问题 选择题

一小球质量为m,用长为L的悬绳(不可伸长,质量不计)固定于O点,在O点正下方处钉有一颗钉子,如图所示,将悬线沿水平方向拉直无初速度释放后,当悬线碰到钉子后的瞬间,下列说法错误的是(  )

A.小球线速度突然增大到原来的2倍

B.小球的角速度突然增大到原来的2倍

C.小球的向心加速度突然增大到原来的2倍

D.悬线对小球的拉力突然增大到原来的2倍

答案

AD

题目分析:在小球通过最低点的瞬间,水平方向上不受外力作用,沿切线方向小球的加速度等于零,因而小球的线速度不会发生变化,故A错误;在线速度不变的情况下,小球的半径突然减小到原来的一半,由v=ωr可知角速度增大为原来的2倍,故B正确;由a=v2/r,可知向心加速度突然增大到原来的2倍,故C正确;在最低点,F-mg=ma,可以看出D错误;故选AD

点评:本题难度较小,本题中要分析出悬线碰到钉子前后的瞬间物理量的变化情况,问题就很好解了,因而,要根据题目的条件分析物理过程后再选用公式,不能随意照套公式

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