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@font-face{font-family:'Poppins';font-weight:700;font-style:normal;font-display:swap;src:url('/physics/assets/fonts/poppins-700.woff') format('woff')}
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--navy:#0D1B2A;--wtext:42rem;--wpanel:47rem;--wfig:58rem}
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article.lesson>*,article.lesson section.lsec>*{max-width:var(--wtext);margin-left:auto;margin-right:auto;padding-left:1.2rem;padding-right:1.2rem}
article.lesson>section.lsec{max-width:none;padding-left:0;padding-right:0}
article.lesson>.panel,article.lesson section.lsec>.panel{max-width:var(--wpanel)}
article.lesson>.eqs,article.lesson section.lsec>.eqs{max-width:var(--wpanel)}
article.lesson>.rulecard,article.lesson section.lsec>.rulecard{max-width:var(--wpanel)}
article.lesson>.tblwrap,article.lesson section.lsec>.tblwrap{max-width:var(--wpanel)}
article.lesson>.pn,article.lesson section.lsec>.pn{max-width:var(--wpanel)}
article.lesson>figure.fig,article.lesson section.lsec>figure.fig{max-width:var(--wfig)}
.panel{border:1px solid var(--edge);background:var(--card);border-radius:20px;padding:1.25rem 1.45rem 1.15rem;margin:2rem auto}
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.p-check .pl{color:var(--cyan)}
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details summary{cursor:pointer;display:inline-flex;align-items:center;gap:.55rem;font-weight:500;color:var(--cyan);
border:1px solid rgba(56,182,255,.45);border-radius:11px;padding:.5rem 1.05rem;list-style:none;user-select:none;
transition:background .18s,border-color .18s}
details summary::-webkit-details-marker{display:none}
details summary::after{content:"";width:.5rem;height:.5rem;border-right:2px solid var(--cyan);border-bottom:2px solid var(--cyan);
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details[open] summary{margin-bottom:.9rem}
details.hintbox summary{color:var(--greytx);border-color:var(--edge)}
details.hintbox summary::after{border-color:var(--greytx)}
details.hintbox summary:hover{background:rgba(245,245,245,.05)}
figure.fig{margin:2.3rem auto;position:relative}
figure.fig svg{width:100%;height:auto;display:block}
figure.fig img{width:100%;height:auto;display:block}
/* The unit-hub guide, below the lesson links: a reference block, not a
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.hubguide{border-top:1px solid var(--hair);margin-top:2.4rem;padding-top:.6rem}
.hubguide h2{font-size:1.06rem;margin:1.5rem 0 .4rem}
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/* A hub equation row is .ieq, which is white-space:nowrap, and often
   carries two equations with a \qquad between them. On a 390px phone
   five of the physics unit hubs pushed the whole page sideways --
   508px on mechanics. The row scrolls within itself now, the same
   treatment a dense figure gets, and fig.js gives it a focus stop and
   a name only while it is actually overflowing. */
.hubeq dt{overflow-x:auto;-webkit-overflow-scrolling:touch}
.hubeq dt:focus-visible{outline:2px solid var(--cyan);outline-offset:2px}
.hubeq dd{margin:0 0 0 0;color:var(--greytx);line-height:1.5}
/* The expand control lives in the caption row below the artwork, never over
   it, and keeps a 44px hit target: 2.75rem at the 16px root. */
.figxbtn{flex:none;margin-left:auto;align-self:center;width:2.75rem;height:2.75rem;border-radius:9px;border:1px solid var(--edge);background:var(--bg);color:var(--greytx);cursor:pointer;display:flex;align-items:center;justify-content:center;padding:0;opacity:.85}
.figxbtn:hover{color:var(--cyan);border-color:var(--cyan);opacity:1}
.figxbtn svg{width:1.05rem;height:1.05rem}
/* A figure whose smallest label would project below 9 CSS px on a phone keeps
