⟁ THREAD 4 — MECHANICS & THE PHYSICS OF WORK · MODULE 2
S ScienceT Technology E EngineeringA Arts M Mathematics

The Gravity-Powered
Kinetic Battery

We have been trained to think of a battery as a toxic sealed box of chemicals. But a battery is simply a device that stores energy for later use. When you lift a heavy weight into the air, you are storing energy in it. When you let it fall in a controlled way, it releases that energy as useful mechanical work.

High School – Adult · Ages 14+ Project Duration: 1 weekend No electricity required
HOW DO YOU LEARN BEST?
Works offline · No login · Yours to keep
⚠ Safety First — Read Before Building This project suspends 20–50 lbs in the air. Never stand under the weight. Use steel axles (not wood dowels) in proper bearings. Balance the fan fly before any load test — an unbalanced fly at ~380 RPM destroys frames. Install the fan fly before releasing the weight. Use bowline knots. Build a structural lumber frame, not light wood. Full detail in Step by Step tab.
CALCULATOR

Potential Energy & Gear Ratio Calculator

M · Mathematics · S · Science

Design your battery before you build it. Adjust the sliders to see how weight, height, and gear ratio interact.

⟁ Design Your Battery

Stored Energy
89 J
PE = mgh
Gear Ratio
4.0:1
Large ÷ Small
Weight (kg)
9.07
lbs × 0.4536
Fan Fly Speed
×4.0
vs spool rotation
Equiv. LED Time
8.9 s
at 10 watts
Free-Fall Time
0.45 s
without governor
The Law — Same as the Lever & Block and Tackle PE = mgh. Double the weight → double the energy. Double the height → double the energy. The gear ratio multiplies fan fly speed by the same factor — but torque is divided by the same factor. Work in = Work out. The law never changes, only the machine that applies it.
DIAGRAM

System Anatomy

T · Technology · E · Engineering
CONTEXT

This Is Happening at Grid Scale Right Now

S · Science
Pumped-Storage Hydropower When grid electricity is cheap and abundant — overnight, during windy days — pumped-storage plants pump water uphill into a reservoir, storing gravitational PE. When demand spikes, they release it through turbines. Pumped-storage currently represents approximately 95% of the world's utility-scale energy storage capacity. Your bench-scale gravity battery and a 300-meter-tall reservoir run the exact same equation: PE = mgh.

The Full Module — Reading Mode

⚠ Safety This project suspends 20–50 lbs in the air. Never stand under the suspended weight. Use steel axle rods and proper bearings — wooden dowels deflect under this load. Balance the fan fly before any load test — unbalanced rotation at high RPM destroys frames. Install fan fly before releasing weight. Use bowline knots on all load connections. Structural lumber frame only.

What This Is

Before electricity, the most reliable way to power a machine automatically was a falling weight. Mechanical clocks, kitchen spit-roasters, and astronomical telescope drives all used this principle: wind a rope around a spool, lifting a weight into the air, storing gravitational potential energy. As the weight falls, it turns gears. A fan fly governs the release speed using air resistance. You are building a working power source that releases energy at a sustained, usable rate.

The Physics of Stored Energy

Gravitational Potential Energy: PE = m × g × h, where m is mass in kilograms, g is 9.81 m/s², and h is height in meters. Double the weight, double the energy. Double the height, double the energy. A 20 lb (9.07 kg) weight lifted 1 meter stores 89 Joules — enough to power a 10-watt LED for about 9 seconds, or run a mechanical clock for hours because mechanical systems use this energy far more efficiently than converting it to electricity.

The Gear Train — Same Law as the Lever

A gear train does the same trade-off as a lever, block and tackle, or any other simple machine: it trades force for distance (or torque for speed). If your large gear has 52 teeth and your small gear has 13 teeth, the gear ratio is 4:1 — the small gear spins four times for every one rotation of the large. Torque is divided by four; speed is multiplied by four. Work in still equals work out. The law hasn't changed since the lever module.

The Fan Fly Governor

Without the fan fly, a 20 lb weight free-falls and reaches the ground in under half a second, destroying everything. The fan fly — flat paddles on the fastest shaft — pushes against air. Drag grows as the square of velocity: double the speed, quadruple the drag. This means the system finds equilibrium automatically: as the weight speeds up, drag increases until it exactly balances the driving force, and the weight descends at a steady, constant, useful speed. This is a self-regulating governor with no external control mechanism.

Historical Context

Weight-driven clocks are documented from 13th century Europe. Kitchen spit-jacks powered by falling weights were common from the 16th to 18th centuries. The fan fly governor predates the centrifugal governor by centuries. Pumped-storage hydropower plants — the dominant form of grid-scale energy storage right now — work on exactly this principle at industrial scale.

