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图示梁的四种截面形状，假定剪力沿垂直方向。横截面上最大切应力(或切应力垂直分量的最大值)的位置为()。
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如图所示，S1、S2，是两个相干光源，它们到P点的距离分别为r1和r2。路径S1P垂直穿过一块厚度为t1、折射率为n1的介质板，路径S2P垂直穿过厚度为t2、折射率为n2的另一介质板，其余部分可看做真空，这两条路径的光程差等于()。
如图所示，正弦电流通过纯电感元件时，下列关系式正确的是()。
Denny
His nickname is Denny. He weighs 400 pounds; he is fearless and he never goes to sleep on the job. An ideal security guard？ For many situations he may be. And if he's so good that you wish you had a dozen like him, just place your order. Denny is a robot guard.
Denny can detect, within a 150-foot radius, the presence of anything or anybody that shouldn't be there. Its swiveling (旋转) head contains microwave and infrared sensors that can detect people as well as smoke. In future editions the head will also contain sensors that can smell the weak smell of a human body.
A high-resolution TV camera in Denny's head is on at all time. When something Unexpected comes into view; the TV transmitter switches on. Thus the human overseer (看管人) in the control center sees the sudden appearance of a picture on the monitor screen. At the same time the picture is automatically videotaped.
Normal speed of the robot guards is about one mile an hour, and. they can even talk: 'you have been detected,' warns the voice from the clever guard. Denny is designed to patrol corridors and other areas after lock-down hours (of course, he can work round the clock when necessary), not to move among people. If, say, a prisoner does get near the corridor where he should not be, it'll immediately tell its base station by radio.
Denny has understandable limitations. He can't open doors or watch stairs, for example, or distinguish friend from enemy. Thus he will have to go about unarmed. And he won't be able to replace human security guards where people move about freely.
Denny is a robot guard, who
Earthquakes and Animals
Scientists who try to predict earthquakes have gotten some new helpers recently—animals. Animals often seem to know in advance that an earthquake is coming, and they show their fear by acting in strange ways. Before a quake (地震) in China in 1975 snakes awoke from their winter sleep early only to freeze to death in the cold air. All the unusual behavior, as well as physical changes in earth, alerted (提醒) Chinese scientists to the coming quake. They moved people away from the danger zone and saved thousands of lives.
One task for scientists today is to learn exactly which types of animal behavior. predict quakes. It is not an easy job. First of all not every animal reacts to the danger of an earthquake. Just before a California quake in 1979, for example, an Arabian horse became very nervous and tried to break out of his stall. The horse next to him, however, remained perfectly calm. It is also difficult sometimes to tell the difference between normal animal calmness and "earthquake nerves." A zookeeper once told earthquake researchers that his cougar (美洲狮) had been acting strangely. It turned out that the cougar had a stomachache.
A second task for scientists is to find out exactly what kinds of warning the animals receive. They know that animals sense far more of the world than humans do. Many animals can see, hear, and smell things that people do not even notice. Some can detect tiny changes in air pressure, gravity, or the magnetism (磁力) of earth. This extra sense probably helps animals predict earthquakes.
A good example of this occurred with a group of dogs. They were closed in an area that was being shaken by a series of tiny earthquakes. Before each quake a low booming sound was heard. Each sound caused the dogs to bark (吠) wildly. The dogs began to bark during a silent period! A scientist who was recording the quakes looked at his machine. He realized that the dogs had reacted to a booming noise. They also sensed the tiny quake that followed it. The machine recorded both, though humans felt and heard nothing.
In this case there was a machine to check what the dogs were sensing. Many times, however, our machines record nothing out of the ordinary, even though animals know a quake-is coming. The animals might be sensing something we do measure but do not recognize as a warning. Discovering what animals sense, and learning how they know danger signals, is a job for future scientists.
During an earthquake in China in 1975, some cows had "earthquake nerves" and some remained calm.
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