Energy: The Quantity That Only Changes Costume
Physics' best accounting trick: a number you can compute before and after any process — and it always balances.
Builds on: 0.2 Acceleration: When Velocity Itself Changes1.1 Newton's Three Laws: The Operating System
Same idea, other course:🧬 Helix 1.2 — your cells spend these same joules, via ATP
Two costumes to start with
Kinetic energy is the energy of motion; potential energy is stored in position — lifted against gravity, for now:
The v² in kinetic energy is the fact behind every speed limit: at double the speed a car carries four times the energy, and needs four times the distance to brake it away. Sixty percent more speed doubles the crash energy.
The conservation law
Drop a ball and PE drains as KE fills, gram for gram, joule for joule. Add them and the total stays fixed — that is conservation of energy, and it is not approximately true, it is exactly true, always, everywhere. Energy is never created or destroyed; it changes costume: motion, height, spring tension, chemical bonds, electricity, light, and — the costume of last resort — heat.
“Losing energy to friction” is really converting it into disordered jiggling of molecules. The books still balance; the energy has just gone somewhere you can no longer conveniently spend. A pendulum dies down, and the room is immeasurably warmer.
Why physicists lead with it
A skateboarder rolls down a curved ramp: computing forces along that curve is miserable. Energy doesn’t care about the path — only start and end:
Same answer for a straight drop, a curved slide or a loop, and no forces computed anywhere. Conservation laws are physics’ way of skipping the hard part of the story and reading the last page.
Power: energy per second
Power is how fast energy converts — joules per second, i.e. watts. A 60 W bulb spends 60 J each second; a fit cyclist sustains ~250 W; a kettle draws 2,000. Your monthly energy bill is literally a joule count (dressed up as kilowatt-hours: one kWh = 3.6 million joules).
A food-label “calorie” (kcal) is 4,184 joules. A 2,000 kcal day is ~8.4 MJ — a continuous burn near 100 W. You are, energetically, a bright old-style light bulb; Helix’s respiration lesson is the story of how that wattage is actually delivered.
⚗️ Lab — The Energy Ramp
A skater on a curved ramp, with live energy bars doing the accounting.
- Watch PE drain into KE on the way down and refill on the way up — the total bar never moves.
- Add friction and watch a third bar (heat) grow at the others’ expense. Total: still fixed.
- Halve the drop height and check the bottom speed: √2 slower, not 2× — the square root at work.
Work it by hand — 0 / 3
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±2% count.