Two Cans & String

{ "thread_id": "6877", "posts": [ { "id": "660378", "time": "1367554313", "html": "Could you please help me with this physics problem (if you can and/or want)?<br />\n<br />\nAlso, a quick explanation for how you got your answer(s) would be greatly appreciated. <br />\n<br />\nYes, this is high school physics. Don't hate me please.<br />\n<br />\n<img src=\"/uploads/files/num7.png\" alt=\"num7.png\" />", "user": "arthurkock" }, { "id": "660379", "time": "1367554520", "html": "So you need us to do your homework for you.", "user": "alpha" }, { "id": "660421", "time": "1367555849", "html": "One problem he's not sure how to start is pretty different from "here can you guys do this entire packet for me?" Not to mention the fact that <a href=\"http://apcentral.collegeboard.com/apc/members/repository/sg_physics_b_01.pdf\">the answers and explanations for all previous AP exams are available online</a> (which Arthur should look at if he hasn't yet). I think this is a reasonable request. If he just wanted an answer, he could google it. He wants a different explanation so he understands it, though, which is admirable. I don't remember enough about this kind of physics to help, though :/", "user": "zia" }, { "id": "660429", "time": "1367557398", "html": "Oh.<br />\n<br />\nMy apologies.", "user": "alpha" }, { "id": "660750", "time": "1367583464", "html": "A) Mass defect is the difference in mass on the two sides of the equations. Plug in numbers, multiply, divide.<br />\n<br />\nB) Energy released must be enough to balance the mass defect. 1u is ~931 MeV/c^2, so you can use this to find the energy in MeV where c is the speed of light in vacuum. The convert to Joules from MeV.<br />\n<br />\nC) Once you do B, you will just multiply and divide.<br />\n<br />\nD) Again, just multiply and divide with the information from B and C. You would do well to see what the actual atomic mass of oxygen is.<br />\n<br />\n<br />\nIf you would like to see the actual process, I am happy to go through it later.", "user": "terrabranford" }, { "id": "660846", "time": "1367604175", "html": "The final answer seems way too high, but I'll show how I would have done this. (It's been, like, 8 years since physics class.) You probably also could reduce the number of steps in part D.<br />\n<br />\na) The mass defect is the amount of mass lost in the equation. Subtract the mass of the products from the mass of the reagents.<br />\n3 * 2.0141u - ( 4.0026u + 1.0078u + 1.0087u ) = <span style=\"font-weight:bold;\">0.0232u</span><br />\n<br />\nb) E = mc²<br />\n0.0232u * (299,792,458 m/s)² = <span style=\"font-weight:bold;\">3.5 * 10^-12J</span><br />\n<br />\nc) Divide energy required by energy-per-reaction, multiply by number of deuterium per reaction.<br />\n10²⁰ / 3.5 * 10^-12 * 3 = <span style=\"font-weight:bold;\">9 * 10³¹</span><br />\n<br />\nd) We are apparently assuming that seawater is 100% H₂0, and the Hydrogen is only either regular-type or deuterium. <br />\n<br />\nThe average molecule of H₂0 weighs:<br />\n(2.0141u * 0.00015 + 1.0078u * .99985) * 2 + 16u = 18.0159u<br />\nThe contribution from deuterium is negligible, so you could probably leave it out.<br />\n<br />\nThe number of H₂0 molecules in a kilogram is then 1kg / 18.0159u, or 3.3427 * 10²⁵, and the number of deuterium is that * 0.015%, or 5.014 * 10²¹.<br />\n<br />\nThe number of kilograms needed is (deuterium needed / deuterium per kg).<br />\n9 * 10³¹ / 5.014 * 10²¹ = <span style=\"font-weight:bold;\">1.795\u00d710¹⁰kg</span>", "user": "fwip" }, { "id": "855065", "time": "1433423113", "html": "Hi! I would like to share some awesome news with you :) Now there is a dedicated website to help us with our homework and there is a <a href=\"http://www.digischool.co.uk/homework/year-11/physics/\">physics homework category</a> :) You can post a homework or a question and you get help for free!! Join now! I have tested it, it's great :)", "user": "steyllen" }, { "id": "1263501", "time": "1674769623", "html": "Almost 10 years later and I'm soon set to defend my doctoral dissertation in physics. Oh how time flies. I miss high school.", "user": "arthurkock" }, { "id": "1264415", "time": "1675340410", "html": "That is epic. Congratulations are in order, and perhaps a welcome back?", "user": "gws" } ], "users": { "arthurkock": { "name": "ArthurKock", "key": "arthurkock", "url": "/users/arthurkock", "avatar": "/images/white.png", "rankClass": "civilian", "rankText": "", "posts": "53" }, "alpha": { "name": "Alpha", "key": "alpha", "url": "/users/alpha", "avatar": "/uploads/drawn/2901.png", "rankClass": "civilian", "rankText": "", "posts": "2700" }, "zia": { "name": "Zia", "key": "zia", "url": "/users/zia", "avatar": "/uploads/drawn/10397.png", "rankClass": "civilian", "rankText": "", "posts": "2319" }, "terrabranford": { "name": "TerraBranford", "key": "terrabranford", "url": "/users/terrabranford", "avatar": "/uploads/drawn/510.png", "rankClass": "civilian", "rankText": "", "posts": "1953" }, "fwip": { "name": "Fwip", "key": "fwip", "url": "/users/fwip", "avatar": "/uploads/drawn/583.png", "rankClass": "admin", "rankText": "Admin", "posts": "3744" }, "steyllen": { "name": "steyllen", "key": "steyllen", "url": "/users/steyllen", "avatar": "/images/white.png", "rankClass": "civilian", "rankText": "", "posts": "2" }, "gws": { "name": "gws", "key": "gws", "url": "/users/gws", "avatar": "/uploads/drawn/29928.png", "rankClass": "civilian", "rankText": "", "posts": "5830" } }, "page_num": 1, "locked": 0, "total_pages": 1 }

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