DENJIRO: This is a model of an Ancient Roman weapon called a "ballista." It was used to shoot arrows or large rocks far away. You spin these gears with the handles, which will pull the drawstring. There's a stopper here so the gear can rotate in only one direction. It's called a "ratchet." Let's place an arrow on this weapon and shoot it! 3, 2, 1, fire!
The arrow flew quite fast! The key is the ropes that are inside the weapon. They work as a torsion spring to create the force needed to shoot an object. Let me explain what a torsion spring is. Ropes do not shrink and stretch like a spring, but they do have a little elasticity. When we twist rope, it gets a little shorter. However, when we twist it while holding the bottom part in place, it cannot get any shorter. That means that the rope is extending to make up for it. The torsion spring takes the elastic force that is generated when the extended rope returns to its normal state, and converts it into a great force.
Look at this device I've created here. I'm going to put this baseball bat in the rope and twist it. Now let me release the baseball bat.
When we pull the arms of the ballista, it twists the ropes. By letting go, the rope tries to go back to its original state and causes the arm to rotate with it. The ballista fires objects by using this force, where the rope tries to unwind itself.
Here, we have an even larger ballista. Here we go! 3, 2, 1, fire!
I hope energy will bring you all happiness. Our magic word is "Happy Energy!"