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Frostbite Theater

Here's a Question! - Coins on a Hoop

Twenty-five pennies are precariously stacked on a plastic hoop. The hoop is carefully balanced on a glass bottle. What will happen if the hoop is suddenly pulled to one side?

Announcer: Frostbite Theater presents... Cold Cuts! No baloney!

Joanna and Steve: Just science!

Joanna: Hi! I'm Joanna!

Steve: And I'm Steve!

Joanna: Here's a question for you...

Steve: A plastic hoop is placed on a glass bottle.

Joanna: 25 pennies are carefully stacked on the hoop.

Steve: The hoop is removed by pulling it quickly, horizontally, to the right.

Joanna: What happens to the pennies?

Steve: Do they stay suspended in mid-air?

Joanna: Do they stay on the hoop?

Steve: Do they fall down and to the right, mostly missing the bottle?

Joanna: Or, do they fall nearly straight down, falling neatly into the bottle?

Pause the video now if you'd like to think it over before we show you what happens.

Joanna: It isn't windy in here, so there's just two forces that we need to consider. The first is gravity, which is pulling straight down on the pennies. If the hoop were just to - poof - vanish from existence, the pennies would fall straight down into the bottle.

Steve: The second force is friction, which doesn't show up unless we're moving the hoop. If we move the hoop slowly, static friction will keep the pennies on the hoop. If we move the hoop quickly, static friction is broken but sliding friction will still act on the pennies. Both forms of friction will act horizontally if we move the hoop horizontally.

Joanna: The trick is to remove the hoop quickly so that the frictional forces don't accelerate the pennies horizontally all that much. The pennies will fall nearly straight down and, hopefully, into the bottle.

Thanks for watching. I hope you'll join us again soon for another question!

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