friction concept 高手請進(urgent)

2011-08-24 7:04 pm

Friction opposes the relative motion of two contacting surfaces. But I still cannot fully understand the concept. Can some1 point out whether I have any misconceptions when explaining the following situations:

a) A ball is sliding down a slope. Because the ball is moving downward relative to the slope (stationary), therefore an upward frictional force acts on the ball. For the slope, since it is stationary and the ball is moving downward, the ball is moving upward relative to the ball, therefore a downward frictional force acts on the slope.

b) A ball is rolling down a slope. Because the contacting surface of the ball is moving upward relative to the slope, a downward frictional force acts on the ball. And the contacting surface of the slope is moving downward relative to the ball, an upward frictional force acts on the slope.

圖片參考：http://imgcld.yimg.com/8/n/HA01097219/o/701108240033913873448340.jpg

Is my concept correct? Can some1 point out the misconceptions above?

thanks

更新1:

to #2, thanks. btw, is case b (rolling) similiar to the case we walk?

更新2:

to doraemonpaul: can u talk more about that 'rolling friction'? and what if I replace the 'ball' with a block in case a, that should be possible right?

回答 (3)

天同

2011-08-24 10:16 pm

✔ 最佳答案

(a) I supose the surface of the slope is rough, such that friction is present.

A ball is sliding down a slope. Because the ball is moving downward relative to the slope (stationary), therefore an upward frictional force, given by the slope , acts on the ball. By action-reaction principle, an reaction, given by the ball, acts on the slope in the downward direction.

(b) A ball is rolling down a slope. Because the contacting surface of the ball is moving upward relative to the slope, a downward frictional force acts on the ball. By the principle of action and reaction, an reaction force, given by the ball, acts on the slope surface in the upward direction.

2011-08-26 14:02:49 補充：
Q: is case b (rolling) similiar to the case we walk?
A: Yes, it is

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doraemonpaul

2011-08-24 10:09 pm

你的concept當然有問題，因為b的那個case一定是考慮rolling friction而不是考慮sliding friction，而a的那個case則永遠沒有可能發生。

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2011-08-24 7:15 pm

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