Field Engineering: Step by Step

⚠ Full Safety Never stand under the suspended weight. Use steel axle rods (3/8-inch min) in proper bearings. Balance fan fly before any load test. Install fan fly before releasing weight under load. Bowline knots on all load connections. Structural lumber frame. Establish and mark a clear drop zone before lifting anything.
STEP 1
Source the Gears. Salvage a bicycle front chainring (~52 teeth, large) and rear cog (~13 teeth, small) with chain. Avoid cheap plastic gears — they shatter under the torque of a 20 lb load. Count the teeth on both gears and calculate your ratio before continuing.
STEP 2
Source Steel Axles and Bearings. 3/8-inch (10mm) steel rod for both axles. Skateboard bearings (608ZZ) are cheap, standard, and fit 8mm rod. A wooden dowel in a drilled hole creates too much friction and fails under continuous load.
STEP 3
Build the Frame. Structural lumber (2×4 minimum). The frame must hold both axles parallel and resist the rotational torque of the weight pulling the spool. Through-bolted connections or mortise-and-tenon joints — not screwed butt joints.
STEP 4
Build the Spool. Short length of PVC pipe with wooden disc end-caps glued and screwed. End-caps prevent the rope from slipping off. Mount spool and large gear on Axle 1.
STEP 5
Build and Balance the Fan Fly. Cut a 20 cm × 5 cm strip of stiff cardboard or thin plywood. Drill a snug center hole, mount on Axle 2. Now: rest the axle horizontally in its bearings and release the fan fly. Whichever end drops is heavier. Add tape to the lighter end, a piece at a time, until it rests horizontal from any release angle. This is not optional.
STEP 6
Align the Gears. Position Axle 1 and Axle 2 so gears mesh or chain tensions correctly. Turn the spool by hand — the fan fly should spin freely and fast. Any binding or sticking means realignment is needed. Friction wastes stored energy.
STEP 7
Rig the Weight. Tie the rope to the spool core with a bowline. Wrap neatly in even layers. Tie the weight end with a bowline to the sandbag or bucket. Confirm end-stops are in place on the spool.
STEP 8
Test the Battery. Clear the drop zone. With fan fly installed, lift the weight to wind the rope (charging). Let go. Weight descends slowly, gears turn, fan fly spins into a steady blur. If too fast, increase fan fly size. If barely moving, reduce it. Find the steady hum.

Build It For Real

Materials

Heavy Weight
20–50 lb sandbag or bucket of sand. Confirmed weight for PE calculation.
Static Rope
Braided nylon. No paracord. No stretchy cord. Check WLL before use.
Bicycle Sprockets + Chain
Front chainring (large) + rear cog (small). Count teeth on both.
Steel Axle Rods
3/8-inch (10mm) minimum. Not wooden dowels — they deflect under load.
Skateboard Bearings (608ZZ)
Fits 8mm rod, cheap and widely available. Essential for low friction.
Structural Lumber
2×4 minimum for frame. Through-bolted or mortised joints.
PVC Pipe + Wooden Discs
For the spool — with end-caps to contain the rope.
Stiff Cardboard or Thin Plywood
For the fan fly. Must be balanced before use.

Field Data Log

MeasurementYour Value
Weight (lbs)
Weight (kg) [× 0.4536]
Drop height (m)
PE = m × 9.81 × h (Joules)
Large gear teeth
Small gear teeth
Gear ratio (Large ÷ Small)
Full-charge run time (s)
Run time after modification (s)

Complete Project Checklist

Phase 1 — Mathematics & Sourcing

  • Weight sourced, kg equivalent calculated
  • Drop height measured, meters calculated
  • PE calculated in Joules (PE = mgh)
  • Gear teeth counted on both gears, ratio calculated
  • Steel axle rods and skateboard bearings sourced

Phase 2 — Engineering the Battery

  • Structural lumber frame built with rigid joints
  • Axle 1 in bearings (spool + large gear)
  • Axle 2 in bearings (small gear + fan fly)
  • Fan fly built and balanced to hang horizontal from any angle
  • Gears aligned, system spins freely with no binding
  • Rope attached with bowline, spool end-stops confirmed

Phase 3 — Testing & Arts-as-Attention

  • Drop zone established and cleared
  • Fan fly installed before weight released under any load
  • Battery charged and released — steady governed descent achieved
  • Run time measured and recorded
  • Engineering modification attempted, new run time recorded
  • Gear ratio and PE calculation on field schematic
  • Complete schematic with all components and dimensions drawn
  • User manual written and tested by family member