问题 选择题

一质量为M的探空气球在匀速下降,若气球所受浮力F始终保持不变,气球在运动过程中所受阻力仅与速率有关,重力加速度为g.现欲使该气球以同样速率匀速上升,则需从气球吊篮中减少的质量为(  )

A.2(M-

F
g

B.M-

2F
g

C.2M-

F
g

D.0

答案

匀速下降时,受到重力Mg,向上的浮力F,向上的阻力f,根据共点力平衡条件

Mg=F+f          ①

气球匀速上升时,受到重力(M-△m)g,向上的浮力F,向下的阻力f,根据共点力平衡条件

(M-△m)g+f=F  ②

由①②式解得

△m=2(M-

F
g

故选A.

阅读理解

The best of Bali in 48 hours

Bali(巴厘岛) is home to most of Indonesia's Hindu minority. Here's how to get the most out of a 48­hour visit:

FRIDAY

3pm—There's nothing like a good meal. La Lucciola is the place to go, with its good food, great service and a beautiful view of the sea.

You can hire a car for some 450,000 rupiah per day. This will help you have more to spare during your trip there.

5pm—About an hour away from La Lucciola is Uluwatu. This southern tip of the island is famous for its temple that lies on the edge, high above the sea. Fire dance is performed here daily at sunset.

8pm—To finish off the evening, head to Jimbaran Bay where restaurants offer candles, dances, boys singing songs. Kick off your shoes, feel the sand, and enjoy the night.

SATURDAY

9:30am—Start the day with another taste of culture by heading for a “barong and keris” dance performance. It's held in many places. One is the Catur Eka Budi in Denpasar, capital of Bali.

12pm—About 50km from Denpasar is Kintamani, a favorite with tourists for the view of active volcano Mt. Batur. It's a good idea to reach Kintamani early as it gets cloudy after 3pm. Have lunch at a local restaurant with a view of Mt. Batur. Tourists can climb to Mt. Batur to catch the sunrise.

6:30pm—Spa(温泉疗养)time! Spa Hati is a good place. Book ahead.

SUNDAY

6am—It's time to go to the sea. Don't forget the camera.

10:30am—Another half­day trip to two of the most famous temples in Bali. Drive down to the Taman Ayun temple at Mengwi.

12am—Next up is the Tanah Lot temple, perhaps the most famous of them all. Built on a rock, it's best seen in the afternoon when it's against the sun or at sunset.

小题1:How can you save time during your trip in La Lucciola according to the text?

A.Having a pair of good shoes.

B.Preparing early and well.

C.Joining a tour group.

D.Hiring a car.小题2:Where can you enjoy fire dance?

A.In La Lucciola.

B.In Uluwatu.

C.At Mengwi.

D.In Denpasar.小题3:Which of the following is probably a better time to visit the famous temple built on a rock?

A.5 am.

B.4 pm.

C.11 am.

D.9 pm.小题4:Which of the following needs to be booked in advance?

A.Joining in a “ barong and keris” dance.

B.Visiting the Tanah Lot temple.

C.Having a spa in Spa Hati.

D.Having lunch in Kintamani.

单项选择题