{"id":2942,"date":"2008-09-17T08:35:32","date_gmt":"2008-09-17T08:35:32","guid":{"rendered":"http:\/\/scienceblogs.com\/principles\/2008\/09\/17\/spherical-cows\/"},"modified":"2008-09-17T08:35:32","modified_gmt":"2008-09-17T08:35:32","slug":"spherical-cows","status":"publish","type":"post","link":"http:\/\/chadorzel.com\/principles\/2008\/09\/17\/spherical-cows\/","title":{"rendered":"Spherical Cows"},"content":{"rendered":"

Two new recent posts take up the question of “spherical cows,” the old joke term for absurd-sounding approximations that physicists make to turn intractable problems into easy ones. First, The First Excited State explains when N=N+1<\/a>:<\/p>\n

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Everybody who’s taken any sort of math class knows that a statement like N+1 = N is simply ridiculous. Everyone, that is, except for the physicist. Let’s say that N is a really huge number, like if someone dumped an entire truckload of M&M’s in your driveway. If you turned your back on me to watch the truck drive away, and I threw another M&M in the pile while you weren’t looking, would you really notice? What if I snuck one while you were looking to the sky to thank God for this miracle? No, you’d really have no idea. So in this case, for all practical purposes, N+1 = N-1 = N. We make this approximation all the time in my statistical mechanics class, where N represents some astronomically huge number, like the number of water molecules in your glass.<\/p>\n<\/blockquote>\n

Then the author links to a post at Shores of the Dirac Sea in which David Berenstein offers his own take<\/a>:<\/p>\n

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Let me give you an example of how spherical cows might be put to some use. Assume that a cow is falling from a cliff onto a body of water like a lake (we don’t want the cow to get hurt after all, we are not cruel). A physics question you might ask is how long does the cow take to fall. This is controlled by gravity and the height of the cliff. One of the things we know is that the shape of the cow does not really change the result much. Sure, there is some air resistance, etc, etc, but these effects are small. Saying an effect is small is stating that you only need an approximate answer and not an exact one. For these situations, given that you don’t have the information of the exact shape of the cow and that you don’t need it, it might as well be spherical. Now, a spherical cow falling on water would be a comical sight. Don’t you think? Well, you might also consider that a lot of flailing legs and a panicked look on the face of a cow is funny.<\/p>\n<\/blockquote>\n

(There’s also a link there to Tom’s old post on the point-cow approximation<\/a>. That’s three spherical cows this week…)<\/p>\n

In related news, I’d like to ask for a moratorium on people starting really good physics blogs that I don’t have time to read. Thank you.<\/p>\n","protected":false},"excerpt":{"rendered":"

Two new recent posts take up the question of “spherical cows,” the old joke term for absurd-sounding approximations that physicists make to turn intractable problems into easy ones. First, The First Excited State explains when N=N+1: Everybody who’s taken any sort of math class knows that a statement like N+1 = N is simply ridiculous.… Continue reading Spherical Cows<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"1","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"class_list":["post-2942","post","type-post","status-publish","format-standard","hentry","category-physics","entry"],"_links":{"self":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/2942","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/comments?post=2942"}],"version-history":[{"count":0,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/2942\/revisions"}],"wp:attachment":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/media?parent=2942"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/categories?post=2942"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/tags?post=2942"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}