问题 选择题

在静止的电梯里放一桶水,把一个轻弹簧的一端连在桶底,另一端连接在浸没在水中的质量为m的软木塞上,轻弹簧处于伸长状态,如图所示.当电梯由静止开始匀加速下降(a<g)时,轻弹簧的长度将发生怎样的变化:( )

A.伸长量保持不变

B.由伸长变为压缩

C.伸长量增加

D.伸长量减小

答案

答案:D

电梯静止时,软木塞受到弹簧的支持力N1,自身的重力G以及浮力F,且G-N1-F=0,得N1=G-F;当电梯加速向下运动时,软木塞受到的弹簧支持力N2,且G-N2-F=ma,得N2=G-F-ma,所以N2<N1,弹簧的形变量变小。选D

单项选择题
填空题


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