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Slowly Dropping Ball

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What kind of experiment is this?

Experimental procedure and explanation:

  • Drop a ball into a measuring cylinder. When the gap between the measuring cylinder and the ball is narrow, the ball drops slowly.
  • When the air under the ball flows through the gap between the ball and the cylinder, viscous friction works to slow the flow. As a result, it takes time for the air to escape from the measuring cylinder, and the ball drops slowly. To put it another way, since it is difficult for the air to pass through the gap, the pressure on the upstream side (in other words, below the ball) increases, and this pressure slows the dropping of the ball.
  • When a cup is dropped and the gap is narrow, the ball drops slowly in the same manner.
  • This same principle is used with door closers (devices that slow the closing of doors).
[Keywords] viscous friction
[Reference] “The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P19
Last update: 1.21.2015