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Thread: Skydiving and gravity

  1. #1 Skydiving and gravity 
    Forum Freshman
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    Hello! I am a skydiver with some questions.

    From what i've read about gravity, an object with twice the mass should fall just as fast as an object with half the mass. From newton's second law of motion, gravity is a constant force. An object with twice the weight has twice the amount of gravity pulling it down. But it also has twice the mass, and these two effects should cancel each other out, causing the acceleration to be the same as the object with half the mass.

    I have a hard time understanding why the fat bastards i skydive with fall faster than i do. I am 5' 10", 135 lb's. Some of the people i skydive with are maybe 6'3" and 180 lb's.

    Now, we should fall at the same rate in terms of gravitational pull. Now, after taking into account air resistance, shouldn't i fall faster? Since i am much smaller and have less air resistance holding me up, shouldn't i have less "drag"?

    The only things i can think of are how we arch. We arch to fall stable, by pushing our pelvises forward and bending our legs. All i can think of is that the other heavier guys are more "rounded" and can more easily "arch" and spill much more air around them than i can.

    Thoughts?


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  3. #2  
    Forum Professor
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    You will probably have less air resistance, but their larger mass means that the air resistance will affect them less. Try working the acceleration equations again, but reduce the force of gravity by 10 newtons for you and for a guy who weighs 50 kg more than you. You should work out that they fall faster.


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  4. #3  
    Forum Masters Degree Numsgil's Avatar
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    Air resistance is a function of surface area and velocity. A guy that weighs 100 extra pounds from you is not going to have that much more surface area. He may experience 25% more air resistance, or something like that. But that's a force, so it gets divided by mass. His fat ass has way more mass, so the air resistance won't affect him as much as you. He'll have to reach a higher velocity before the air resistance can balance gravity and he'll stop accelerating.
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  5. #4  
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    Good explanations, thank you.
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