   a build-computed minimum artwork width (--figmin) and scrolls sideways
   instead of shrinking its labels past reading size. fig.js makes the
   viewport a named, keyboard-focusable region only while it actually
   overflows, so it is never a tab stop that goes nowhere. */
.figscroll{overflow-x:auto;-webkit-overflow-scrolling:touch}
.figscroll svg{min-width:var(--figmin,0)}
.figscroll:focus-visible{outline:2px solid var(--cyan);outline-offset:2px}
dialog.figdlg{background:var(--bg);border:1px solid var(--edge);border-radius:14px;padding:0;width:min(96vw,1150px);max-height:92vh;margin:auto;color:var(--ink)}
dialog.figdlg[open]{display:flex;flex-direction:column}
dialog.figdlg::backdrop{background:rgba(4,10,17,.82)}
.figdlgbar{display:flex;align-items:center;gap:1rem;justify-content:space-between;padding:.55rem .55rem .55rem .95rem;border-bottom:1px solid var(--hair)}
.figdlgbar p{margin:0;font-size:.85rem;color:var(--greytx);line-height:1.35}
.figdlgclose{flex:none;border:1px solid var(--edge);background:none;color:var(--ink);font:inherit;font-size:.85rem;border-radius:8px;padding:.35rem .8rem;cursor:pointer}
.figdlgclose:hover{border-color:var(--cyan);color:var(--cyan)}
.figdlgpan{overflow:auto;flex:1;min-height:0;padding:.9rem;-webkit-overflow-scrolling:touch}
.figdlgpan svg{width:900px;max-width:none;display:block;margin:0 auto}
@media print{.figxbtn,dialog.figdlg{display:none}}
figure.fig figcaption{display:flex;gap:.7rem;align-items:baseline;border-top:1px solid var(--hair);
margin-top:.7rem;padding-top:.55rem;color:var(--greytx);font-size:.9rem;line-height:1.55}
figure.fig .fign{color:var(--cyan);font-weight:700;font-size:.74rem;letter-spacing:.14em;flex:none}
.figsvg .sk-ink{stroke:var(--ink)}.figsvg .sk-cyan{stroke:var(--cyan)}.figsvg .sk-grey{stroke:var(--grey)}
.figsvg .sk-amber{stroke:var(--amber)}.figsvg .sk-coral{stroke:var(--coral)}.figsvg .sk-bg{stroke:var(--bg)}
.figsvg .fl-ink{fill:var(--ink)}.figsvg .fl-cyan{fill:var(--cyan)}.figsvg .fl-grey{fill:var(--grey)}
.figsvg .fl-amber{fill:var(--amber)}.figsvg .fl-coral{fill:var(--coral)}.figsvg .fl-bg{fill:var(--bg)}
rect.fl-bg[stroke-width]{stroke:var(--ink)}
.figsvg text{font-family:'InkGlyphs','Poppins',sans-serif}
.qbar{text-decoration:overline}
/* The combining overline that travels with the clipboard: kept in the text
   flow so copy picks it up, collapsed so the CSS bar above is the only one
   drawn. display:none would remove it from the clipboard as well. */
.qbarc{font-size:0;speak:never}
@keyframes an-swing{from{transform:translate(var(--ox),var(--oy)) rotate(var(--a0)) translate(var(--nox),var(--noy))}to{transform:translate(var(--ox),var(--oy)) rotate(var(--a1)) translate(var(--nox),var(--noy))}}
.an-swing{animation:an-swing 1.9s ease-in-out infinite alternate}
@keyframes an-flip{from{transform:translate(var(--ox),var(--oy)) scaleY(1) translate(var(--nox),var(--noy))}to{transform:translate(var(--ox),var(--oy)) scaleY(-1) translate(var(--nox),var(--noy))}}
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@keyframes an-jit{0%{transform:translate(0,0)}25%{transform:translate(3.5px,-2.6px)}50%{transform:translate(-2.9px,1.9px)}75%{transform:translate(1.9px,3.2px)}100%{transform:translate(0,0)}}
.an-jit1{animation:an-jit 1.1s linear infinite}
.an-jit2{animation:an-jit 1.6s linear infinite reverse}
.an-jit3{animation:an-jit 2.3s linear infinite}
@keyframes an-walk{0%{transform:translate(0,0)}13%{transform:translate(8px,-5px)}26%{transform:translate(3px,7px)}40%{transform:translate(-7px,11px)}55%{transform:translate(-12px,2px)}68%{transform:translate(-4px,-9px)}82%{transform:translate(6px,-12px)}100%{transform:translate(0,0)}}
.an-walk{animation:an-walk 8.5s linear infinite}
