问题 问答题

如图所示,工人用定滑轮匀速提升重为180N的物体,所用的拉力为200N,在5s内物体升高的高度为2m.此过程中,求:

(1)有用功;

(2)总功的功率;

(3)定滑轮的机械效率.

答案

(1)工人做的有用功:

W=Gh=180N×2m=360J;

(2)使用的是定滑轮,拉力移动的距离s=h=2m,

工人做的总功:

W=Fs=200N×2m=400J

工人做功的功率

P=

W
t
=
400J
5s
=80W;

(3)定滑轮的机械效率:

η=

W
W
×100%=
360J
400J
×100%=90%.

答:(1)工人做的有用功为360J;

(2)工人做功的功率为80W;

(3)定滑轮的机械效率为90%.

阅读理解

Modern life is impossible without travelling. The fastest way of travelling is by plane. With a modern airline you can travel in one day to places which it took a month or more to get to hundred years ago.

Travelling by train is slower than by plane, but it has its advantages(便利). You can see the country you are travelling through. Modern trains have comfortable seats and dining-cars. They make even the longest journey enjoyable.

Some people prefer to travel by sea when possible. There are large liners and river boats. You can visit many other countries and different parts of your country on them. Ships are not so fast as trains or planes, but travelling by sea is a very pleasant way to spend a holiday.

Many people like to travel by car. You can make your own timetable. You can travel three or four hundred miles or only fifty or one hundred miles a day, just as you like. You can stop wherever you wish where there is something interesting to see, at good restaurant where you can enjoy a good meal, or at a hotel to spend the night. That is why travelling by car is popular for pleasure trips, while people usually take a train or plane when they are travelling on business.

小题1:From the passage, we know the fastest way of travelling is          .

A.by train

B.by sea

C.by plane

D.by car小题2:If we travel by car, we can           .

A.make the longest journey enjoyable

B.travel to a very far place in several minutes

C.make our own timetable

D.travel only fifty or one hundred miles a day小题3:When people travel on business, they usually take               .

A.a plane or a car

B.a car or a boat

C.a boat or a train

D.a train or a plane小题4:How many ways of travelling are mentioned in the passage?

A.Four

B.Three

C.Two

D.Six

选择题