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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

admin2016-08-19  5
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.
Which of the following is not the reason why Lyderic Bocquet supposed that spin is important?
选项 A、Because it keeps the stone fairly flat from one bounce to the next.
B、Because the spin can create a gyroscopic effect.
C、Because the gyroscope can prevent the stone from tipping and falling sideways into the water.
D、Because the spin can reduce the water drag.
答案D
解析 推断题。根据第四段可知，为什么Lyderic Bocquet认为石子旋转如此重要，“because itkeeps the stone fairly flat from one bounce to the next．The spin has a gyroscopic effect，preventing the stone from tipping and falling sideways into the water．”所以A、B、C的选项内容都有涉及，而D没有提到，故选D。
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