@keyframes an-strike{0%{transform:translate(0,0)}40%{transform:translate(var(--sx),var(--sy))}52%{transform:translate(calc(var(--sx)*0.86),calc(var(--sy)*0.86))}64%{transform:translate(var(--sx),var(--sy))}76%{transform:translate(calc(var(--sx)*0.3),calc(var(--sy)*0.3))}100%{transform:translate(0,0)}}
.an-strike{animation:an-strike 2.6s linear infinite}
@media (prefers-reduced-motion: reduce){.figsvg g[class^="an-"]{animation:none !important;transform:none !important}}
.figsvg .tx-ink{fill:var(--ink)}.figsvg .tx-cyan{fill:var(--cyan)}.figsvg .tx-grey{fill:var(--greytx)}
.figsvg .tx-amber{fill:var(--amber)}.figsvg .tx-coral{fill:var(--coral)}.figsvg .sk-greytx{stroke:var(--greytx)}.figsvg .fl-greytx{fill:var(--greytx)}
@media (prefers-reduced-motion: reduce){.animfig *{animation:none !important}}
@media print{.animfig *{animation:none !important}}
/* Figures animated through the global an-* classes must print still too. */
@media print{.figsvg g[class^="an-"]{animation:none !important;transform:none !important}}
.eqs{text-align:center;padding:.4rem .2rem .2rem;margin:1.9rem auto;overflow-x:auto;overflow-y:hidden}
.eqs .eq{display:block;font-size:clamp(1.15rem,4.6vw,1.5rem);font-weight:500;letter-spacing:.01em;line-height:1.35;white-space:nowrap}
.eqs .dstag{display:inline-block;margin-top:.7rem;font-size:.68rem;font-weight:700;letter-spacing:.15em;
border-radius:99px;padding:.22rem .8rem}
.eqs .ds-on{color:var(--amber);border:1px solid rgba(255,183,3,.45)}
.eqs .ds-off{color:var(--greytx);border:1px solid var(--edge)}
.ieq{font-weight:500;white-space:nowrap}
/* The two semantic twins of an equation: the MathML a screen reader reads and
   the linear text a copy takes. Hidden by clipping and taken out of the flow,
   not by display:none or visibility:hidden, either of which would drop the
   MathML out of the accessibility tree along with the pixels.

   Out of the flow is the load-bearing part. Hiding an inline twin in place
   with font-size:0 reads the same and copies the same, but Chromium still
   gives a <math> containing an <mfrac> a two-pixel minimum width, and two
   pixels beside an inline equation move the rest of the line. Absolute
   positioning is the only version that is provably zero. */
.eqml,.eqtx{position:absolute;width:1px;height:1px;margin:-1px;padding:0;border:0;
overflow:hidden;clip:rect(0 0 0 0);clip-path:inset(50%);white-space:nowrap}
/* Only the visual layer that has a semantic twin opts out of selection, so a
   copy takes the unambiguous .eqtx once instead of the picture and the MathML
   twice. Bare affixes ("R{{_{2}}}") carry no aria-hidden and stay selectable. */
.eqml,.eq[aria-hidden],.ieq[aria-hidden]{-webkit-user-select:none;user-select:none}
.frac{display:inline-flex;flex-direction:column;align-items:center;vertical-align:middle;line-height:1.25;margin:0 .1em}
.frac .n{border-bottom:1.6px solid currentColor;padding:0 .28em}
.frac .d{padding:0 .28em}
.rt{margin-right:.05em}
.rad{border-top:1.6px solid currentColor;padding:0 .15em}
.rt.big{font-size:2.05em;font-weight:400;line-height:1;vertical-align:middle;margin-right:.02em}
.rad.tall{display:inline-flex;align-items:center;vertical-align:middle;border-top:1.6px solid currentColor;padding:.12em .22em .12em .12em}
sup,sub{line-height:0}
.tblwrap{overflow-x:auto;margin:1.6rem auto}
.dtable{width:100%;border-collapse:collapse;font-size:.95rem}
.dtable th{font-size:.73rem;letter-spacing:.14em;text-transform:uppercase;color:var(--greytx);text-align:left;padding:.5rem .8rem;border-bottom:2px solid rgba(110,125,140,.5);font-weight:700}
.dtable td{padding:.55rem .8rem;border-bottom:1px solid rgba(110,125,140,.28)}
.dtable td:first-child{font-weight:700}
