问题 计算题

(10分)某飞机场利用如图所示的传送带将地面上的货物运送到飞机上,传送带与地面的夹角θ= 30°,传送带两端A、B的长度L = 10m。传送带以v = 5m/s的恒定速度匀速向上运动。在传送带底端A轻轻放一质量m = 5kg的货物,货物与传送带间的动摩擦因数。求货物从A端运送到B端所需的时间。(g取10m/s2

答案

3s

(10分)

mgμcos30°-mgμsin30°= ma…………………………………………………… 2分

解得a =" 2.5" m / s2………………………………………………………………………1分

货物匀加速运动的时间…………………………………………………1分

货物匀加速运动的位移………………………………2分

随后货物做匀速运动。

运动位移  S2 = L-S1 =" 5m" ……………………………………………………………1分

匀速运动时间 …………………………………………………………1分

t = t1 + t2 =" 3s" ……………………………………………………………………………2分

多项选择题
阅读理解

If you walk slowly through downtown Helsinki(赫尔辛基)during the day, taking in the splendid 19th century buildings, white boats and noise of passing trams(有轨电车), you will start to understand why it is called a city of two colors: white and blue.

The sea is always present in Helsinki. When you take a walk over the great open space of the central square, you will hear seabirds screaming. When you take the tram, suddenly and unexpectedly, you are faced with a calm, shining blue sea. You may notice that people in Helsinki do not rush about as in other cities. Instead, they walk along the roads, politely letting other people by.

A usual way to see Helsinki for the first time is to start out by the boats. You will walk by the elderly women selling fish and vegetables in the market square and find yourself in front of a beautiful park. You may enjoy a pleasant walk in the park for a few hours and then take the tram. Trams are the perfect way to get around in Helsinki. Watching the old houses, parks, theatres, churches, shops, restaurants and people in the streets, you may have a slightly sad film feeling to it.

The pale summer nights are another wonder in the city. Following the waterfront(滨水区)of the city after sunset, you couldn’t help stopping and listening to the sweet silence, interrupted only by the screaming seabirds and leaving fishing boats.

However, in some way, Helsinki is also the most modern city in northern Europe. You will surely want to visit the white Glass Palace, the modern art museum, and all those extremely popular cafes and design stores.

68.Helsinki is called a city of two colors mainly because of the colors of its ________.

A.19th century buildings, boats and parks

B.19th century buildings, boats and seabirds

C.old houses, parks and trams

D.old buildings, boats and the sea

69.The best way to see most of Helsinki is to go ________.

A.by boat     B.by bus      C.by tram    D.on foot

70.The 19th century buildings, the White Glass Palace, popular cafes and design shops in Helsinki all show that Helsinki is _________.

A.both splendid and traditional   B.both quiet and noisy

C.both historical and modern     D.both old and new

71.This passage is most likely to be found in _________.

A.a story-book           B.a geography textbook

C.a research report             D.a travel magazine