问题 多选题

如图所示,放置在水平地面上的直角劈质量为M,上面放一个质量m的物体,若m在其上匀速下滑,M仍保持静止,下列正确的说法是(  )

A.M对地面的压力等于(M+m)g

B.M对地面的压力大于(M+m)g

C.地面对M没有摩擦力

D.地面对M有向左的摩擦力

答案

m在斜面上匀速下滑,所以m受力平衡,对m受力分析如图:

因为m处于平衡状态,故支持力N与摩擦力f的合力F′等于mg,方向竖直向上,即:斜面对m的作用力竖直向上大小为mg.

对斜面受力分析如图:

斜面对m的作用力竖直向上大小为mg,所以m对斜面的作用力F′大小为mg,方向竖直向下.

由平衡条件得:地面对斜面没有摩擦力的作用,且:

N=(M+m)g

故:AC正确,BD错误.

故选:AC

单项选择题

This was one of my experiences (1) . When I was in my 30s, I was working for a large international company which had its head office in London. I was working in the (2) department.

The company had a training centre which was just outside London. Different courses were held there throughout (3) and I (4) go on one training course a year. These usually started (5) a Sunday evening and lasted six days.

Once I was (6) a week’s training course with about forty (7) sales people. I was in the bar on the Friday evening and suddenly a woman who was (8) behind the bar asked mea curious question. "I hope you don’t (9) my asking," she said, "but I’ve been (10) about it all week. Have you got a sister (11) Mary" The woman’s name was June. And she used to do various jobs at the training centre. She worked in the office, she organized all the food and drink for the centre and she worked in the bar at lunchtime and in the evening.

My answer (12) June’s question was "Yes", and June said, "I thought (13) . I met her last September when she was organizing a course here. " At that time, my sister was working for (14) company but in a different part of the country. That was (15) she came to be at the training centre.

(16) surprised me was how June guessed we were brother and sister. Three things made it even more (17) . First, she had met my sister six months before she met me. Second, they have about sixty new people every week at the training centre. So about one thousand five hundred people had (18) the centre in those six months. And (19) , my sister is married, so she doesn’t have the same family name as me.

We’re not (20) , but I guess we must look quite like each other.

2().

A.selling

B.sales

C.sale

D.sold

问答题 简答题