Jimmy Says It Rolls Uphill…

And He Almost Convinces Us

Why it’s not what you see, but what you measure

“Sir,” Jimmy said, “I’ve found something that shouldn’t be possible.”

He placed a wooden contraption on the desk, attracting the attention of the class.

“It rolls uphill,” he stated excitedly.

The object looked harmless enough. It was shaped like a spinning top stretched at both ends; wider in the middle, narrowing toward each side.

Beside it sat a pair of rails, slightly sloped and set further apart at one end than the other.

“I saw this done online,” Jimmy said.

He placed the object at the lower end of the rails.

True to his word, the object rolled up the sloping plane.

Students cheered in amazement and disbelief.

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Strong reactions from the class.

“No way!” “That’s impossible!” “Do it again!”

Jimmy did.

Up it went again, smooth and steady, as if gravity had quietly stepped aside.

Jimmy took a bow.

“Well, sir?” he said.

“Let’s have a closer look,” I suggested.

I picked up the object and showed it to the class, turning it slowly in my hands.

“What do you notice?” I asked.

“It goes uphill,” Jimmy said immediately.

“That’s not what I mean, Jimmy.”

“It’s wider in the middle,” he said.

“Good,” I replied, placing the object back on the table.

“Now, put it back on the rack. But this time, don’t watch the ends.”

“What else do I watch?” Jimmy asked in a puzzled voice.

“The part that matters.”

The class eagerly looked on.

The object rolled again. Up… or so it seemed.

“Look at the middle as the object moves,” I prompted.

Then someone spoke.

“It’s lower.”

Jimmy turned.

“No, it’s not.”

We repeated the experiment, but this time I held a ruler beside it as it moved.

Very slightly — but clearly — the centre was dropping.

“The ends go up,” I said. “But the centre goes down.”

Jimmy stared at it.

“That doesn’t make sense.”

“It does if you’re watching the right thing.”

I turned to the board and drew a quick sketch.

“As the rails spread apart,” I explained, “the middle of the object sinks lower between them — even while the ends rise.”

I tapped the centre of the diagram.

“This is what matters.”

“It’s not really going uphill, then?” Jimmy asked.

“Not in any way that physics cares about,” I said. “The centre of mass is moving down. Gravity is doing exactly what it should.”

Jimmy leaned back, still studying it.

“It still looks wrong.”

“Of course it does,” I said. “You trusted your eyes. Think about a tightrope walker.”

“Why?” Jimmy asked, thinking this to be an irrelevant comment. “Are you taking us to the circus?”

Laughter.

“Why do they carry a long pole,” I finally managed to ask.

“To look impressive?” Jimmy concluded.

“To control their centre of mass,” I replied.

I held my arms out like a balancing pole.

“They keep their centre directly above the rope. That’s what keeps them steady.”

“Then it’s the same idea here?”

“Exactly,” I said. “Whether something balances, falls or appears to go uphill, depends on where the centre is.”

“Yes. Back in a minute,” I promised, dashing out of the room.

I ransacked the cupboard in the adjacent science room, returning with a cup, two forks and a bamboo skewer.

As the class watched, I intertwined the forks and placed a bamboo skewer between them. The contrivance was then carefully placed on the cup and was held in equilibrium by the skewer.

It leaned precariously, but didn’t tip.

Everyone stared.

“That should fall over,” Jimmy declared.

“Should it?” I asked.

I pointed to the setup.

“All the weight is arranged so that the centre of mass is directly above the point where it’s supported.”

“Even though it looks like it’s hanging off?”

“Exactly. It only looks wrong if you’re watching the wrong part,” I finished.

Jimmy looked back at the ramp, smiling.

“Sir,” he said, “if I get the wrong answer in a test…”

I waited for the other shoe to drop.

“…maybe I was just watching the wrong part?”

Sometimes, the mistake isn’t in the mathematics… it’s in deciding what to measure in the first place.

If you enjoyed this, you might like Jimmy’s earlier attempt to prove that falling objects never stop speeding up.