Welcome to CK-12 Foundation | CK-12 Foundation

<!DOCTYPE html>     <html class="no-js" lang="en">  <head> <title>Welcome to CK-12 Foundation | CK-12 Foundation</title> <meta property="og:site_name" itemprop="name" name="author" content="CK-12 Foundation" /> <meta property="fb:app_id" content="159884644067996" /> <meta itemprop="url" content="https://www.ck12.org" /> <meta itemprop="telephone" content="(650) 494-1302" /> <meta charset="utf-8" />  <meta http-equiv="x-dns-prefetch-control" content="on"> <link rel="dns-prefetch" href="//static1.ck12.org"> <link rel="dns-prefetch" href="//static2.ck12.org"> <link rel="dns-prefetch" href="//static3.ck12.org"> <link rel="dns-prefetch" href="//image1.ck12.org"> <link rel="dns-prefetch" href="//image2.ck12.org"> <link rel="dns-prefetch" href="//image3.ck12.org"> <link rel="dns-prefetch" href="//api-prod.ck12.org"> <link rel="canonical" href="https://www.ck12.org/c/physical-science/"/> <meta property="og:url" content="https://www.ck12.org/c/physical-science/" /> <meta name="viewport" content="width=device-width" /> <meta name="google-site-verification" content="wHx0f-Udlfd7ZNYNrIXpJVpjjO8nSH5RE0jIX8V-jIM" /> <meta name="google-translate-customization" content=""/> <meta name="application-name" content="CK-12 Foundation"/> <meta name="msapplication-TileColor" content="#003300"/> <meta name="msapplication-TileImage" content="//image1.ck12.org/media/build-20241120063108/images/ck12-logo-livetile.png"/> <link rel="apple-touch-icon" href="//image1.ck12.org/media/build-20241120063108/images/ck12-logo-livetile-57x57.png" /> <link rel="apple-touch-icon" sizes="72x72" href="//image1.ck12.org/media/build-20241120063108/images/ck12-logo-livetile-72x72.png" /> <link rel="apple-touch-icon" sizes="114x114" href="//image3.ck12.org/media/build-20241120063108/images/ck12-logo-livetile-114x114.png" /> <link rel="shortcut icon" href="/favicon.svg" type="image/x-icon" /> <link rel="search" type="application/opensearchdescription+xml" href="/opensearch.xml" title="CK-12 Foundation" /> <link rel="stylesheet" type="text/css" media="all" href="//static2.ck12.org/media/build-20241120063108/common/vendor/jquery-ui/css/ui-lightness/jquery-ui-1.10.2.custom.min.css" /> <link rel="stylesheet" type="text/css" media="all" href="//static1.ck12.org/media/build-20241120063108/course-book-app/dist/main.css" /> <link rel="stylesheet" type="text/css" media="all" href="//static2.ck12.org/media/build-20241120063108/common/css/master.css" /> <link rel="stylesheet" type="text/css" media="all" href="//static2.ck12.org/media/build-20241120063108/common/css/tangerine-ui.css" /> <link href='https://fonts.googleapis.com/css?family=Pacifico' rel='stylesheet' type='text/css'> <link rel="stylesheet" type="text/css" media="all" href="//static3.ck12.org/media/build-20241120063108/duplicateUsernames/css/content.css" />  <script src="/media/buildnumber.js"> </script>  <script src="//static3.ck12.org/media/build-20241120063108/outdatedbrowser/outdatedbrowser.js"></script> <script type="application/ld+json"> { "@context": "https://schema.org", "@type": "Organization", "name": "CK-12 Foundation", "url": "https://www.ck12.org/", "logo": "https://www.ck12.org/media/common/images/logo_ck12.svg", "sameAs": [ "https://www.facebook.com/CK12Foundation", "https://twitter.com/CK12Foundation", "https://www.linkedin.com/company/ck-12-foundation/", "https://www.pinterest.com/ck12foundation/" ] } </script>  <script> if (window.navigator.userAgent.toLowerCase().indexOf('prerender') !== -1) { window.SC_DISABLE_SPEEDY = true; 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expires=' + now.toUTCString() + '; path=/'; } function addEvent(obj, evt,fn,useCapture){ if(obj.addEventListener){ obj.addEventListener(evt,fn,useCapture); return true; }else if(obj.attachEvent){ var r=obj.attachEvent('on'+evt,fn); return r; }else this['on'+evt]=fn; } addEvent(window, "load", adsOnload); var existingOnBeforeUnloadHandler = window.onbeforeunload; function handleExistingBeforeUnload() { var returnValue; if (existingOnBeforeUnloadHandler){ returnValue = existingOnBeforeUnloadHandler(event); } adsOnBeforeUnload(); if (returnValue) { return returnValue; } } addEvent(window, "beforeunload", handleExistingBeforeUnload);  var cookieEnabled = (navigator.cookieEnabled) ? true : false; if (typeof navigator.cookieEnabled == "undefined" && !cookieEnabled) { var cookieName = "CK12-Cookie-Test"; document.cookie= cookieName; cookieEnabled = (document.cookie.indexOf(cookieName) != -1) ? true : false; if (cookieEnabled){ document.cookie = cookieName + "=; expires=Thu, 01 Jan 1970 00:00:01 GMT;"; 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