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How to calculate distance required for observable difference

  1. May 26, 2015 #1
    Me and my grandfather got into a "debate" (more like one-sided argument) on whether objects fall at the same rate on earth independent of air resistance or not. He claims that "If you drop 2 objects from an airplane -- like a car and a marble, both objects would hit the ground at the same time.", I say no, because the car has a larger surface area and the marble would hit first due to less air resistance (Neither of us really know anything about physics [that's why I'm here]).

    I've sent him tons of links on the subject but he's still determined he's right because "that's the way he was taught in school". There's not many high places here for me to drop 2 objects off of to see the results, so my question is:

    How high must we be to be able to observe a noticeable difference in when the two objects hit the ground? How do I calculate the approximate difference of objects hitting the ground? For simplicity let's say the 2 objects I am dropping are: a 2kg 1m square, and a 2kg 1m diameter sphere.
     
  2. jcsd
  3. May 26, 2015 #2

    Drakkith

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    As a kid I had a toy paratrooper with a plastic parachute that I would fling into the air and wait for him to parachute down. I was able to tell the difference pretty easily between my paratrooper and a rock. :wink:

    So you don't really need to even get into the math. Just have your grandfather go throw a leaf and a rock in the air and see which one hits the ground first.
     
  4. May 26, 2015 #3
    Try to explain to your father what is the "school science".:wink:
     
  5. May 26, 2015 #4
    Something similar was literally one of the first things I tried, I suggested dropping a rock and a feather but he claims a feather has no mass. o0) Regardless, this is something I find interesting and would like to know. I've found calculators that could calculate it for me but I don't understand how it arrives at the conclusion.

    That's it. That's how I'm gonna prove it to him, I used to do the same thing but that was so long ago I forgot!!
     
  6. May 26, 2015 #5

    Drakkith

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    Ask him if a paratrooper also has no mass!

    Not sure how much I can help you, as I don't fully understand all the details myself. Give this wiki article a read and see if it helps: http://en.wikipedia.org/wiki/Terminal_velocity
    The 'Physics' section contains the math needed to find the terminal velocity of the object based on its mass, drag coefficient (highly dependent on shape), and projected area (the part of the object facing into the wind). Note that for a real, non-simple object (i.e. not a simple cube, sphere, cone, or other basic shape) finding the terminal velocity is much more complicated, since its drag coefficient isn't easily found. Still, you should be able to get to some reasonable approximation with it.
     
  7. May 26, 2015 #6

    CWatters

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    Ask your grandfather why parachutists bother to pull their rip cords if air resistance doesn't slow them down.
     
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