# Swinging into Physics

A gorgeous Spring day, perfect for taking my 7-year-old niece to the park. We politely say “Hello” to the geese and then head to the playground. Of course she runs straight to the swing set. “Help me onto the high one, Uncle Sy!”

“Why that one, Teena? Your feet won’t reach the ground and you won’t be able to kick the ground to get going.”

“The high one goes faster,”

“How do you know that?”

“I saw some kids have races and the kid on the high swing always did more back-and-forths. Sometimes it was a big kid, sometimes a little kid but they always went faster.”

“Good observing, Sweetie. OK, upsy-daisy — there you are.”

“Now give me pushes.”

“I’m not doing all the work. Tell you what, I’ll give you a start-up shove and then you pump to keep swinging.”

“But I don’t know how!”

“When you’re going forward, lean way back and put your feet up as high as you can. Then when you’re going backward, do the opposite — lean forward and bend your knees way back. Now <hnnnhh!> try it.

<creak … creak> “Hey, I’m doing it! Wheee!”

<creak> “Good job, you’re an expert now.”

“How’s it work, Uncle Sy?”

“It’s a dance between kinetic energy, potential energy and momentum.”

“I’m just a little kid, Uncle Sy, I don’t know what any of those things are.”

“Mmm… Energy is what makes things move or change. You know your toy robot? What happens when its batteries run down?”

“It stops working, silly, until Mommie puts its battery in the charger overnight and then it works again.”

“Right. Your robot needs energy to move. The charger stores energy in the battery. Stored energy is called potential which is like ‘maybe,’ because it’s not actually making something happen. When the robot gets its full-up battery back and you press its GO button, the robot can move around and that’s kinetic energy. ‘Kinetic’ is another word for ‘moving.'”

“So when I’m running around that’s kinetic energy and when I get tired and fall asleep I’m recharging my potential energy?”

“Exactly. You’re almost as smart as your Mommie.”

“An’ when I’m on the swing and it’s moving, that’s kinetic.”

“You’ve got part of it. Watch what’s happening while you swing. Are you always moving?”

<creak … creak> “Ye-e—no! Between when I swing up and when I come down, I stop for just a teeny moment at the top. And I stop again between backing up and going forward. Is that when I’m potential?”

“Sort of, except it’s not you, it’s your swinging-energy that’s all potential at the top. Away from the top you turn potential energy into kinetic energy, going faster and faster until you’re at the bottom. That’s when you go fastest because all your potential energy has become kinetic energy. As you move up from the bottom you slow down because you’re turning your kinetic energy back into potential energy.”

<creak> “Back and forth, potential to kinetic to potential, <creak> over and over. Wheee! Mommie would say I’m recycling!”

“Yes, she would.”

<creak> “Hey, Uncle Sy, how come I don’t stop at the bottom when I’m all out of potential?”

“Ah. What’s your favorite kind of word?”

M-words! I love M-words! Like ‘murmuration‘ and ‘marbles.'”

“Well, I’ve got another one for you — momentum.”

“Oh, that’s yummy — mmmo-MMMENN-tummmm. What’s it mean?”

“It’s about how things that are moving in a straight line keep moving along that line unless something else interferes. Or something that’s standing still will just stay there until something gives it momentum. When we first sat you in the swing you didn’t go anywhere, did you?”

“No, ’cause my toes don’t reach down to the ground and I can’t kick to get myself started.”

“That would have been one way to get some momentum going. When I gave you that push, that’s another way.”

“Or I could wear a jet-pack like Tony Stark. Boy, that’d give me a LOT of momentum!”

“Way too much. You’d wrap the swing ropes round the bar and you’d be stuck up there. Anyway, when you swing past the bottom, momentum is what keeps you going upward.”

“Yay, momentum!” <creak>

~~ Rich Olcott