@font-face{font-family:Archivo Variable;font-style:normal;font-display:swap;font-weight:100 900;src:url(/_astro/archivo-vietnamese-wght-normal.XAtsl5Q_.woff2) format("woff2-variations");unicode-range:U+0102-0103,U+0110-0111,U+0128-0129,U+0168-0169,U+01A0-01A1,U+01AF-01B0,U+0300-0301,U+0303-0304,U+0308-0309,U+0323,U+0329,U+1EA0-1EF9,U+20AB}@font-face{font-family:Archivo Variable;font-style:normal;font-display:swap;font-weight:100 900;src:url(/_astro/archivo-latin-ext-wght-normal.C4zznr8T.woff2) format("woff2-variations");unicode-range:U+0100-02BA,U+02BD-02C5,U+02C7-02CC,U+02CE-02D7,U+02DD-02FF,U+0304,U+0308,U+0329,U+1D00-1DBF,U+1E00-1E9F,U+1EF2-1EFF,U+2020,U+20A0-20AB,U+20AD-20C0,U+2113,U+2C60-2C7F,U+A720-A7FF}@font-face{font-family:Archivo Variable;font-style:normal;font-display:swap;font-weight:100 900;src:url(/_astro/archivo-latin-wght-normal.E0tuGl4L.woff2) format("woff2-variations");unicode-range:U+0000-00FF,U+0131,U+0152-0153,U+02BB-02BC,U+02C6,U+02DA,U+02DC,U+0304,U+0308,U+0329,U+2000-206F,U+20AC,U+2122,U+2191,U+2193,U+2212,U+2215,U+FEFF,U+FFFD}@font-face{font-family:Atkinson Hyperlegible;font-style:normal;font-display:swap;font-weight:400;src:url(/_astro/atkinson-hyperlegible-latin-ext-400-normal.DRk46D-x.woff2) format("woff2"),url(/_astro/atkinson-hyperlegible-latin-ext-400-normal.Bbz-b3yf.woff) format("woff");unicode-range:U+0100-02BA,U+02BD-02C5,U+02C7-02CC,U+02CE-02D7,U+02DD-02FF,U+0304,U+0308,U+0329,U+1D00-1DBF,U+1E00-1E9F,U+1EF2-1EFF,U+2020,U+20A0-20AB,U+20AD-20C0,U+2113,U+2C60-2C7F,U+A720-A7FF}@font-face{font-family:Atkinson Hyperlegible;font-style:normal;font-display:swap;font-weight:400;src:url(/_astro/atkinson-hyperlegible-latin-400-normal.BrHNak5F.woff2) format("woff2"),url(/_astro/atkinson-hyperlegible-latin-400-normal.BbWidj28.woff) format("woff");unicode-range:U+0000-00FF,U+0131,U+0152-0153,U+02BB-02BC,U+02C6,U+02DA,U+02DC,U+0304,U+0308,U+0329,U+2000-206F,U+20AC,U+2122,U+2191,U+2193,U+2212,U+2215,U+FEFF,U+FFFD}@font-face{font-family:Atkinson Hyperlegible;font-style:normal;font-display:swap;font-weight:700;src:url(/_astro/atkinson-hyperlegible-latin-ext-700-normal.BoVPHkS0.woff2) format("woff2"),url(/_astro/atkinson-hyperlegible-latin-ext-700-normal.CKkU2Dpt.woff) format("woff");unicode-range:U+0100-02BA,U+02BD-02C5,U+02C7-02CC,U+02CE-02D7,U+02DD-02FF,U+0304,U+0308,U+0329,U+1D00-1DBF,U+1E00-1E9F,U+1EF2-1EFF,U+2020,U+20A0-20AB,U+20AD-20C0,U+2113,U+2C60-2C7F,U+A720-A7FF}@font-face{font-family:Atkinson Hyperlegible;font-style:normal;font-display:swap;font-weight:700;src:url(/_astro/atkinson-hyperlegible-latin-700-normal.GZI4o3u0.woff2) format("woff2"),url(/_astro/atkinson-hyperlegible-latin-700-normal.BK6Glc0m.woff) format("woff");unicode-range:U+0000-00FF,U+0131,U+0152-0153,U+02BB-02BC,U+02C6,U+02DA,U+02DC,U+0304,U+0308,U+0329,U+2000-206F,U+20AC,U+2122,U+2191,U+2193,U+2212,U+2215,U+FEFF,U+FFFD}@font-face{font-family:Courier Prime;font-style:normal;font-display:swap;font-weight:400;src:url(/_astro/courier-prime-latin-ext-400-normal.B-EsvyE4.woff2) format("woff2"),url(/_astro/courier-prime-latin-ext-400-normal.CKOCNFvK.woff) format("woff");unicode-range:U+0100-02BA,U+02BD-02C5,U+02C7-02CC,U+02CE-02D7,U+02DD-02FF,U+0304,U+0308,U+0329,U+1D00-1DBF,U+1E00-1E9F,U+1EF2-1EFF,U+2020,U+20A0-20AB,U+20AD-20C0,U+2113,U+2C60-2C7F,U+A720-A7FF}@font-face{font-family:Courier