问题 计算题

一列简谐波沿x轴方向传播,已知x轴上x1=0和X2=1 m两处质点的振动图象分别如图甲、乙所示,求此波的传播速度。

    

答案

解:由所给出的振动图象可知周期T=4×10-3 s。由图象可知t=0时刻,x1=0的质点P在正最大位移处,x2=1 m的质点Q在平衡位置向y轴负方向运动,所以当简谐波沿x轴正向传播时PQ间距离为;当波沿x轴负向传播时PQ 间距离为

因为,所以

同理因为,所以

因此可得,波沿x轴正向传播时的波速

波沿x轴负向传播时的波速

单项选择题
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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 Johnston 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 travelling side by side with whales.

● When whales are travelling 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.

1. For whom is this text written?

A. Tour guides.

B. Whale watchers.

C. Vessel operators.

D. Government officials.

2. 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

3. 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

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

A. 400 metres.

B. 300 metres.

C. 200 metres.

D.100 metres.