如图所示,用滑轮组提升重为630N的物体,绳子自由端拉力F为300N,重物以0.

题型:问答题

问题:

如图所示,用滑轮组提升重为630N的物体,绳子自由端拉力F为300N,重物以0.2m/s的速度匀速上升,不计摩擦和绳重.求:

(1)滑轮组的机械效率;

(2)拉力做功的功率;

(3)在不超过绳子能承受的最大拉力的情况下,用此滑轮组提起较重的物体,滑轮组的机械效率是否变化?为什么?

考点:功率的计算滑轮(组)的机械效率
题型:问答题

阅读理解。

     Fish have started protecting us. It seems unlikely, but the Intelligent Automation Corporation has developed

a system that uses fish to protect our water supplies. Cities and towns share water with millions of people, so

it's important that the water is clean and safe to drink. Bluegills, which are small enough to fit in your hand, are

the fish that are protecting our water. They are common freshwater fish native to America and are also known

as bream or sunfish. According to the Associated Press, the bluegills were originally used by the Army to make

sure water was safe. Now San Francisco, New York City, Washington D.C and several other cities use the bluegill monitoring system. The bluegill system works by constantly filling the fish tank with city or town water. A

small group of bluegills swim in the local water and are closely monitored.

     Bluegills are very sensitive to their environment. Changes in the environment cause their behavior and

breathing patterns to change. They cough if the water is polluted. Fish cough by flexing their gills (鳃) to get

rid of unwanted particles, like grain of sand, from their breathing passages. If the bluegills are coughing the

computer sends an alarm via email.

     The bluegills in New York City discovered an diesel spill (柴油外溢) hours before the usual monitors.

Adding the bluegill system to regular water protection methods resulted in an early warning and safer water.

1. People use the bluegill system to monitor water supplies_____.

A. because they have high sensibility to their environment

B. because they are used to swimming in the fresh water

C. because they can cough by means of their gills

D. because they are able to send e-mails by computers

2. Which of the following statements about Bluegills is NOT true?

A. They are about the size of a human hand.

B. They are freshwater fish easily found in America.

C. They were used to safeguard water in the army.

D. They are smarter than any other living thing on earth.

3. People decide whether the water is polluted by means of _____.

A. information sent by the Army

B. the warning e-mail the computer sends

C. observing the activities of bluegills

D. constantly filling the fish tank with water

题型:问答题

浓香型白酒的主体香气成分是(),清香型白酒的主体香气成分是()和()。

题型:问答题

哪一项不是鼠疫的传播途径

A.输血
B.呼吸道
C.鼠蚤叮咬
D.皮肤黏膜
E.直接接触或剥食疾病鼠

题型:问答题

瘀血阻滞型腹痛的代表方剂是()。

A.血府逐瘀汤

B.桃红四物汤

C.少腹逐瘀汤

D.大黄牡丹皮汤

E.桃核承气汤

题型:问答题

商品货架摆放的安全标准是什么?

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