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Acceleration Due to Gravity

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Dangerous Pennies
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Dangerous Pennies

Credit: Robert Paul Young
Source: http://www.flickr.com/photos/robertpaulyoung/73844757/
License: CC BY-NC 3.0

The Empire State Building stands 381 meters tall in Midtown Manhattan. Every year, visitors throw pennies from the top of the American landmark to see for themselves if the pennies end up embedded in the sidewalk below.

Amazing But True

  • Most people believe that when a penny is thrown from a high building, it accelerates towards the ground until impact. If this were true, the penny would reach a velocity of approximately 86.4 meters per second or 193.2 miles per hour. In reality, the falling penny's terminal velocity is only 29.0 meters per second or 65.0 miles per hour. The large difference in the velocities is due to the drag force, or air resistance, that acts upward on the penny as it falls.
  • As the penny falls, the gravitational force of the Earth pulls the penny downward, but air resistance pushes upward. As the penny's velocity increases, the air resistance increases as well. After falling approximately 43 meters, the force of gravity and the force due to air resistance on the penny become equal. Since the forces are equal but opposite in direction, the penny stops accelerating and the velocity becomes constant. This is what is known as an object's terminal velocity.
  • Watch the MythBusters test this urban myth at the following link: http://www.youtube.com/watch?v=PHxvMLoKRWg

Show What You've Learned

Using the information provided above, answer the following questions.

  1. If the size of a penny was tripled, would the terminal velocity be different?
  2. How could you devise a setup where the penny achieved a velocity greater than terminal velocity?
  3. What is the rate of acceleration of the penny as it falls?

Image Attributions

  1. [1]^ Credit: Robert Paul Young; Source: http://www.flickr.com/photos/robertpaulyoung/73844757/; License: CC BY-NC 3.0

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