问题 填空题

如图不计摩擦和绳重,用100N的力拉着重250N的物体匀速上升,物体上升速度是0.3m/s,该滑轮组的机械效率为___________,动滑轮重为______N,拉力做功功率为_________W。

答案

83.3 %,50 ,90   

由图知,使用该滑轮组承担物重的绳子股数,n=3

滑轮组的机械效率:

η=W有 /W总 ×100%="Gh" /Fs ×100%="Gh" /F•3h ×100%="G" /F×3 ×100%="250N" /100N×3 ×100%≈83.3%;

动滑轮的重力:G=nF-G物=3×100N-250N=50N

绳子自由端移动的速度:

v′=3v=0.3m/s×3=0.9m/s,

拉力的功率:

P=W总 /t ="Fs" /t =Fv=100N×0.9m/s=90W.

填空题
阅读理解

C

Conjoined twins lead separate but active lives.

Lori and Reba have never been alone and they want to keep it that way.

The 40-year-old conjoined twins in Pennsylvania,US,have read about others locked together by an accident of birth who have been medically separated.But they insist only death can part them.

“We’ve never imagined a life divided,”Lori said.

Lori and Reba are joined together at the side of their heads,facing in opposite directions.Their blood supply is partly linked,but each has a separate brain.

Reba suffers from a disease which has left her completely dependent on her sister.

Lori wheels her everywhere they go in different professional directions.Lori worked in a laundry room at a hospital for six years before giving that up to allow Reba to try to become a country singer.

Her first song is called “The Fear of Being Alone”,which was inspired by Reba and Lori’s desire for togetherness.

Reba’s career has taken her across the United States,Europe and parts of Asia.

While the twins receive some money from the government,the earning from Reba’s concerts go to AIDS and cancer charities(慈善机构).

The sisters have set goals for themselves.Reba looks forward to a successful career as a country singer,while Lori says she hopes to get married and have children.Lori said,”So when it comes,it comes.Meanwhile,I live wonderfully from day to day.”

46.When Lori said “We’ve never imagined a life divided”,she meant that      .

A.they don’t wish to be made separate        

B.they think their case is beyond medical help

C.independent life would be too good to think of 

D.they would die once separated

47.By leading separate lives,the topic means that       .

A.either of the two can look after herself   B.the twins do different things

C.they face in opposite directions         D.they are no longer conjoined now

48.When Reba gives performances as a country singer,Lori       .

A.works in a hospital                   B.continues her college education

C.brings up her children      D.looks after her sister

49.The twins lead active lives because they       .

A.are tired of staying at home doing nothing 

B.need more money  

C.want to prove themselves as useful citizens

D.have to earn their own bread

50.From the last paragraph we can understand that Lori       .

A.thinks it’s too late for her to have children 

B.is sure she’ll get married and have children

C.prefers to have things go naturally     

D.would rather not change her way of life