estimation of the terminal velocity of an Iphone 5s

Discussion in 'Physics & Math' started by Repo Man, Sep 1, 2014.

  1. Repo Man Valued Senior Member

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    After viewing this video on YouTube ( https://www.youtube.com/watch?v=82SDk1kInvI ) I noticed a commenter who said he was surprised that anyone was allowed to bring an iPhone, and it could cause a lethal injury to a passerby if dropped from that height. I replied that in my estimation it could cause a head injury, but would be very unlikely to be fatal. The commenter had some math skills, and replied "A 112 gram iPhone 5s falling 650 meters (height of Shanghai tower) would be travelling 113 meters per second and generate a force of approximately 713 joules... which is about 526 ft/lbs. The shape and density of that phone, along with the steel casing would most definitely kill someone."

    Terminal velocity is based on a drag to weight ratio. An iPhone has very little drag, but it also has very little weight. It seems quite possible to me that turbulent air could cause the phone to tumble (while it has a small area, and is pretty aerodynamically clean, it also doesn't weigh much), and that tumbling could significantly increase the drag, and lower the speed. Is the original commenter too confident in his numbers, or do you think that he's correct?
     
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  3. TBodillia Registered Senior Member

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    113 m/s IF it were falling in a vacuum. That's the same urban legend as a penny falling from a skyscraper will kill you.

    Terminal Velocity calculator

    Dropping the phone screen up, looking at dimensions of 123.8mm x 58.6 mm x 7.6mm, mass 112 g...the real unknown is drag coefficient. A rectangular box has a coefficient of 2.1. The calculator shows terminal velocity of 0.84 m/s. A streamlined body has a coefficient of 0.04 and the terminal velocity is 6.10 m/s.

    Drag Coefficients

    But, better yet is a story from 2011 when a, US Airman dropped his Iphone 4s from 1000ft/304.8m and it landed safely in the forest.
    iPhone 4 falls from plane, lives to teach us physics

    You also have the GoPro camera dropped from a plane in February of this year.
    You’re Going To Want To Watch This: Camera Falls From Airplane, Lands In Pig Pen, Keeps Recording

    And there is one more thing: Just because you dropped it, it doesn't mean it will hit the ground. In 1979, Elvita Adams jumped from the 86th floor of the Empire State Building. The wind blew her back onto the 85th floor. She broke a hip.
     
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  5. Repo Man Valued Senior Member

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    Thanks for the reply.
     
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