MC04 No.8 - Ans.fyi 參考答案

MC04 No.8

dom123

2008-02-11 2:00 am

The photograph shows a sucker sticking a hook to a vertical wall.
Which of the following forces balances the gravitational force acting on the sucker and hook by the earth
A the frictional force between the sucker and the wall
B the force exerted by the air molecules on the sucker
C the normal reaction exerted by the wall on the sucker
D the gravitational force acting on the earth by the sucker and the hook
我想問下其實個free body diagram 係點, 我不係好明點解會黏到係個wall 度
呢個情況有冇wall 的reaction force?
thx

更新1:

嗯, 謝謝那麼這裏net force 不就是reaction force嗎? 如果reaction force 沒有被balance 的話, sucker 會向左加速吧? 所以是不是會有force balance 左reaction force?

回答 (1)

Audrey Hepburn

2008-02-11 2:48 am

✔ 最佳答案

Free body diagram~~~

圖片參考：http://i182.photobucket.com/albums/x4/A_Hepburn_1990/A_Hepburn01Feb101846.jpg?t=1202640396

The answer is A. the frictional force between the sucker and the wall balances the gravitational force acting on the sucker and hook by the earth.

For A, frictional force and gravitational force are in same magnitude and opposite direction. However, they are not action and reaction pair since the two forces act on the same object.

For C, normal reaction exerted by the wall on the sucker is perpendiuclar to the weight. So, normal reaction does not balance the weight.

For D, they are action and reaction pairs.

2008-02-10 19:30:50 補充：
首先，讓我解釋為何會有reaction force就是因為sucker向wall exert一個action force，所以根據牛頓第三定律wall會向sucker exert一個reaction force所以，sucker並不會向左加速

參考: Myself~~~

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