<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Diver Velocity Calculation</title>
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
      font-family: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
    }

    body {
      width: 100%;
      background: #f8fafc;
      color: #1e2945;
      padding: 16px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      min-height: 100vh;
    }

    .calc-wrapper {
      width: 100%;
      max-width: 560px;
      display: flex;
      flex-direction: column;
      gap: 12px;
    }

    /* Header title */
    .calc-header {
      display: flex;
      align-items: center;
      justify-content: space-between;
      gap: 8px;
    }

    .calc-title {
      font-size: 15px;
      font-weight: 700;
      color: #1e2945;
      letter-spacing: -0.2px;
    }

    /* Controls header */
    .controls-row {
      display: flex;
      align-items: center;
      justify-content: space-between;
      gap: 8px;
      width: 100%;
      flex-wrap: wrap;
    }

    .step-pills {
      display: flex;
      gap: 6px;
      flex-wrap: wrap;
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    .step-btn {
      background: #ffffff;
      border: 1.5px solid #cbd5e1;
      color: #475569;
      font-size: 12.5px;
      font-weight: 600;
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      transition: background 0.2s ease, border-color 0.2s ease;
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    .play-btn:hover {
      border-color: #22b7ff;
      background: #f0f9ff;
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    /* Steps stack */
    .steps-container {
      display: flex;
      flex-direction: column;
      gap: 8px;
      width: 100%;
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    .calc-step {
      background: #ffffff;
      border: 1.5px solid #e2e8f0;
      border-radius: 10px;
      padding: 10px 14px;
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      align-items: center;
      justify-content: space-between;
      gap: 12px;
      transition: all 0.25s ease;
      cursor: pointer;
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      box-shadow: 0 3px 12px rgba(34, 183, 255, 0.2);
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    .calc-step.passed {
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      background: #fafbfc;
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    .calc-step.inactive {
      background: #f8fafc;
      border-color: #e2e8f0;
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    .step-tag-group {
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      min-width: 90px;
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      font-weight: 600;
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      font-family: 'Cambria Math', 'STIX Two Math', 'Times New Roman', serif;
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      padding-right: 3px;
      display: inline-flex;
      align-items: center;
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    /* Callout box */
    .callout-box {
      background: #f0fdf4;
      border: 1.5px solid #86efac;
      border-radius: 10px;
      padding: 10px 14px;
      display: flex;
      align-items: center;
      gap: 10px;
      box-shadow: 0 1px 4px rgba(22, 163, 74, 0.08);
    }

    .badge-icon {
      background: #16a34a;
      color: #ffffff;
      width: 22px;
      height: 22px;
      border-radius: 50%;
      display: flex;
      align-items: center;
      justify-content: center;
      font-size: 13px;
      font-weight: bold;
      flex-shrink: 0;
      line-height: 1;
    }

    .callout-text {
      font-size: 13.5px;
      font-weight: 600;
      color: #14532d;
      line-height: 1.35;
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    @media (max-width: 480px) {
      body {
        padding: 10px;
      }
      .calc-step {
        padding: 8px 10px;
        gap: 8px;
      }
      .step-tag-group {
        min-width: 75px;
      }
      .step-num {
        font-size: 10px;
      }
      .step-desc {
        font-size: 10px;
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      .step-math {
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      .math-unit {
        font-size: 12px;
      }
      .radical-symbol {
        font-size: 17px;
      }
      .step-btn {
        font-size: 11.5px;
        padding: 4px 9px;
      }
      .callout-text {
        font-size: 12.5px;
      }
    }
  </style>
</head>
<body>

  <div class="calc-wrapper">
    
    <div class="controls-row">
      <div class="step-pills">
        <button class="step-btn active" data-step="0">1. Formula</button>
        <button class="step-btn" data-step="1">2. Substitute</button>
        <button class="step-btn" data-step="2">3. Multiply</button>
        <button class="step-btn" data-step="3">4. Solve</button>
      </div>
      <button class="play-btn" id="playToggle" title="Play / Pause" aria-label="Play / Pause">⏸</button>
    </div>