.rulecard{border:2px solid rgba(56,182,255,.55);border-radius:22px;text-align:center;padding:1.6rem 1.3rem;margin:2.7rem auto 0}
.rulecard p{margin:0;font-weight:700;font-size:clamp(1.1rem,3.6vw,1.32rem);line-height:1.5}
.rulecard .r2{color:var(--cyan)}
.videotop{margin:.9rem 0 0;font-size:.95rem}
.videotop .pi{width:.85rem;height:.85rem;vertical-align:-.1em;margin-right:.4rem}
.videotop a{color:var(--cyan);text-decoration:underline;text-underline-offset:3px}
.next{display:inline-flex;align-items:center;gap:.6rem;border:1px solid var(--edge);border-radius:14px;
padding:.7rem 1.2rem;color:var(--ink);font-weight:500;transition:border-color .18s,transform .18s cubic-bezier(.2,.9,.3,1.25)}
.next:hover{border-color:var(--cyan);text-decoration:none;transform:translateY(-1px)}
.next svg{width:1rem;height:1rem;flex:none}
.pn{display:flex;gap:.9rem;justify-content:space-between;flex-wrap:wrap;margin:2.4rem auto 0}
.pn .next{max-width:100%}
.pn .rev svg{transform:scaleX(-1)}
.pn .pnl{font-size:.68rem;font-weight:700;letter-spacing:.15em;color:var(--greytx);display:block}
.cards{display:grid;grid-template-columns:repeat(auto-fill,minmax(13.5rem,1fr));gap:.95rem;margin:1.5rem 0}
.card{border:1px solid var(--edge);background:var(--card);border-radius:20px;padding:1.05rem 1.15rem;color:var(--ink);display:block;transition:transform .18s cubic-bezier(.2,.9,.3,1.25),border-color .18s}
a.card:hover{transform:translateY(-2px);border-color:rgba(56,182,255,.5);text-decoration:none}
.card h3,.card h2{font-size:1.02rem;font-weight:700;margin:0 0 .25rem}
/* The first card or section on an index page is an H2, not an H3: a page that
   goes H1 -> H3 leaves a hole in the outline a screen reader has to guess at.
   The level is semantics; the size is this rule. Anywhere a card-level H2
   appears it must lose the section dot and the flex row an H2 normally gets. */
.card h2,.rpcard h2,.cklesson h2,.ki h2,.eqgrp h2{display:block;line-height:1.3;gap:0;letter-spacing:normal}
.card h2::before,.rpcard h2::before,.cklesson h2::before,.ki h2::before,.eqgrp h2::before{content:none;display:none}
.card .meta{color:var(--greytx);font-size:.84rem;font-weight:500}
ul.topics{list-style:none;padding:0;margin:.8rem 0}
ul.topics li{padding:.62rem .2rem;border-bottom:1px solid var(--hair);display:flex;align-items:baseline;gap:.7rem;flex-wrap:wrap}
ul.topics .tag{font-size:.7rem;font-weight:700;letter-spacing:.12em;color:var(--greytx);border:1px solid var(--edge);border-radius:6px;padding:.06rem .48rem;flex:none}
ul.topics .writing{color:var(--greytx)}
.search{width:100%;background:var(--card);border:1px solid var(--edge);border-radius:12px;color:var(--ink);font:inherit;padding:.62rem .95rem;margin:.4rem 0 .8rem}
.search::placeholder{color:var(--greytx)}
.unitline{color:var(--greytx);font-weight:700;font-size:.78rem;letter-spacing:.14em;margin:1.5rem 0 .1rem}
h2.unitline{display:block;line-height:1.5}
h2.unitline::before{display:none}
.jumps{color:var(--greytx);font-size:.88rem;line-height:2;margin:.2rem 0 1.2rem}
.jumps a{color:var(--greytx);text-decoration:underline;text-underline-offset:3px;white-space:nowrap}
.jumps a:hover{color:var(--cyan)}
ul.topics .tag a{text-decoration:none;display:inline-block;padding:.3rem .25rem;min-height:24px;box-sizing:border-box}
ul.topics .tag a:hover{color:var(--cyan);text-decoration:underline;text-underline-offset:3px}
@media (max-width:560px){
.jumps{line-height:2.35}
.jumps a{display:inline-block;border:1px solid var(--edge);border-radius:99px;padding:0 .68rem;text-decoration:none;margin:0 .06rem;line-height:1.9}
ul.topics li.flt{display:block}
ul.topics li.flt .tag{display:table;margin-top:.45rem;font-size:.74rem}
ul.topics li.flt .tag a{padding:.4rem .3rem}
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