Ch.1 · Before the formulas
What does a joule actually feel like?
Nobody has intuition for "1 kJ." So we build it from one thing your body already knows — lifting a water bottle.
PART 1 · HOW BIG IS AN ENERGY NUMBER?
Rung 1 of 7
One lift
A 1 litre water bottle weighs about 1 kg. Drag it up to the line — one metre — and let go. That's it. That's the unit.
0 lifts0 J
That's one lift → what's a kilojoule?
Rung 2 of 7
Build a kilojoule
One lift ≈ 10 joules. So a kilojoule is 100 lifts. Hold the button and stack them up — notice how long 1 kJ actually takes.
0 lifts0 J
HOLD TO KEEP LIFTINGeach press = one bottle, one metre
That is one kilojoule. 100 full lifts of a water bottle.1 kJ = 100 liftsEvery kJ you'll ever see in a thermo problem is this much real, physical effort. Keep that picture.
Now compare it to food →
Rung 3 of 7
One biscuit
A single Parle-G style biscuit holds about 100 kJ. You just felt 1 kJ. Hold again and try to fill the biscuit — go on.
🍪
One small biscuit
≈ 100 kJ · ≈ 25 kcal
each square = 1 kJ = the 100 lifts you just did
0 lifts0 / 100 kJ
holding at 50 lifts/sec — faster than any human could actually lift
HOLD TO KEEP LIFTINGstill one bottle, one metre, each time
Tired? You're barely started. One biscuit is10,000 liftsLifting that bottle once a second, non-stop, it would take you almost 3 hours to work off one biscuit. That's why energy density in food is enormous — and why "just burn it off" is harder than it sounds.
Now use your own numbers →
Rung 4 of 7
Your day, in bottle-lifts
Drag these to match your real life. Everything converts back to the one unit you now have a feel for.
A day of your food
8,400 kJ
= 840,000 bottle-lifts
Your kettle run
270 kJ
= 27,000 bottle-lifts · about 3% of your day's food
This is the whole point of the unit. A joule isn't abstract — it's one small physical act, and everything else is just how many of them.
But energy doesn't only mean lifting →
Rung 5 of 7 · Energy changes form
Now drop it
Same bottle. Same joules you just spent lifting it. Drag it to any height and let go — then watch the two bars while it falls.
↕ DRAG TO A HEIGHT, THEN RELEASE
thud — warmth + sound
🍶
Watch the top two bars trade places while the bottom bar never moves. The energy didn't get used up or created — it changed form. Height-energy became motion-energy, joule for joule.9.81 J → 9.81 JThat single sentence, made rigorous, is the First Law of Thermodynamics.
Same idea, no gravity at all →
Rung 6 of 7 · Energy changes form
Rub your hands
No lifting, no falling, no height. Swipe left and right across the box — and do it with your real hands too, right now.
+0.0 °C
🤚✋
↔ SWIPE BACK AND FORTH
0 strokes+0.0 °C
Not one bit of that came from height. You pushed against friction, and the motion turned into warmth — a completely different form, by a completely different route.motion → heatStop swiping and the warmth leaks away to the room. It didn't vanish; it spread out. That leak is the seed of the Second Law, and you just felt it.
One last case — and this one's a puzzle →
Rung 7 of 7 · Energy changes form
Where did the rest go?
A 1 kg ball dropped from 1 m comes back to only 0.8 m. Tap to drop it and watch.
dropped from 1.00 m — 9.81 J
came back to 0.80 m — 7.85 J
⚪
TAP TO DROP THE BALL
The ball started with 9.81 J and came back with 7.85 J. 1.96 J is unaccounted for. Drag it to where it actually went.
It was destroyed
Warmth in the floor & air
Still inside the ball
Energy can't be destroyed, and the ball is measurably not holding it. The only place left is the one nobody looks at: the floor, the air, and the ball's own material are all very slightly warmer.1.96 J → heatEvery real machine leaks like this. The energy is never lost — it just ends up somewhere useless. Hold on to that; it's the whole reason Chapter 3 exists.
Foundation complete. You now have a physical feel for how big a joule is, and you've watched the same joules change form three different ways — through gravity, through friction, and through a bounce. Nothing was ever created. Nothing was ever destroyed.