    
    <div class="steps-container">
      
      <div class="calc-step active" id="step-0" data-step="0">
        <div class="step-tag-group">
          <span class="step-num">Step 1</span>
          <span class="step-desc">Formula</span>
        </div>
        <div class="step-math">
          <span class="math-var">v</span>&nbsp;=&nbsp;
          <span class="radical-wrap">
            <span class="radical-symbol">√</span>
            <span class="radicand">2<span class="math-var">g</span><span class="math-var">h</span></span>
          </span>
        </div>
      </div>

      
      <div class="calc-step inactive" id="step-1" data-step="1">
        <div class="step-tag-group">
          <span class="step-num">Step 2</span>
          <span class="step-desc">Substitute</span>
        </div>
        <div class="step-math">
          <span class="math-var">v</span>&nbsp;=&nbsp;
          <span class="radical-wrap">
            <span class="radical-symbol">√</span>
            <span class="radicand">2 × 9.8<span class="math-unit">N/kg</span> × 5.0<span class="math-unit">m</span></span>
          </span>
        </div>
      </div>

      
      <div class="calc-step inactive" id="step-2" data-step="2">
        <div class="step-tag-group">
          <span class="step-num">Step 3</span>
          <span class="step-desc">Multiply</span>
        </div>
        <div class="step-math">
          <span class="math-var">v</span>&nbsp;=&nbsp;
          <span class="radical-wrap">
            <span class="radical-symbol">√</span>
            <span class="radicand">98</span>
          </span>
        </div>
      </div>

      
      <div class="calc-step inactive" id="step-3" data-step="3">
        <div class="step-tag-group">
          <span class="step-num">Step 4</span>
          <span class="step-desc">Evaluate</span>
        </div>
        <div class="step-math">
          <span class="math-var">v</span>&nbsp;≈&nbsp;9.9<span class="math-unit">m/s</span>
        </div>
      </div>
    </div>

    
    <div class="callout-box">
      <div class="badge-icon">✓</div>
      <div class="callout-text">Notice: The diver's mass was not needed!</div>
    </div>
  </div>

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    const STEP_DURATION = 3200;

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    const stepCards = document.querySelectorAll('.calc-step');
    const playToggle = document.getElementById('playToggle');

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      timer = setInterval(nextStep, STEP_DURATION);
      isPlaying = true;
      playToggle.textContent = '⏸';
      playToggle.setAttribute('title', 'Pause');
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      isPlaying = false;
      playToggle.textContent = '▶';
      playToggle.setAttribute('title', 'Play');
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</body>
</html>

Equating Gravitational Potential Energy and Kinetic Energy in Falling Bodies

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Conservation of Energy - Roller Coaster</title>
  <style>
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body {
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
      background: #ffffff;
      color: #0f172a;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: flex-start;
      min-height: 100vh;
      padding: 8px;
      user-select: none;
    }

    .main-container {
      width: 100%;
      max-width: 580px;
      display: flex;
      flex-direction: column;
      gap: 8px;
    }

    /* Track Viewport */
    .viewport {
      position: relative;
      width: 100%;
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      overflow: hidden;
    }

    svg {
      display: block;
      width: 100%;
      height: auto;
    }

    /* Formula Panel */
    .calc-box {
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      padding: 8px 10px;
      display: flex;
      flex-direction: column;
      gap: 6px;
    }

    .calc-header {
      font-size: 11.5px;
      font-weight: 700;
      text-transform: uppercase;
      letter-spacing: 0.4px;
      color: #475569;
      display: flex;
      flex-wrap: wrap;
      justify-content: space-between;
      gap: 4px;
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    .calc-flow {
      display: flex;
      align-items: center;
      justify-content: center;
      flex-wrap: wrap;
      gap: 5px;
      font-family: "SF Mono", Monaco, Inconsolata, Consolas, monospace;
      font-size: 12px;
      color: #0f172a;
    }

    .calc-step {
      background: #ffffff;
      padding: 4px 7px;
      border: 1px solid #cbd5e1;
      white-space: nowrap;
    }

    .calc-step.highlight {
      background: #eff6ff;
      border-color: #2563eb;
      color: #1d4ed8;
      font-weight: 700;
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    .arrow { color: #64748b; font-weight: bold; font-size: 12px; }

    /* Controls */
    .controls {
      display: flex;
      flex-wrap: wrap;
      justify-content: center;
      gap: 6px;
    }

