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A Cup that Rolls toward You

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Watch the video on YouTube.

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

Last Update:9.7.2013