What is stored energy?

Energy can be saved up in a spring and let out later. When you pull the car backwards, a spring inside the pull-back motor winds up tight. It is storing your energy! Let go and the spring unwinds, turning the wheels and zooming the car forward.

Try Pull-back Racer for free

Complete MightyCog booklet collection: 82 PDF booklets for $12.99. Instructions only; individual booklets remain free.

Explore the printable pack

Try it yourself

Pull back a short way, then a long way. Which goes further?

Build it: 13 builds that teach stored energy

  • Pull-back Racer🏁 Pull-back Racer: Pull it back, let it go — ZOOM! A racer with a spring for an engine. (level 2 of 5, about 12 minutes, 10 steps)
  • 🛺 Pull-back Trike: A three-wheeler with a spring engine — twist the front wheel and it drives in circles! (level 2 of 5, about 20 minutes, 9 steps)
  • 🔥 Big-Wheel Dragster: Giant back wheels, tiny front wheels — the same spring pushes this car much further! (level 2 of 5, about 22 minutes, 10 steps)
  • 💨 Gear-Up Speeder: The spring engine sits up high and turns a BIG gear, which spins the wheels faster! (level 2 of 5, about 25 minutes, 11 steps)
  • 🐢 Pull-back Tortoise: Slow but super-strong: a spring car that can climb up a book! (level 2 of 5, about 30 minutes, 12 steps)
  • 🌀 Flywheel Friction Car: Vroom it on the floor, let go — and it keeps on rolling! (level 2 of 5, about 25 minutes, 5 steps)
  • 🎯 Spring Catapult: Cock the arm, click the trigger, FIRE! A spring flings the ammo across the room. (level 2 of 5, about 25 minutes, 14 steps)
  • 🎚️ Two-Speed Pull-back: Choose your gear before the race: CLIMB or SPRINT! Slide the back axle to swap gears. (level 3 of 5, about 35 minutes, 13 steps)
  • 🚀 Launch-Lock Racer: Wind it up, and a little catch holds the spring tight until YOU press the launch button! (level 3 of 5, about 40 minutes, 21 steps)
  • 🐾 Twin-Spring 4×4: Two spring engines — one for the front wheels, one for the back. All four wheels push! (level 3 of 5, about 35 minutes, 16 steps)
  • 🌀 Pull-back Diff Racer: A spring racer with a real differential, so it zooms round curves without skidding. (level 3 of 5, about 40 minutes, 23 steps)
  • 🏹 Counterweight Trebuchet: The heavy battery box drops, the long arm whips up — and your bush flies across the room! (level 3 of 5, about 40 minutes, 17 steps)
  • 🕰️ Spring Escapement Clock: A ticking pin-wheel clock powered by the pull-back motor's spring. Tick… tock… tick… (level 4 of 5, about 50 minutes, 22 steps)

In the classroom

Common engineering questions

How can I try stored energy at home?

Pull back a short way, then a long way. Which goes further?

Which builds teach stored energy?

13 MightyCog builds teach it: Pull-back Racer, Pull-back Trike, Big-Wheel Dragster, Gear-Up Speeder and Pull-back Tortoise and more. Start with Pull-back Racer: 10 steps, about 12 minutes.

Ideas that go with it