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Acceleration Due to Gravity

Understand and state acceleration due to gravity and the value of g

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Dangerous Pennies

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 in Midtown Manhattan. Every year, dozens of individuals visit this American landmark and throw pennies from the highest possible vantage point. What these individuals are doing is attempting to test an urban myth: That a penny tossed from the top of the Empire State building will create a crater in the sidewalk below.

Amazing But True

  • What most people believe is that once the penny is thrown, it will accelerate 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 mph. In reality though, the penny only reaches a maximum velocity of 29.0 meters per second or 65.0 mph. The reason for such a large difference in the velocities is due to the balance of the forces on the penny as it falls.
  • As the penny falls, the force of gravity pulls the penny downward while air resistance pushes upward. The greater the velocity of the penny the greater the air resistance becomes. After approximately 43 meters from being dropped, the force of gravity and the force due to air resistance balance out. 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 objects terminal velocity.
  • Watch the Mythbusters test this urban myth at the link below: 


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 that the penny falls at?

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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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