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  3. Everyone knows a stone bounces best on water if it’s round and flat, and spun towards the water as fast as possible. Some enthus

Everyone knows a stone bounces best on water if it’s round and flat, and spun towards the water as fast as possible. Some enthus

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问题     Everyone knows a stone bounces best on water if it’s round and flat, and spun towards the water as fast as possible. Some enthusiasts even travel to international stone-skimming competitions, like world champion Jerdone Coleman-McGhee, who made a stone bounce 38 times on Blanco River, Texas, in 1992.
    Intuitively, a flat stone works best because a relatively large part of its surface strikes the water, so there’s more bounce. Inspired by his eight-year-old son, physicist Lyderic Bocquet wanted to find out more. He tinkered with some simple equations describing a stone bouncing on water in terms of its radius, speed and spin, and taking account of gravity and the water’s drag.
The equations showed that the faster a spinning stone is travelling, the more times it will bounce. To bounce at least once without sinking, Bocquet found the stone needs to be travelling at a minimum speed of about 1 kilometre per hour.
    The equations also backed his hunch (直觉) that spin is important because it keeps the stone fairly flat from one bounce to the next. The spin has a gyroscopic (陀螺的) effect, preventing the stone from tipping and falling sideways into the water.
    To match the world record of 38 bounces using a 10-centimetre-wide stone, Bocquet predicts it would have to be travelling at about 40 kilometres per hour and spinning at 14 revolutions a second. He adds that drilling lots of small pits in the stone would probably help, by reducing water drag in the same way that dim pies on a golf ball reduce air drag.
According to Lyderic Bocquet, which of the following is NOT mentioned as a factor influencing the bounces of a stone on water?
选项 A、The clearness of the water.
B、The gravity of the Earth.
C、The shape of the stone.
D、The speed and spin.
答案A
解析 文中提到了很多影响水漂次数的因素(石头的形状、大小、转速、重力因素、水的阻力等)，但没有提到水的清澈度对打水漂次数的影响。
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