题目

A Spanish company Siesta &Go has opened the first public napping (小睡) space in Madrid. It promises people a quiet rest in the middle of the city’s business center.Napping spaces are nothing new; places to rest are already set up in other big cities in the world. But the idea would appear to work well with the culture of Spain, where people traditionally take an afternoon nap, called siesta.The Madrid nap space recently opened with 19 beds. People can use them by the minute or by the hour, and they can also choose either a private or shared room. An hour of napping inside a private ro1．竖直细杆上套有一个0.1kg的小圆环，圆环左侧系住一劲度系数k=50N/m的轻弹簧，已知弹簧与竖直方向的夹角为θ=37°，圆环始终静止，则以下分析正确的是（g取10m/s2）（　　）A．当弹簧伸长量x=2.5 cm时，圆环与竖直杆的摩擦力为零B．当弹簧伸长量x=5 cm时，圆环与竖直杆的弹力F=1.5 NC．保持弹簧伸长量不变，适度减小θ，圆环与细杆之间的摩擦力不变D．保持弹簧伸长量不变，适度减小θ，圆环与细杆之间的摩擦力变小