Not Your Problem

The ball has been rolling around in Nora's head for three evenings. A steel ball, a wooden ramp, half a meter high, a photogate at the bottom. That's what it says on the first-round problem set of the physics competition she signed up for, voluntarily, because her teacher said, “This is right up your alley, Nora.” Calculate the speed at the bottom of the ramp. Explain the difference from the measurement. She can do the calculation in two minutes: a little over three meters per second. Except when she rebuilt the setup in the physics lab, the photogate said two and a half. Three times in a row.

At home, David has been singing in the shower for twenty minutes, the same chorus over and over. So Nora grabs her papers and crosses the street to the Kellers' garage, which smells like chain oil, where Alex is kneeling on the floor truing the spokes on a back wheel. It's her bike. He's been fixing it for a week, because she asked him to. She sits on the workbench, one leg tucked under her, and does the problem for the fourth time.

“Friction,” she says after a while, mostly to herself. “It has to be friction. But then it'd be a ridiculous amount of friction, and steel on varnished wood isn't…” She breaks off and pushes her bangs aside with the back of her hand. Alex looks up. “Want me to take a look?”

“No.” It comes out fast. Too fast. He sets down the spoke wrench. “Maybe you just messed up somewhere. I do it all the time, a plus sign where there should be a minus…” “I don't mess up.” Nora hears how her voice sounds and can't stop anyway. “I've done it four times, Alex. In my head and on paper. It's not a sign error. You don't have to help me. It's not your problem.”

She doesn't totally know why it stings so much. Maybe because physics is her thing. She's good at math, but in physics she's the one explaining to people at lunch why a moving bike doesn't fall over. She's even explained to Alex why his smallest gear is useless on a hill, and he listened without needing to know better. And now she's stuck, of all places right here, and he's kneeling down there asking if she messed up. The way you'd ask a little kid if they washed their hands.

Alex stands up, wipes his fingers on a rag, and leans against the workbench next to her, not too close. “I don't think you messed up,” he says. “That was a dumb question. We never did rolling balls. If you handed me that sheet, I probably wouldn't even understand the question.” He pauses, the way he always does right before the part that matters. “I wasn't going to do the math. I figured you could explain it to me. It helps me when I have to explain something to somebody who has no clue. And I really have no clue.”

Nora looks at him. Her left eyebrow goes up all on its own. “You want me to explain a problem I can't solve.” “You could do that,” Alex says. Coming from him, that usually means something is a bad idea. This time the corner of his mouth twitches.

So she explains. She sketches the ramp in pencil on the back of an old repair receipt. “At the top, the ball has potential energy. The higher it is, the more it has. At the bottom the height is gone, so the energy is all in the motion now. Everything that was up there turns into speed down here, minus a little friction.” Alex doesn't nod. He just listens. “And friction isn't enough?” “Not even close. More than a third of the energy is missing. Where does that much go? The ball just slides down the ramp and…”

She stops. The pencil stays where it is. “Wait. Again.” Alex doesn't say anything. He doesn't fill the silence; he never has. “It doesn't slide,” Nora says slowly. “It rolls. It doesn't just go down, it spins on the way. And spinning takes energy too. Part of what was up there isn't in the speed at the bottom at all. It's in the spin.” Her lips move; she's calculating. “For a solid ball, two sevenths. That leaves five sevenths for the speed, square root of that… two point six. Minus a little friction.”

Then she laughs, loud, and at herself first. “Three evenings. Over something that spins.” Alex looks at her back wheel in the stand and gives it a push. The spokes flicker, the hub ticks softly. “Like that?” he asks. “Exactly like that,” Nora says. “For the record, I couldn't have told you that,” he says. “I know,” she says. “You just kept asking what comes next. That was enough.”

She writes the whole solution out again, neatly, this time with the spin, and Alex kneels back down next to the wheel. For a while there's nothing but the scratch of the pencil and the small click whenever he tightens a spoke. It's the kind of quiet where two people are working together without doing the same thing.

When she gathers up her papers at ten, she stops at the garage door. “What I said before wasn't fair,” she says. “The ‘not your problem’ thing.” Alex doesn't look up, but he smiles on the one side he always smiles on. “Figuring out the friction tomorrow?” “Yeah.” “Need somebody with no clue for that?” “Maybe,” Nora says. And then, because she always says thank you out loud and right away, and almost forgot this time: “Thanks.”