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Momentum and Impulse

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It Is All A Matter Of Impulse

It Is All A Matter Of Impulse

Credit: Charlie Cowins
Source: http://en.wikipedia.org/wiki/File:Peng_Shuai_-_Flickr_-_chascow.jpg
License: CC BY-NC 3.0

As a tennis ball is struck by a racket, an extremely large force is applied to the ball over a very small amount of time. Assuming the average impact time is on the order of 1 millisecond, this would mean that the force exerted on a tennis ball during a 140 mph serve is approximately 140,000 N!

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  • The significant force on the ball can be explained by the concept of impulse. The impulse is an interaction between a system and its environment, where during this interaction the momentum of the system changes. How much the momentum changes by is dependent upon how long the system is acted on and by how much force is applied on the system. Therefore, the change in momentum p_{(\text{final})} - p_{(\text{initial})} or the impulse I, equals the force F applied times how long it is applied, t.

\Delta p = p_{\text{final}} - p_{\text{initial}} = I=F \Delta t

  • Credit: Mike Babcock
    Source: http://www.flickr.com/photos/13613374@N00/3098836311
    License: CC BY-NC 3.0

    Air bags help decrease harsh impact force on the human body [Figure2]


  • It is because our understanding of impulse that thousands of lives are saved every year in automobile accidents. During an accident, a passenger in a car would experience the same change in momentum whether the air bags are installed or not. The benefit of the air bags is that they allow the passenger to go through the same change in momentum but over a much longer time when compared to an accident where no air bags deploy. The increased time provided by the air bags allows for a much smaller force on the human body, preventing sometimes fatal injuries.

Show What You Know

Using the information provided above, answer the following questions.

  1. Consider two object of equal mass and shape, object 1 and object 2. If both objects experience the same change in momentum but object 1 experience the change in half the time, which object would have a greater force applied to it?
  2. Why is there no mention of external forces in the description of impulse, i.e. gravitational, electrical, etc?
  3. Name other possible methods that could be used to increase the amount of time a person in a car accident could be brought to a stop.

Image Attributions

  1. [1]^ Credit: Charlie Cowins; Source: http://en.wikipedia.org/wiki/File:Peng_Shuai_-_Flickr_-_chascow.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: Mike Babcock; Source: http://www.flickr.com/photos/13613374@N00/3098836311; License: CC BY-NC 3.0


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