    .btn {
      background: #ffffff;
      border: 1px solid #cbd5e1;
      padding: 6px 12px;
      font-size: 12.5px;
      font-weight: 600;
      color: #334155;
      cursor: pointer;
      display: inline-flex;
      align-items: center;
      gap: 4px;
      transition: background 0.15s, border-color 0.15s, color 0.15s;
    }
    .btn:hover { background: #f1f5f9; border-color: #94a3b8; }
    .btn.active {
      background: #2563eb;
      border-color: #2563eb;
      color: #ffffff;
    }

    @media (max-width: 440px) {
      .calc-header { font-size: 10.5px; }
      .calc-flow { font-size: 10.5px; gap: 3px; }
      .calc-step { padding: 3px 5px; }
      .arrow { font-size: 10.5px; }
      .btn { font-size: 11px; padding: 5px 8px; }
    }
  </style>
</head>
<body>

  <div class="main-container">
    
    <div class="viewport">
      <svg viewBox="0 0 560 220" id="scene">
        <defs>
          
          <pattern id="grid" width="20" height="20" patternUnits="userSpaceOnUse">
            <path d="M 20 0 L 0 0 0 20" fill="none" stroke="#e2e8f0" stroke-width="0.8"/>
          </pattern>
          
          <marker id="arrowhead" markerWidth="8" markerHeight="6" refX="7" refY="3" orient="auto">
            <polygon points="0 0, 8 3, 0 6" fill="#1d4ed8" />
          </marker>
        </defs>

        
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        <line x1="20" y1="185" x2="540" y2="185" stroke="#64748b" stroke-width="1.5" stroke-dasharray="5 4" />
        <text x="270" y="205" font-size="11" font-weight="700" fill="#64748b" text-anchor="middle">Ground Reference: h = 0 m</text>

        
        <g transform="translate(14, 14)">
          <rect width="102" height="54" rx="3" fill="#ffffff" fill-opacity="0.96" stroke="#cbd5e1" stroke-width="1.2" />
          <text x="7" y="16" font-size="10" font-weight="700" fill="#0f172a">▲ Peak (v = 0 m/s)</text>
          <text x="7" y="32" font-size="11" font-weight="700" fill="#0284c7">E<tspan font-size="8.5" dy="2">p</tspan><tspan font-size="11" dy="-2"> = max</tspan></text>
          <text x="7" y="47" font-size="11" font-weight="700" fill="#d97706">E<tspan font-size="8.5" dy="2">k</tspan><tspan font-size="11" dy="-2"> = 0</tspan></text>
        </g>

        
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        <line x1="123" y1="45" x2="137" y2="45" stroke="#0284c7" stroke-width="1.5"/>
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        <rect x="120" y="105" width="20" height="20" fill="#f8fafc" opacity="0.85" />
        <text x="127" y="119" font-size="11" font-weight="700" fill="#0284c7" text-anchor="end">h = 40 m</text>

        
        <path d="M 150 45 C 260 45, 360 185, 490 185" fill="none" stroke="#cbd5e1" stroke-width="10" stroke-linecap="round" />
        <path d="M 150 45 C 260 45, 360 185, 490 185" fill="none" stroke="#334155" stroke-width="3.5" stroke-linecap="round" id="trackPath" />

        
        <g transform="translate(432, 114)">
          <rect width="112" height="54" rx="3" fill="#ffffff" fill-opacity="0.96" stroke="#cbd5e1" stroke-width="1.2" />
          <text x="7" y="16" font-size="10" font-weight="700" fill="#0f172a">▼ Trough (h = 0 m)</text>
          <text x="7" y="32" font-size="11" font-weight="700" fill="#0284c7">E<tspan font-size="8.5" dy="2">p</tspan><tspan font-size="11" dy="-2"> = 0</tspan></text>
          <text x="7" y="47" font-size="11" font-weight="700" fill="#d97706">E<tspan font-size="8.5" dy="2">k</tspan><tspan font-size="11" dy="-2"> = max</tspan></text>
        </g>

        
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        <text id="velocityLabel" x="0" y="0" font-size="11.5" font-weight="800" fill="#1d4ed8" opacity="0">v = 28 m/s</text>

        
        <g id="cart" transform="translate(165, 41)">
          
          <rect x="-18" y="-14" width="36" height="17" rx="3" fill="#ea580c" stroke="#9a3412" stroke-width="2" />
          
