我想問一條F.6 Physics關於relative velocity既問題（高考題目）請求幫忙＞＜

2007-08-19 9:12 am

A man swims across a river 100m wide always leaning upstream at an angle of 30 with the bank.

1.How long does he take to across the river if he swims at 4m/s and the water flows at 16m/s?
2.How far upstream from the point at which he would be reached if the water is still,assuming that he keeps swimming straight.

其實我想知ｌｅｅ個ｔｏｐｉｃ大概會點問，有冇特定方法做，係咪顯顯差唔多，識做ｌｅｅ題係咪等於識計其他？（我備緊課，望有高人指點＞＿＿＜）

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更新1:

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回答 (2)

Audrey Hepburn

2007-08-19 10:48 pm

✔ 最佳答案

1. Relative to the river, the man is swimming at 4 m/s at an angle of 30* wih the bank.

So, speed of the man that is used to cross the river
= 4sin30*
= 2 m/s

Therefore, the time taken to cross the river
= Distance/speed
= 100 / 2
= 50 s

2. Horizontal speed of the man
= 4cos30*
= 3.464 m/s

So, the required distance
= speed X time
= 3.464 X 50
= 173.2 m

參考: Myself~~~

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max

2007-09-28 11:02 pm

但係佢個個,,,river flows at 16m/s 你好似無計呢!

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