As in the field of space travel, so in undersea exploration new technologies continue to appear. They share a number of similarities with each other — as well as some important differences.
Manned submersibles (潜水器), like spaceships, must maintain living conditions in an unnatural environment. But while a spaceship must simply be sealed against the vacuum of space, a submersible must be able to bear extreme pressure if it is not to break up in deep water.
In exploring space, unmanned vehicles were employed before astronauts. In undersea exploration, on the other hand, men paved the way, only recently have unmanned remote-operated vehicles (ROVs) been put to use.
One reason for this is that communicating with vehicles in orbit is much easier than talking to these underwater. A vacuum am ideal medium for radio communications, but underwater communications are limited to much slower sound waves. Thus, most undersea vehicles—particularly ROVs&mdash
A. the communication problem
B. the ocean depths
C. the movement of waves
D. the problem of vacuum
As in the field of space travel, so in undersea exploration new technologies continue to appear. They share a number of similarities with each other — as well as some important differences.
Manned submersibles (潜水器), like spaceships, must maintain living conditions in an unnatural environment. But while a spaceship must simply be sealed against the vacuum of space, a submersible must be able to bear extreme pressure if it is not to break up in deep water.
In exploring space, unmanned vehicles were employed before astronauts. In undersea exploration, on the other hand, men paved the way, only recently have unmanned remote-operated vehicles (ROVs) been put to use.
One reason for this is that communicating with vehicles in orbit is much easier than talking to these underwater. A vacuum am ideal medium for radio communications, but underwater communications are limited to much slower sound waves. Thus, most undersea vehicles—particularly ROVs&mdash
A. more interesting
B. more demanding
C. more attractive
D. more’dangerous
| Experienced baseball fielders can tell
how far a ball is going to travel just by listening to the crack of the bat. If
they didn’t, they wouldn’t stand a chance of catching it, claims a physicist in
New York. "When a baseball is hit straight at an outfielder, he cannot quickly judge the angle of the scent and the distance the ball will travel," says Robert Adair, a physicist at Yale University. If he relied purely upon visual information, the fielder would have to wait for about one-and-a-half seconds before he could tell accurately if the pitcher hit the ball long or short. By this time the ball may have travelled too far for him to reach it in time. To stand a fighting chance of catching it, according to Adair, fielders must listen to the sound the ball hitting the bat to judge how far it will travel. There is anecdotal A. Because it may take longer time for him to exert judgment. B. Because the ball may change directions during flight. C. Because the ball may be hit either long or short. D. Because light travels faster than sound. [单项选择]已知y’+y=x的一个解为y1=x，y"+y=2ex的一个解为y2=ex，则方程y"+y=x+2ex的通解为
。
A. x+ex B. c1cosx+c2sinx C. c1cosx+c2sinx+x+ex D. c1cosx+c2sinx+x 我来回答: 提交
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