This square shape can roll, but why?

Originally published at: https://boingboing.net/2020/09/01/this-square-shape-can-roll-bu.html

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He kinda botched that explanation. The rounded shape part of the explanation is correct. And he’s correct for friction being useful for rolling, but it’s actually not necessary for the explanation. The rebound explanation is off. It’s true that you want to avoid rebound because it wastes energy, but that’s all.

Really it’s a matter of the kinetic energy being greater than the next potential energy peak. A ball on a level surface, that potential energy peak is zero, so any kinetic energy will keep it rolling. For a rippled surface plus ball or an oval cross section on a level surface, the potential energy is going up and down. As long as there is enough kinetic energy to roll over that next “hump”, it keeps rolling.

The Lego rolling was misleading because the center of gravity was over the leading edge. i.e. it’s falling over. The potential energy is lower if it rolls even if it starts with zero kinetic energy.

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So, as long as it moves forward fast enough, it will keep rolling for a while, right?

I admit I couldn’t have explained it in technical terms as eloquently as you did, but I felt something was missing.

So, I guess its much more a question of what the cross-section of the object looks like. This seems to be rather rounded, so more of an elliptical shape than rectangular.
Looks like you’re not rolling a hard-edged block like the LEGO brick, but rather a rugby ball end-over-end. And all of a sudden it sounds a lot more feasable than roling a square shape…

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Early in the explanation he says “I seen” and it immediately made me less interested.

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Doesn’t “rolling” mean the axis stays put? This is flipping.

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Plus, its kind of deceptive to talk about “square” or “hexagon” shapes. All that matters is the elliptical cross section. It could be a “circle” (ie, a “coin”)— if it had that elliptical cross section.

It’s “rolling” because the shape’s surface is in continuous contact with the ground and not “slipping.”

The human body is similarly elliptical on the side. I was trying to think where I have seen this motion before and it was a human rolling sideways (down a hill since the soft tissue make for higher rolling resistance than metal).

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