glasses Sunscreen/UV damage Life, other planets, galaxies Unusual lights (black lights, neon) UCSD: Physics 8; 2 Bad Movies (for physics) Though entertaining (and I like some of these), among the most guilty betrayers of physics are: Armageddon! (at the top of the list for a reason) Mummy movies Tomb Raider The Day After Tomorrow The Core Cliffhanger (couldn't even bear the previews) Mission Impossible (any of them) Speed (fun, but wrong) Mel Gibson, Schwarzenegger, James Bond movies Interesting case study: Armageddon vs. Deep Impact Deep Impact hired science consultants and did okay UCSD: Physics 8; 2 Themes of physics misrepresentation Everything goes BOOM, and explodes in huge fireball Real life more often just crunches (we're not loaded with dynamite) Momentum seldom conserved Bullet sends victim flying out window, shooter remains motionless Hearing sound in space no air to carry sound waves Seeing laser beams in space What are they reflecting off of? Is it smoky? Aerodynamic spaceships, airplane-like maneuvering Exploding rather than imploding submarines Fake props: wrong inertia properties Raiders of the Lost Arc: swiped huge gold statue like it was nothing! UCSD: Physics 8; 2 Movie Examples... Goldeneye, catch up to airplane Terminal velocity of human: 50 m/s, up to 70 m/s if you reduce your effective area by a factor of two Terminal velocity of plane in dive configuration: about 90 m/s (more with engine at full power) Got a late start, too... Also problem pulling out of dive! Plane terminal velocity: given best glide 10:1 at 30 m/s (on the slow side): drops at 3 m/s = v/10, so mgh mgv/10 Watts expended drag force F over v m/s Fv Watts = mgv/10 F = mg/10 at best glide speed F proportional to v2, so F = mg terminal velocity condition is met at about 3 times best glide speed > 90 m/s UCSD: Physics 8; 2 Examples, continued Speed, bus jump 150-200 ft, level (call it 45 m) bus at 30 m/s (67 mph): takes 1.5 seconds to cross drops 11 m (36 ft) in 1.5 s could work, at 15-20 degree launch angle, no air drag Websites www.pbs.org/teachersource/whats_new/science/aug01.shtm www.space.com/opinionscolumns/opinions/plait_000217.html www.badastronomy.com/ UCSD: Physics 8; 2 3D glasses: Stereo Vision left-eye view right-eye view from http://en.wikipedia.org/wiki/Stereo_vision UCSD: Physics 8; 2 Putting it together 3D films are shot with two cameras side-by-side mimicking your eyes If projected on the same screen, must somehow let your left eye know to pay attention to left image, and right eye know to pay attention to right image Can use color: blue image/blue filter for one eye, red image/red filter for other but can't do this for color movie! Can use polarization: could do vertical for left, horizontal for right or 45 one way vs. the other way glasses will appear gray In both cases, projection and detection must be separated into different "channels" UCSD: Physics 8; 2 3D Glasses Example red/blue scheme actually, the glasses pictured below go the wrong way: the right eye wants to see red how do we tell? Look at the left ear: for which eye is the background more blocked? UCSD: Physics 8; 2 What does UV do that's so bad anyway? Light comes in little bundles called photons The energy of a photon is proportional to its frequency UV is short wavelength, thus high frequency thus high-energy UV photons have enough energy to destroy chemical bonds changes chemistry pigments broken up colors fade can cause cancerous change to DNA in skin cell used in autoclaves to sterilize equipment (UV kills microbes) UCSD: Physics 8; 2 Sunscreen A coating of highly UV-absorbing molecules UVB: 290320 nm: sunburn UVA: 320400 nm: long-term skin damage/aging SPF: protection factor if you burn in half-hour, SPF 10 will protect you for 5 hours a thin, white t-shirt may be only SPF 4 or so sitting in shade but with lots of blue sky exposure may be SPF 4 All that absorption in such a thin layer?! Ozone is already SPF 10, and only 3 mm thick (if concentrated to one layer) in our atmosphere if you burn in 30 minutes, that'll be 3 minutes in space! put in liquid form (density) and now only 3 microns thick! What's responsible for selective absorption? Carotene UCSD: Physics 8; 2 makes carrots orange, tomatoes red, daffodils yellow, leaves turn must absorb blue light Long, organic molecular chain most dyes, pigments are such resonances in optical light Sunscreen works the same way: the molecules contained in sunscreen have a resonance absorption in the ultraviolet The absorbed UV turns into molecular vibration heat UCSD: Physics 8; 2 Paper Analog to Sunscreen
