f4 phy & one extra question...

Crystal

2012-02-28 6:44 am

1.A football player heads a ball as shown.
According to Newton's third law,which of the following pairs of forces is an action-and-reaction pair?
Ans:The gravitational force exerted by the Earth on the player and the gravitational force exerted by the player on the Earth

點解 player 會exert gravitational force on the Earth?
唔係得Earth(or other planet) 先會exert gravitational force咩?

2.花灑again.....

圖片參考：http://imgcld.yimg.com/8/n/HA04937267/o/701202270094013873442470.jpg

藍色線=花灑,
紅線=assume 無air resistance,無weight既condition下,水既movement
A=point that water changes its direction(即係水經過e點會momentarily at rest,velocity=0)
紫色=weight exerted on water by Earth
我想問,點解水經過A 呢一點之後,佢既path仍然係parabolic?而唔係垂直?

更新1:

thx a lot!

回答 (1)

天同

2012-02-28 7:35 am

✔ 最佳答案

1. Any object with a mass possesses gravitational attractive force, no matter it is a table, a chair, a desk, a car, a train or a person.

2. There is always a horizontal component of the velocity of the water stream when it leaves the shower head. As it is assumed that there is no air resistance, such horizontal velocity component would NOT disappear. It therefore carries the water to move horizontally. When combining with the vertical velocity of the water given by gravity, a parabolic path is formed.

Notice that the water stream would NEVER become vertical due to the presence of the horizontal velocity component.

Also note that at the point A, the velocity of water is NOT zero. That is, it is not momentarily at rest, as given in your question. At that point, the water possesses the horizontal velocity component only.

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