搜题请搜索小程序“爱下问搜题”
问题 选择题

质量为m的滑块沿高为h，长为l的粗糙斜面匀速下滑，在滑块从斜面顶端滑至低端的过程中（　　）

A．重力对滑块所做的功为mgh

B．滑块克服摩擦所做的功为mgl

C．滑块的机械能保持不变

D．滑块的重力势能增加了mgh

答案

A、重力对滑块所做的功W=mgsinθ•L=mgh．故A正确．

B、由题，滑块匀速下滑，动能的变化量为零，根据动能定理得，mgh-Wf=0，则滑块克服阻力所做的功Wf=mgh．故B错误．

C、滑块匀速下滑，动能不变，重力势能减小，则机械能减小．故C错误．

D、重力势能的变化量△EP=-WG=-mgh，所以滑块的重力势能一定减少mgh，故D错误．

故选A

单项选择题 A1/A2型题

引起红细胞沉降率加快最常见的原因为（）.

A.各种炎症

B.组织坏死

C.恶性肿瘤

D.红细胞增多症

E.肝病
查看答案
填空题

语法填空。

     At 3:42 a.m. everything began to shake. It seemed l_____ the world was 2_____ an end! Eleven kilometers

3_____ (direct) below the city the greatest earthquakes of the 20th century began. It was heard in Beijing,

4_____ is one hundred kilometers away. One-third of the nation felt it. A huge crack that was eight kilometers

long and thirty meters wide cut 5_____ houses, roads and canals. Steam burst from 6_____ (hole) in the

ground. Hard hills of rock became nvers of dirt. In fifteen terrible seconds a large city lay in ruins. Two-thirds

of the people died or 7_____ (injure) during the earthquake. Thousands of families 8_____ (kill) and many

children were left 9_____ parents. The number of people 10_____ were killed or injured reached more than

400,000.

查看答案