Prime;font-style:normal;font-display:swap;font-weight:400;src:url(/_astro/courier-prime-latin-400-normal.BbyBr73r.woff2) format("woff2"),url(/_astro/courier-prime-latin-400-normal.BAlbUm6l.woff) format("woff");unicode-range:U+0000-00FF,U+0131,U+0152-0153,U+02BB-02BC,U+02C6,U+02DA,U+02DC,U+0304,U+0308,U+0329,U+2000-206F,U+20AC,U+2122,U+2191,U+2193,U+2212,U+2215,U+FEFF,U+FFFD}@font-face{font-family:Courier Prime;font-style:normal;font-display:swap;font-weight:700;src:url(/_astro/courier-prime-latin-ext-700-normal.ByMJlNdM.woff2) format("woff2"),url(/_astro/courier-prime-latin-ext-700-normal.BIFoAzHx.woff) format("woff");unicode-range:U+0100-02BA,U+02BD-02C5,U+02C7-02CC,U+02CE-02D7,U+02DD-02FF,U+0304,U+0308,U+0329,U+1D00-1DBF,U+1E00-1E9F,U+1EF2-1EFF,U+2020,U+20A0-20AB,U+20AD-20C0,U+2113,U+2C60-2C7F,U+A720-A7FF}@font-face{font-family:Courier Prime;font-style:normal;font-display:swap;font-weight:700;src:url(/_astro/courier-prime-latin-700-normal.D1YCjmaD.woff2) format("woff2"),url(/_astro/courier-prime-latin-700-normal.CVvp4Sof.woff) format("woff");unicode-range:U+0000-00FF,U+0131,U+0152-0153,U+02BB-02BC,U+02C6,U+02DA,U+02DC,U+0304,U+0308,U+0329,U+2000-206F,U+20AC,U+2122,U+2191,U+2193,U+2212,U+2215,U+FEFF,U+FFFD}:root{--paper: #faf6ee;--surface: #ffffff;--ink: #16161a;--ink-soft: #6f6d65;--rule: #16161a;--rule-soft: #d6d2c6;--hatch: rgba(166, 122, 44, .07);--blue: #21509f;--red: #ad2e1d;--green: #1f7a3f;--gold: #c6952c;--shadow: 4px 4px 0 rgba(22, 22, 26, .12);--font-display: "Archivo Variable", "Helvetica Neue", Arial, sans-serif;--font-body: "Atkinson Hyperlegible", "Helvetica Neue", Arial, sans-serif;--font-mono: "Courier Prime", "Courier New", monospace;color-scheme:light}*{box-sizing:border-box}html{background-color:var(--paper);background-image:repeating-linear-gradient(135deg,var(--hatch) 0,var(--hatch) 1.5px,transparent 1.5px,transparent 13px);color:var(--ink);font-family:var(--font-body);font-size:112.5%;line-height:1.55}body{margin:0;min-height:100vh;display:flex;flex-direction:column}h1,h2,h3,h4{font-family:var(--font-display);line-height:1.12;letter-spacing:-.015em;margin:0;text-wrap:balance}p{margin:0}a{color:var(--blue)}:focus-visible{outline:3px solid var(--blue);outline-offset:2px}@media(prefers-reduced-motion:reduce){*,*:before,*:after{animation:none!important;transition:none!important}}.kicker{font-family:var(--font-mono);font-weight:700;font-size:.72rem;letter-spacing:.16em;text-transform:uppercase;color:var(--ink-soft)}.shell{width:100%;max-width:74rem;margin:0 auto;padding:0 1.5rem}.sheet{background:var(--surface);border:1px solid var(--rule);box-shadow:var(--shadow)}.btn{display:inline-flex;align-items:center;justify-content:center;gap:.6rem;font-family:var(--font-body);font-weight:700;font-size:1.05rem;line-height:1.2;text-decoration:none;color:var(--ink);background:var(--surface);border:2px solid var(--ink);box-shadow:3px 3px 0 var(--ink);padding:.8rem 1.5rem;min-height:3rem;cursor:pointer}.btn:active{translate:2px 2px;box-shadow:1px 1px 0 