DENJIRO: Do you know what this is? It’s a tool known as a “sling.” Back in the day, people would use this to throw rocks when hunting. I’m going to place the rock in this leather part here and hold the ends of the ropes. The famous sculpture David is holding a sling. Let me show you how to use it. For safety reasons, I’m using a ball instead of a rock. I’m going to swing it around and let go of one of the ropes at the right timing.
BOY: That’s so fast!
DENJIRO: Since you get a longer reach with the ropes, the end of the ropes becomes even faster. I slowly spin it so the rope is stretched out, and when the timing is right, I accelerate and throw it. The timing of the release is very important. If you let go when the ball is next to your face, it will fly straight.
Let me use this model to explain. A metal ball is spinning inside a ring, and now we’re going to remove the ring. The ball rolls in a straight line. If we look at the path it took, we see that it traveled in the tangential direction of the rotation. This demonstrates the timing of when you want to release the sling.
A device that applies the same mechanism as throwing a ball with a sling is the catapult. Rope is attached to a rotating arm to throw an object. The arm gets rotated by this weight here. The potential energy of the weight is converted into the arm’s rotational energy to throw the rock. Now, let’s try using this big catapult. Fire! Catapults were used during warfare to make an attack on castles, and they’re known as siege engines. They were mainly used in Europe and ancient China.
I hope energy will bring you all happiness. Our magic word is "Happy Energy!”