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Types of Friction

Introduction to static, sliding, rolling, and fluid friction.

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Speeding Up by Falling Down

Speeding Up by Falling Down

Credit: David Shankbone
Source: http://www.flickr.com/photos/27865228@N06/8463943114
License: CC BY-NC 3.0

Look up—way, way up. This photo shows New York City’s iconic Empire State Building. It rises majestically for a total of 102 stories above street level.

The Back Story

  • According to an urban myth, a penny dropped from a skyscraper like the Empire State Building will turn into a lethal weapon by the time it reaches the ground.
  • Credit: Kevin Dooley
    Source: http://www.flickr.com/photos/pagedooley/4085876710/
    License: CC BY-NC 3.0

    A typical penny weighs 2.5 grams [Figure2]

  • The reason? Acceleration due to gravity will increase the speed of the penny so much that it will be speeding like a bullet by ground level.
  • Is this just a myth, or is it true? That’s what Discovery Channel’s Mythbusters wanted to know.
  • The Mythbusters accelerated a penny to the maximum speed it would have if it had dropped from the Empire State Building to the ground. Watch this video to see what happens to concrete, skulls, and skin when struck by the speeding penny:


Can You Apply It?

At the link below, learn more about how things speed up by falling down. Then answer the questions that follow.

  1. What is terminal velocity? What is the terminal velocity of a penny that falls off a skyscraper?
  2. Identify and explain the two forces that act on a falling object and determine its terminal velocity. When does terminal velocity occur?
  3. How does a penny’s shape and weight affect its terminal velocity?
  4. Review velocity-time graphs at the following link:  http://www.saburchill.com/physics/chapters/0002.html. Then sketch a graph to show how the velocity of a penny changes as it falls from a skyscraper to the ground.
  5. Describe what happens when a penny at terminal velocity strikes concrete, a skull, and skin.

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Image Attributions

  1. [1]^ Credit: David Shankbone; Source: http://www.flickr.com/photos/27865228@N06/8463943114; License: CC BY-NC 3.0
  2. [2]^ Credit: Kevin Dooley; Source: http://www.flickr.com/photos/pagedooley/4085876710/; License: CC BY-NC 3.0

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