          <circle cx="-7" cy="-7" r="3.2" fill="#ffffff" stroke="#9a3412" stroke-width="1" />
          <circle cx="7" cy="-7" r="3.2" fill="#ffffff" stroke="#9a3412" stroke-width="1" />
          
          <circle cx="-10" cy="5" r="4.5" fill="#0f172a" stroke="#cbd5e1" stroke-width="1.5" />
          <circle cx="10" cy="5" r="4.5" fill="#0f172a" stroke="#cbd5e1" stroke-width="1.5" />
        </g>
      </svg>
    </div>

    
    <div class="calc-box">
      <div class="calc-header">
        <span>Conservation of Energy: E<sub>p</sub> = E<sub>k</sub> &rarr; mgh = ½mv²</span>
        <span>g = 9.8 m/s²</span>
      </div>
      <div class="calc-flow">
        <div class="calc-step" id="step1">h = v² / 2g</div>
        <span class="arrow">&rarr;</span>
        <div class="calc-step" id="step2">h = (28)² / (2 × 9.8)</div>
        <span class="arrow">&rarr;</span>
        <div class="calc-step" id="step3">h = 784 / 19.6</div>
        <span class="arrow">&rarr;</span>
        <div class="calc-step highlight" id="step4">h = 40 m</div>
      </div>
    </div>

    
    <div class="controls">
      <button class="btn active" id="btnPeak" onclick="setPos(0)">1. Peak (0 m/s)</button>
      <button class="btn" id="btnMid" onclick="setPos(0.5)">2. Descent</button>
      <button class="btn" id="btnTrough" onclick="setPos(1)">3. Trough (28 m/s)</button>
    </div>
  </div>

  <script>
    const path = document.getElementById('trackPath');
    const cart = document.getElementById('cart');
    const vVector = document.getElementById('velocityVector');
    const vLabel = document.getElementById('velocityLabel');
    const pathLength = path.getTotalLength();

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      document.getElementById('btnPeak'),
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      return { x: p.x, y: p.y, angle: angle };
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    function renderCart(t) {
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      cart.setAttribute('transform', `translate(${x}, ${y - 4}) rotate(${angle})`);

      // Velocity Vector (scaled with sqrt(t) as v = sqrt(2gh))
      const speed = Math.round(28 * Math.sqrt(t));
      const vectorLen = 14 + t * 38;

      if (t > 0.03) {
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        const vy2 = (y - 4) + Math.sin(rad) * vectorLen;

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        vVector.setAttribute('y1', y - 4);
        vVector.setAttribute('x2', vx2);
        vVector.setAttribute('y2', vy2);
        vVector.setAttribute('opacity', '1');

        vLabel.setAttribute('x', vx2 + 6);
        vLabel.setAttribute('y', vy2 - 4);
        vLabel.textContent = `v = ${speed} m/s`;
        vLabel.setAttribute('opacity', '1');
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        vLabel.setAttribute('opacity', '0');
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      animId = requestAnimationFrame(animate);
    }

    // Initialize at Peak
    renderCart(0);
  </script>
</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="1:1">
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
      -webkit-tap-highlight-color: transparent;
    }
    body {
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
      background: transparent;
      color: #1e2945;
      width: 100%;
      height: 100%;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: space-between;
      padding: 12px;
      user-select: none;
    }

    /* Top Equation Banner */
    .header-bar {
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 8px;
      font-size: 15px;
      font-weight: 700;
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      border-radius: 12px;
      padding: 6px 14px;
      width: 100%;
      max-width: 440px;
    }
    .tag-ep { color: #10b981; }
    .tag-ek { color: #22b7ff; }

    /* Main Stage */
    .stage {
      display: flex;
      width: 100%;
      max-width: 440px;
      flex: 1;
      gap: 12px;
      margin: 10px 0;
      min-height: 240px;
    }

