A Cup that Rolls toward You

Let's take a look!

What kind of experiment is this?

Experimental procedure and explanation:

• Materials required for the experiment: Two paper cups (or styrene foam cups) and a roll of scotch tape.
• Align the bottoms of the cups and tape them together using scotch tape.
• On a table, place the cups on their sides. Using a straw, blow air on the front at an angle. The cup will roll away from you.
• Question: Try rolling the cup toward you, while blowing air at the front at an angle.
• Answer: Blow air at the middle section of the front, toward the bottom. You can picture it as if you are blowing air through the bottom gap.
• As you pull the straw toward you in a parallel motion, the cup will roll toward you.
• The direction of the flow changes along the face of the cup (Coanda effect), and as a reaction to the force acting on the flow, there is a force acting on the cup that is toward you and downward.

[Keywords]	Coanda effect, momentum theory
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Koudansha Blue Backs, pp. 120-127

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