英语时差：跳伞的科学依据

Why would you want to jump out of a perfectly good airplane? Well, you get to the ground much faster, and the trip down is a whole lot more interesting. Skydiving has become an increasingly popular sport, although most of us haven’t yet taken the plunge and only enjoy it from the sidelines. If you’ve seen video of skydivers in action, especially skydiving teams that link up to create formations, you might wonder how they do it. It’s a matter of elementary physics. Any falling object experiences basically two forces: The downward tug of gravity, and the upward push of air resistance. When a skydiver first leaps out of a plane, she begins accelerating rapidly downward, tugged down by gravity. If you or I fell out of a tree, we would experience the same thing–this is the usual experience of falling. After about ten seconds though, she stops accelerating downward. She’s still falling of course, usually at around a hundred and fifteen to a hundred and thirty miles per hour, but her downward speed is no longer increasing.