    /* Left: Ruler and Falling Ball */
    .track-card {
      flex: 1;
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      border-radius: 12px;
      position: relative;
      display: flex;
      padding: 16px 8px 16px 36px;
    }
    .ruler {
      position: absolute;
      left: 12px;
      top: 24px;
      bottom: 24px;
      width: 2px;
      background: #cbd5e1;
    }
    .tick {
      position: absolute;
      left: 0;
      display: flex;
      align-items: center;
      transform: translateY(-50%);
    }
    .tick::before {
      content: "";
      width: 8px;
      height: 2px;
      background: #94a3b8;
    }
    .tick-label {
      font-size: 12px;
      font-weight: 600;
      color: #64748b;
      margin-left: 6px;
    }
    .track-line {
      position: absolute;
      right: 28px;
      top: 24px;
      bottom: 24px;
      width: 2px;
      background: repeating-linear-gradient(to bottom, #cbd5e1 0, #cbd5e1 4px, transparent 4px, transparent 8px);
    }
    .ball {
      position: absolute;
      right: 17px;
      width: 24px;
      height: 24px;
      border-radius: 50%;
      background: radial-gradient(circle at 35% 35%, #475569, #0f172a);
      box-shadow: 0 4px 8px rgba(15, 23, 42, 0.25);
      transform: translateY(-50%);
      transition: top 0.05s linear;
      z-index: 2;
    }
    .ground {
      position: absolute;
      bottom: 12px;
      left: 10px;
      right: 10px;
      height: 4px;
      background: #94a3b8;
      border-radius: 2px;
    }

    /* Right: Energy Charts */
    .energy-card {
      flex: 1.3;
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      border-radius: 12px;
      padding: 12px;
      display: flex;
      flex-direction: column;
      justify-content: space-between;
    }
    .energy-row {
      display: flex;
      flex-direction: column;
      gap: 6px;
    }
    .energy-header {
      display: flex;
      justify-content: space-between;
      align-items: baseline;
      font-size: 13px;
      font-weight: 700;
    }
    .formula {
      font-size: 11px;
      font-weight: 600;
      color: #64748b;
    }
    .bar-track {
      width: 100%;
      height: 18px;
      background: #e2e8f0;
      border-radius: 9px;
      overflow: hidden;
      position: relative;
    }
    .bar-fill {
      height: 100%;
      width: 50%;
      border-radius: 9px;
      transition: width 0.05s linear;
    }
    .fill-ep { background: #10b981; }
    .fill-ek { background: #22b7ff; }
    .fill-total { background: #8b5cf6; }

    .summary-badge {
      text-align: center;
      font-size: 12px;
      font-weight: 700;
      color: #1e2945;
      padding: 6px;
      background: #ffffff;
      border: 1px solid #e2e8f0;
      border-radius: 8px;
    }

    /* Controls */
    .controls {
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 6px;
      width: 100%;
      max-width: 440px;
    }
    .btn {
      background: #ffffff;
      border: 1px solid #cbd5e1;
      color: #1e2945;
      font-size: 13px;
      font-weight: 600;
      padding: 6px 10px;
      border-radius: 8px;
      cursor: pointer;
      display: inline-flex;
      align-items: center;
      justify-content: center;
      outline: none;
    }
    .btn:active {
      background: #f1f5f9;
    }
    .btn.active {
      background: #1e2945;
      color: #ffffff;
      border-color: #1e2945;
    }
    .btn-play {
      padding: 6px 14px;
      background: #22b7ff;
      color: #ffffff;
      border-color: #22b7ff;
      font-weight: 700;
    }
  </style>
</head>
<body>

  <div class="header-bar">
    <span class="tag-ep">E<sub>p</sub></span> + 
    <span class="tag-ek">E<sub>k</sub></span> = 
    <span>E<sub>total</sub> (100%)</span>
  </div>

  <div class="stage">
    
    <div class="track-card">
      <div class="ruler">
        <div class="tick" style="top: 0%"><span class="tick-label">20 m</span></div>
        <div class="tick" style="top: 25%"><span class="tick-label">15 m</span></div>
        <div class="tick" style="top: 50%"><span class="tick-label">10 m</span></div>
        <div class="tick" style="top: 75%"><span class="tick-label">5 m</span></div>
        <div class="tick" style="top: 100%"><span class="tick-label">0 m</span></div>
      </div>
      <div class="track-line"></div>
      <div id="ball" class="ball" style="top: 24px;"></div>
      <div class="ground"></div>
    </div>

    
    <div class="energy-card">
      
      <div class="energy-row">
        <div class="energy-header">
          <span class="tag-ep">E<sub>p</sub> (mgh)</span>
          <span id="epVal" class="tag-ep">100%</span>
        </div>
        <div class="bar-track">
          <div id="epBar" class="bar-fill fill-ep" style="width: 100%;"></div>
        </div>
      </div>

