Riddles in the Woods: The Trickster Fairy of Newton’s Third Law

One day, lost in the woods, Kip came across a fairy with a mischievous glint in her eye. She looked like the type of person who would know her way out of the woods. He had been lost for many weeks, and he hoped this fairy would tell him the way out of the gloomy forest. Not in the least because there was something with glowing red eyes following him. But the fairy refused, unless Kip could correctly answer her questions.

She said she had been reading the news out of the capital and had become particularly interested in the idea of Newton’s Third Law and force pairs.

“I will give you three pairs of forces,” she said. “You must correctly tell me whether these are third-law pairs or not.”

Kip hid a smile. She didn’t know that he was the one who had discovered the law in the first place. But still, figuring out whether forces were pairs was hard. For example, he knew that if he pushed on a tree, the tree would push back on him. Those were pairs. But he also knew that, when he was standing on the ground, gravity was pulling him down, and the ground was holding him up. Those were forces that balanced each other, but they weren’t third-law pairs, because they acted on the same object—him! Third-law pairs were always “a acts on b” and “b acts on a”. Two different objects, one pushing on the other and the other pushing back.

“If you fail,” the fairy said, her expression suddenly serious, “you must agree to stay in the woods for ten years and ten days.”

Kip swallowed hard. Ten years and ten days was a lot of time. He would surely be able to find his way out of the woods on his own at that rate. But the thing with the eyes was closing in. He knew he could do it. He’d discovered the law, after all.

“I agree,” Kip said.

Question One: She grinned, showing her tiny, sharp teeth. “Very well. I stomp my foot on the ground. The ground pushes back on my foot.”

Answer to Question One: Kip’s heart rate slowed a little. He knew this one. “Yes, they’re third-law pairs,” he said.

Question Two: She frowned, and her green eyes narrowed. “I tie a rope to a tree. I pull on the rope. The rope pulls on the tree.”

Answer to Question Two: Kip licked his lips nervously, but he knew this one, too. This one was tricky, because there were forces acting on different objects, just like you’d usually see. But they weren’t two objects: one pushing and the second pushing back. A third-law pair would be if she pulled on the rope and the rope pulled back on her. Or, if the rope pulled on the tree and the tree pulled on the rope. “They’re not third-law pairs,” Kip said around the lump in his throat.

“Correct,” she said, disappointed.

Question Three: The fairy thought long and hard before at last asking her third and final question. “The earth pulls on the moon, and the moon pulls on the earth.”

Answer to Question Three: Kip grinned widely, and a rush of warmth went through him. He knew this. Newton talked about gravity, and about the dance between the earth and the moon, constantly. “Yes, those are third law pairs,” he said.

The fairy frowned, her wings drooping. “Yes, that’s correct.”

She flung herself onto the ground, pulling her knees into her chest and burying her head in her arms.

“Um, are you all right?” Kip asked.

She looked up, and her eyes were filled with tears. “I was hoping you’d have to stay. My home tree blew over in the last windstorm and I need someone to help me lift it up again.”

“Oh, well, I’ll help you with that,” Kip said.

The fairy zipped up, her wings sending off a delighted shower of sparks. “Thank you!” she said. “Craig’s been trying to help, but he’s too gelatinous.”

Kip got an uncertain feeling in his stomach. “Craig?”

She pointed behind him, and he spun around to see what looked like a large, wet pile of mud with glowing red eyes.

“Er, hello. Craig?” Kip said. The gelatinous pile waved something arm-shaped at him.

Kip, the trickster fairy, and Craig together tied a rope around the fairy’s home tree and managed to pull it upright. That evening they spent some time singing and telling stories. Craig had a surprisingly nice singing voice. The next day, the fairy showed Kip the way out of the forest, and he continued his adventures.