问题 多选题

下列“画阴影”的物体受力分析正确的是(  )

A.

接触面光滑

B.

光滑斜面

C.

物体冲上粗糙斜面

D.

一起向右匀速

答案

A、物体保持静止,合力为零,无向右上方的支持力,故A错误;

B、物体不受支持力,受重力和拉力平衡,故B正确;

C、物体冲上粗糙斜面,滑动摩擦力与相对运动相反,故C错误;

D、一起向右匀速,处于平衡状态,故受重力和支持力而平衡,故D正确;

故选BD.

单项选择题
单项选择题

Like many of my generation, I have a weakness for hero worship. At some point, however, we all to question our heroes and our need for them. This leads us to ask: What is a hero

Despite immense differences in cultures, heroes around the world generally share a number of characteristics that instruct and inspire people.

A hero does something worth talking about. A hero has a story of adventure to tell and community who will listen. But a hero goes beyond mere fame.

Heroes serve powers or principles larger than themselves. Like high-voltage transformers, heroes take the energy of higher powers and step it down so that it can be used by ordinary people.

The hero lives a life worthy of imitation. Those who imitate a genuine hero experience life with new depth, enthusiasm, and meaning. A sure test for would-be heroes is what or whom do they serve What are they willing to live and die for The answer or evidence suggests they serve only their own fame, they may be famous persons but not heroes. Madonna and Michael Jackson are famous, but who would claim that their fans find life more abundant

Heroes are catalysts (催化剂) for change. They have a vision from the mountaintop. They have the skill and the charm to move the masses. They create new possibilities. Without Gandhi, India might still be part of the British Empire. Without Rosa Parks and Martin Luther King, Jr., we might still have segregated (隔离的) buses, restaurants, and parks. It may be possible for largescale change to occur without leaders with magnetic personalities, but the pace of change would be slow, the vision uncertain, and the committee meetings endless.

Madonna and Michael Jackson are not considered heroes because ().

A. they are popular only among certain groups of people

B. their performances do not improve their fans morally

C. their primary concern is their own financial interests

D. they are not clear about the principles they should follow