var(--ink)}.btn.primary{color:#fff;background:var(--blue);border-color:var(--blue);box-shadow:3px 3px 0 var(--ink)}.btn.quiet{box-shadow:none;border-color:var(--rule-soft);font-weight:400}.pro-badge{font-family:var(--font-mono);font-size:.68rem;font-weight:700;letter-spacing:.1em;color:var(--red);border:1.5px solid var(--red);padding:.05rem .3rem;white-space:nowrap}.topic-mark{display:inline-flex;align-items:center;gap:.5rem}.topic-mark:before{content:"";width:.65rem;height:.65rem;background:var(--topic-color, var(--ink));flex:none}[data-topic="Motion & forces"]{--topic-color: var(--blue)}[data-topic="Energy & momentum"]{--topic-color: var(--green)}[data-topic=Collisions]{--topic-color: var(--red)}[data-topic="Waves & sound"]{--topic-color: var(--gold)}[data-topic="Simple harmonic motion"]{--topic-color: var(--ink)}.masthead{border-bottom:3px solid var(--ink);background:var(--surface)}.masthead-inner{display:flex;flex-wrap:wrap;align-items:flex-end;gap:.5rem 2.5rem;padding-top:1.3rem}.brand-block{margin-right:auto;padding-bottom:1rem}.wordmark{position:relative;display:inline-block;font-family:var(--font-display);font-weight:800;font-size:1.65rem;line-height:.95;letter-spacing:-.02em;color:var(--ink);text-decoration:none;padding-bottom:10px}.wordmark .dim{font-weight:500;color:var(--ink-soft);letter-spacing:-.01em}.wordmark:after{content:"";position:absolute;left:0;right:0;bottom:0;height:4px;background:linear-gradient(135deg,#d6361f 0 25%,#1f9b4a 25% 50%,#2663cf 50% 75%,#e0a52c 75% 100%);clip-path:polygon(0 0,100% 0,calc(100% - 4px) 100%,0 100%)}.brand-block .kicker{display:block;margin-top:.55rem}.masthead nav{display:flex;flex-wrap:wrap;align-items:center;gap:.2rem;padding-bottom:.7rem}.masthead nav a,.masthead nav .signin{display:inline-flex;align-items:center;min-height:2.8rem;padding:.3rem .8rem;font-weight:700;font-size:1rem;color:var(--ink);text-decoration:none}.masthead nav a:hover,.masthead nav .signin:hover{color:var(--blue)}.masthead nav a[aria-current=page]{text-decoration:underline;text-decoration-thickness:3px;text-underline-offset:8px;text-decoration-color:var(--blue)}.masthead nav .pro-badge{margin-left:.4rem}.masthead nav .signin{margin-left:.8rem;border:2px solid var(--rule-soft);font-weight:400}.signin-state{display:inline-flex;align-items:center;gap:.75rem;margin-left:.8rem}.signin-state .signin{margin-left:0}.signin-state .kicker{max-width:14rem;overflow:hidden;text-overflow:ellipsis;white-space:nowrap}button.signin{font:inherit;background:none;cursor:pointer;border-radius:0}.site-footer{margin-top:auto;border-top:1px solid var(--rule-soft);padding:1.6rem 0 2.2rem}.site-footer .shell{display:flex;flex-wrap:wrap;gap:.4rem 2rem;justify-content:space-between}dialog.mock-note{border:2px solid var(--ink);box-shadow:var(--shadow);background:var(--surface);color:var(--ink);max-width:26rem;padding:1.6rem 1.7rem 1.7rem}dialog.mock-note::backdrop{background:#16161a59}dialog.mock-note h2{font-size:1.35rem;margin-bottom:.6rem}dialog.mock-note p{color:var(--ink-soft);margin-bottom:1.3rem}@media print{html{background:#fff;font-size:100%}.masthead nav,.site-footer,.no-print{display:none!important}.masthead{border-bottom-width:2px}.sheet{border:none;box-shadow:none}}