问题 选择题

如图,质量为70kg的工人站在水平地面上,用滑轮组把货物运到高处。已知货物质量为160kg,人用力F匀速拉绳,人拉绳的功率P,货箱以0.1m/s的速度匀速上升,地面对工人的支持力为N,滑轮组的机械效率η为80%(不计绳重及滑轮摩擦,g取10N/kg),则下列选项正确的是

[ ]

A.工人的拉力F大小为400N

B.工人拉绳的功率P为40W

C.地面对工人的支持力N为1100N

D.动滑轮的质量为400kg

答案

答案:A

单项选择题

When I was young, my parents ran a snack bar in our small town.

One evening in early April, my mother told me to fill in at the snack bar (1) a worker who had the flu. I told her I would mess it up, (2) I had never worked at the bar before. I (3) that instead of making money, I would end up owing it.

"You can do it, " said my mother, " (4) , you won’t get much business until lunch. "

"But I’ll never remember the orders, and I’m no good (5) money. Please, Mom, don’t (6) me. "

"Then I’ll help you", she said.

I shrugged my shoulders. I thought my mother’s (7) was a bad one, but I (8) .

When I got to the bar the next day, I found my mother was (9) . Because the weather that day was rainy and cold, people wanted hot snacks and drinks. (10) , I was really slow at taking the orders and making change. The line of people grew, and everybody seemed (11) , I was so nervous that my hands shook, and I (12) a cup into pieces. What a mess! Then my mother came to (13) me, and she also showed me how to make (14) If someone gave me $ 5 for something that cost $ 3.25, I handed over (15) quarters and a dollar and said, "75 cents makes four dollars, plus one dollar makes five. " Things went more (16) after that.

By the end of the day, I could remember orders, (17) the bill, and make change quickly with a smile. I was even a little (18) when the sun came out and dried up business. My mother said she was proud of me, and when she (19) that I work at the snack bar again next year, I did not even shrug. I was too busy (20) the restaurant I would open one day.

7()。

A. idea

B. bar

C. day

D. answer

材料题