Spinning Cup

[FunInTheSunParticipant 8286]

I don’t understand how this works, but it looks fun. Maybe someone who understands physics can explain it.

—Fun

"I saw that there comes a point, in the defeat of any man of virtue, when his own consent is needed for evil to win-and that no manner of injury done to him by others can succeed if he chooses to withhold his consent. I saw that I could put an end to your outrages by pronouncing a single word in my mind. I pronounced it. The word was ‘No.’" (Atlas Shrugged)

[sidecarParticipant 35862]

There’s a bunch of different things happening here.

Have you ever noticed how it can take a lot more force to get something sliding than to keep it sliding once it’s going? That’s because there’s a lot less friction between sliding things than stationary things.

They spin the top cup to lower that coefficient of friction between it and the lower cup so that it’s easier to slip out of the lower cup. At the same time that spinning ads in a precessional movement which lifts it out of the lower cup because they are both slightly conical. The small amount of bumping and rattling also helps to keep the cups separated.

Blowing across the cups does then does a few things. The moving air across the top of the upper cup has a lower pressure than stationary air. Check out Bernoulli’s principle for a better explanation than I want to give here. That’s part of what lifts up the upper cup. At the same time some of that blown air is traveling down the gap between the cups caused by the spinning, increasing the pressure between the two cups like in a pneumatic piston causing the upper cup to rise. Since the cups are slightly conical, the upper cup rising even only a small amount increases the gap between the cups, causing more blown air to flow in between them, causing the cup to rise even faster and so on until the upper cup launches out of the lower. That’s most of what’s lifting the upper cup. And the cup is stabilized in flight by the spin it already has.

From there catching it with the other cup is basically just a matter of aim, skill, and luck.

[Anonymous 43]

great, another stupid challenge for my kids to do.

my favorite was the pencil eraser challenge, rubbing a pencil eraser on the back of the hand until you bleed. kids still have scars from that stupidity.

[FunInTheSunParticipant 8286]

Thank you, sidecar! Your explanation made me think about pistons in engines. All that air pressure plus an explosive spark gets that car going!

"I saw that there comes a point, in the defeat of any man of virtue, when his own consent is needed for evil to win-and that no manner of injury done to him by others can succeed if he chooses to withhold his consent. I saw that I could put an end to your outrages by pronouncing a single word in my mind. I pronounced it. The word was ‘No.’" (Atlas Shrugged)

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