问题 计算题

渔船常利用超声波来探测远处鱼群的方位已知某超声波频率为1.0×l05Hz,某时刻该超声波在水中传播的波动图像如图所示

(1)从该时刻开始计时,画出x =7.5×10-3m处质点做简谐运动的振动图像(至少一个周期)。

(2)现测得超声波信号从渔船到鱼群往返一次所用时间为4s,求鱼群与渔船间的距离(忽略船和鱼群的运动)。

答案

解:(1)该波的周期为

由波动图像知,此时x=7. 5×10-3m处的质点位于负的最大位移处,

所以,从该时刻开始计时,该质点的振动图像,如图所示。

(2)由波形图读出波长λ=15×10-3m

由波速公式得v=λf ①

鱼群与渔船的距离为

联立①②式,代入数据得x=3000m③。

阅读理解

Do’s and Don’ts in Whale(鲸) Watching

The Department of Fisheries and Oceans has developed guidelines for whale watching in Johnstone Strait, where killer whales are found on a daily basis each summer. It is strongly recommended that vessel(船只) operators follow these guidelines for all kinds of whales.

· Approach whales from the side, not from the front or the back.

· Approach no closer than 100 metres, then stop the boat but keep the engine on.

· Keep noise levels down — no horns, whistles or racing of engines.

· Start your boat only after the whales are more than 100 metres from your vessel.

· Leave the area slowly, gradually moving faster when you are more than 300 metres from the whales.

· Approach and leave slowly, avoiding sudden changes in speed or direction.

· Avoid disturbing groups of resting whales.

· Keep at low speeds and remain in the same direction if traveling side by side with whales.

· When whales are traveling close to shore, avoid crowding them near the shore or coming between the whales and the shore.

· Limit the time spent with any group of whales to less than 30 minutes at a time when within 100 to 200 metres of whales.

· If there is more than one vessel at the same observation spot, be sure to avoid any boat position that would result in surrounding the whales.

· Work together by communicating with other vessels, and make sure that all operators are aware of the whale watching guidelines.

64. For whom is this text written?

A. Tour guides.                             B. Whale watchers.       

C. Vessel operators.                       D. Government officials.

65. When leaving the observation areas, the vessel should ______.

A. move close to the beach             B. increase speed gradually

C. keep its engine running slowly   D. remain at the back of the whales

66. When going side by side with whales, the vessel should ______.

A. keep moving in the same direction

B. surround the whales with other boats

C. travel closer and closer to the shore

D. take a good viewing position

67. What is the shortest safe distance from the whales?

A. 400 metres.         B. 300 metres.              C. 200 metres.              D. 100 metres.

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