      
      <div class="energy-row">
        <div class="energy-header">
          <span class="tag-ek">E<sub>k</sub> (½mv²)</span>
          <span id="ekVal" class="tag-ek">0%</span>
        </div>
        <div class="bar-track">
          <div id="ekBar" class="bar-fill fill-ek" style="width: 0%;"></div>
        </div>
      </div>

      
      <div class="energy-row">
        <div class="energy-header">
          <span style="color: #64748b;">Total Mechanical</span>
          <span style="color: #1e2945;">100%</span>
        </div>
        <div class="bar-track">
          <div class="bar-fill fill-total" style="width: 100%;"></div>
        </div>
      </div>

      <div class="summary-badge">
        <span class="tag-ep">E<sub>p (top)</sub></span> = <span class="tag-ek">E<sub>k (bottom)</sub></span>
      </div>
    </div>
  </div>

  
  <div class="controls">
    <button id="playBtn" class="btn btn-play">⏸ Pause</button>
    <button class="btn h-btn active" data-h="20">20m</button>
    <button class="btn h-btn" data-h="10">10m</button>
    <button class="btn h-btn" data-h="0">0m</button>
  </div>

  <script>
    const ball = document.getElementById('ball');
    const epBar = document.getElementById('epBar');
    const ekBar = document.getElementById('ekBar');
    const epVal = document.getElementById('epVal');
    const ekVal = document.getElementById('ekVal');
    const playBtn = document.getElementById('playBtn');
    const hBtns = document.querySelectorAll('.h-btn');

    let height = 20; // 0 to 20m
    let isPlaying = true;
    let direction = -1; // -1 falling, 1 rising
    let lastTime = null;

    // Range bounds in pixels for ball within track
    const topPx = 24;
    let trackHeight = 0;

    function updateTrackHeight() {
      const card = document.querySelector('.track-card');
      trackHeight = card.clientHeight - 48; // 24px padding top and bottom
    }

    function render(h) {
      height = Math.max(0, Math.min(20, h));
      const epPercent = Math.round((height / 20) * 100);
      const ekPercent = 100 - epPercent;

      // Update bars
      epBar.style.width = epPercent + '%';
      ekBar.style.width = ekPercent + '%';
      epVal.textContent = epPercent + '%';
      ekVal.textContent = ekPercent + '%';

      // Update ball position (top = 20m, bottom = 0m)
      const fractionDropped = (20 - height) / 20;
      ball.style.top = (topPx + fractionDropped * trackHeight) + 'px';

      // Update button active state
      hBtns.forEach(btn => {
        const bh = parseFloat(btn.dataset.h);
        if (Math.abs(bh - height) < 1) {
          btn.classList.add('active');
        } else {
          btn.classList.remove('active');
        }
      });
    }

    function loop(time) {
      if (!lastTime) lastTime = time;
      const dt = (time - lastTime) / 1000;
      lastTime = time;

      if (isPlaying) {
        // Drop speed: 6 meters per second
        height += direction * (5 * dt);

        if (height <= 0) {
          height = 0;
          isPlaying = false;
          playBtn.textContent = '▶ Replay';
          playBtn.classList.remove('active');
        } else if (height >= 20) {
          height = 20;
        }
        render(height);
      }

      requestAnimationFrame(loop);
    }

    // Interactive button triggers
    playBtn.addEventListener('click', () => {
      if (height <= 0) {
        height = 20;
        direction = -1;
        isPlaying = true;
        playBtn.textContent = '⏸ Pause';
      } else {
        isPlaying = !isPlaying;
        playBtn.textContent = isPlaying ? '⏸ Pause' : '▶ Play';
      }
    });

    hBtns.forEach(btn => {
      btn.addEventListener('click', () => {
        isPlaying = false;
        playBtn.textContent = '▶ Play';
        render(parseFloat(btn.dataset.h));
      });
    });

    window.addEventListener('resize', () => {
      updateTrackHeight();
      render(height);
    });

    // Init
    updateTrackHeight();
    render(20);
    requestAnimationFrame(loop);
  </script>
</body>
</html>

UK-GCSE-SCIENCE-PHYS-ENG-03-01

Aligned learning resources

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