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Frostbite Theater

Here's a Question! - Finger in a Cup

A cup of water is placed on a digital scale. Steve puts his finger in the water without touching the cup. What happens to the reading displayed on the scale?

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...

A cup of water is placed on a digital scale.

Steve: What happens if I put my finger in the water without touching the cup?

Does the reading stay the same?

Joanna: Does the reading decrease?

Steve: Does the reading increase?

Joanna: Or is Steve electrocuted by the scale?

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

Steve: Placing my finger in the cup displaces some of the water. This creates a buoyant force that pushes up on my finger.

Joanna: For every action, there's an equal and opposite reaction. If the water pushes up on Steve's finger, then Steve's finger pushes down on the water.

Steve: This downward force is ultimately transmitted to the scale. The scale is squished by an additional amount, which causes the reading to increase.

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

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