(6分)用如图所示的滑轮组吊起重物,不计绳重及摩擦. (1)当物重为150 N

题型:计算题

问题:

(6分)用如图所示的滑轮组吊起重物,不计绳重及摩擦.

(1)当物重为150 N时,绳端所用的拉力是60 N,重物被匀速提高2m时,求拉力F所做的功是多少?

(2)此时机械效率是多少?

(3)当所吊重物变为300 N,完成上述过程,滑轮组的机械效率是多少?

考点:杠杆及其五要素
题型:计算题

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题型:计算题

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.

 

第(7)空应选择()

A.idea

B.bar

C.day

D.answer

题型:计算题

Begg细丝弓矫治技术中,如果希望后牙前移关闭拔牙间隙和调整磨牙关系,则牵引力是()

A.50~70g

B.200~280g

C.150~300g

D.100~150g

E.500~700g

题型:计算题

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