Reflected light (in this case, sunlight) off of paper appearing: blue green yellow orange red black Note pigment in yellow paper is good at absorbing 400500 nm Sunscreen is similar, but optimized for UVA + UVB white paper would be a flat line at 100% UCSD: Physics 8; 2 The UV sky
The blue sky has evermore flux at UV wavelengths, so that it is not a negligible source of sunburning light. On top of the blue sky spectrum is plotted a theoretical 1/4 model for molecular scattering, which fits reasonably well. This means the sky is 16 times brighter at UVB (300 nm) than at 600 nm sky Though not in words, this explains why the sky is blue! The Place of Humans in the Cosmos
Life in the Universe? UCSD: Physics 8; 2 Our Place in Space The universe is unimaginably big our galaxy is one of > 100 billion visible to us our sun is one of ~100 billion stars in the galaxy if earth is the size of a BB, the sun is a beach ball 100 m away, and the next star is 3/4 of the way around the earth even in the solar system, earth is only a grain earth mass is <0.0003% of solar system mass and humans are tiny compared to the earth We are not at the center of: the solar system the galaxy the universe attention UCSD: Physics 8; 2 Our Place in Time Modern humans have been around maybe 200,000 years This is about 0.001% the age of the universe 2 105/2 1010 = 10-5 flash in the pan Compared to distance scale, this is sort-of like the size of a galaxy compared to the size of the whole universe Feeling Insignificant? UCSD: Physics 8; 2 Are We Alone? Hard to believe that we are Assumptions (restrictive version): must have solid planet to start life planet must be in habitable zone (liquid water) >10% of stars have planets already see >5%, and just getting started life forms given energy input and non-destructive environment no supernovae nearby, no heavy comet bombardment, etc. UCSD: Physics 8; 2 100 billion stars in Milky Way 10% with planetary systems 10 billion planetary systems The Numbers Say 1% of planetary systems have habitable planets 100 million planets Pick very long odds for life formation: one-in-a-million now 100 life-bearing planets in Milky Way Now multiply by 100 billion galaxies in visible universe 10 trillion life-bearing planets in visible universe How many have (or have at one time had) intelligent life? very difficult to know--related question: how long does intelligent life persist? Why don't they visit? same reason we haven't gone farther than our own moon: space is way too vast we may never venture even to the nearest star UCSD: Physics 8; 2 Planetary systems known to date 146 planetary systems discovered in last 10 years 170 planets total 18 multi-planet systems Discovered by seeing star wiggle under gravitational influence of planet tends to find BIG planets CLOSE to the parent star (biased) red points are individual measurements (with error bars) black line is best-fit elliptical orbit 8 MJUP at 2.88 A.U., 0.29 ecc. sun's path in 65 years UCSD: Physics 8; 2 http://exoplanets.org UCSD: Physics 8; 2 Galaxies: Islands of stars Central Virgo Cluster Andromeda Galaxy: our closest "big" neighbor UCSD: Physics 8; 2 How do we know: by their spectra UCSD: Physics 8; 2 Galaxies as far as the "eye" can see Montage of the Hubble Ultra-Deep Field: ALL are galaxies! UCSD: Physics 8; 2 Fluorescent lights Fluorescent lights stimulate emission among atoms like argon, mercury, neon they do this by ionizing the gas with high voltage as electrons recombine with ions, they emit light at discrete wavelengths, or lines Mercury puts out a strong line at 254 nm (UV) this (65%) and other lines hit the phosphor coating on the inside of the tube and stimulate emission in the visible part of the spectrum Fluorescent Spectrum UCSD: Physics 8; 2 http://mo-www.harvard.edu/Java/MiniSpectroscopy.html http://en.wikipedia.org/wiki/Image:Fluorescent_lighting_spect
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Hg: mercury; Tb: Terbium; Eu: Europium Hg Hg Tb Tb Hg Hg Eu Tb Eu Eu Eu Eu UCSD: Physics 8; 2 Black Lights http://en.wikipedia.org/wiki/Black_light standard Hg-driven flourescence single europium-based phosphor (SrB4O7F:Eu2+) also uses glass that blocks visible light > 400 nm many objects absorb the UV light and themselves fluoresce 1. Eu 2. Hg UCSD: Physics 8; 2 Assignments HW8 due 6/8: 14.E.3, 14.E.8, 14.E.10, 14.E.11, 14.E.12, plus additional required questions accessible on website EC due by Thursday (start today if not already!!) Q/O # 5 due Friday 6/9 Final Exam Wed 6/14 3-6 PM WLH 2005 #2 pencil and light-green scantron form required calculator okay will have study guide and review session as for midterm ...
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