雞毛與鐵球誰先落地呢?

2006-11-20 2:18 am
雞毛輕,鐵球重,從同樣的高度向下落,是雞毛先落地,還是鐵球先落地?

回答 (5)

2006-11-21 11:41 am
✔ 最佳答案
In theory, the only force acting on both 雞毛 and 鐵球 is the gravitational force in the vertical direction, so that there is only gravitational acceleration in the vertical direction.

s=ut+at^(1/2) => displacement = initial velocity * time + acceleration * square root of time

From the formulae above, time taken for those 2 objects to reach the ground is not depending on the mass or weight of the object.
It depends on the initial velocity and the acceleration of the object.
Since displacement and acceleration for both 雞毛 and 鐵球 is the same, there shouldn't be any difference between the time taken for them to travel from the same height to ground if the initial velocity of them are the same as well.

However, in reality, there is drag force or air resistance force acting on those 2 objects, so that they will reach the ground at different time.

Drag equation
Drag force = -1/2 *density * velocity^2 * Area * drag coefficient

Drag force is a force opposes the motion of the object when it moves through a fluid such as air or water. Drag force depends on the shape of the object, the properties of the fluid, and the speed of the object relative to the fluid.
Drag coefficients for rough unstreamlined objects can be 1 or more, for smooth object much less.

In this case, since the motion of the 2 objects is going downwards, therefore drag force is acting upwards. 鐵球 should be a more streamlined object compare to a 雞毛, so the drag coefficient of the 鐵球should be smaller than 雞毛. However, the area of a 雞毛 may be smaller than a 鐵球.
(You really need to consider these points when you think about this question.)

By assuming that the area of the 雞毛 and 鐵球 are the same. I'm quite sure that a 鐵球 would reach the ground first.

2006-11-21 08:59:46 補充:
the 1st formulae should be s = ut 1/2 at^2sorry that i've made such silly mistakes......
2006-11-20 5:39 am
係真空入面 兩者係同時著地的
因為根據伽利略既實驗指出
S=ut+1/2at^2

兩者u一樣
so that
S=1/2at^2
兩者S,a 一樣
so that 時間一樣依個情況只會係真空入面出現

點解現實唔會??
因為有空氣阻力 空氣阻力令到佢地既速率改變=>t增加
所以...佢地係不同時著地的
2006-11-20 3:17 am
沒錯...跟據伽利略的說法,即使兩樣重量不同的物質,假如相同的高度、地方放下,會同時落到地面。

可是不要忘記得是他用的都是鐵球,只是重量不同。

不過雞毛和鐵球不同的地方就不只是重量,而且有其他的差別。

就是它們的構造了!!!

雞毛是由纖維造成的,空氣可穿過纖維,所以會成為一個阻力,並且有可能改變雞毛落下的方向,減慢速度。

鐵球是由金屬造成的,並不可能讓空氣穿過,可無視空氣的阻力。
參考: 我的腦袋
2006-11-20 2:28 am
雞毛輕,鐵球重,從同樣的高度向下落,是雞毛先落地,還是鐵球先落地?
答案有3個:一,鐵球先落地;二,同時落到地上;三,雞毛先落地。
讀者請先想一想,然后再做出選擇。
這裡先說一下,這是一個古老的問題,提出問題的人是大名鼎鼎的古希腊哲學家亞里士多德。
公元前300多年的時候,亞里士多德說,物体下落的快慢與它的重量成正比,重的物体落得快。這樣,鐵球比雞毛重,自然是先落地的了。這是第一種答案。
由于亞里士多德被西方人尊奉為科學權威,在長達1900多年的時間裡,沒有人懷疑過亞里士多德的說法,直到16世紀,才有人出來推翻亞里士多德的答案。
這個人是大家熟悉的伽利略。伽利略在《兩種新科學的對話》中,用兩個人對話的形式,做了一段精彩的推理,推翻了亞里士多德的答案。這段對話,用現代語言簡化得通俗一些,大約是這樣的:

甲:物體A比物体B重得多,哪個先落地?
乙:按亞里士多德的說法,物體A先落地。
甲:現在把A和B捆在一起,成為A+B呢?
乙: A+B比A重,應該比物体A先落地。
甲:請注意,B比A輕,按亞里土多德的說法,B下落比A慢, A+B以后,B抱著A,下落速度減慢了。 A+B應該比物体A 後落地。
乙:啊呀,得出了兩個相反的結論,亞里士多德的話不能再相信了。

就這樣,伽利略巧妙地否定了對亞里士多德的迷信,第一個答案不對了,伽利略提出了第二個答案:同時落到地上。伽利略說,輕重不同的物体從同一高度下落,應當同時落地。當時,伽利略是個二十五、六歲的青年,別人都懷疑他的說法,伽利略決心登上比薩斜塔,當著眾人進行一次實驗。他一手拿著1磅重的鐵球,另一手拿著10磅重的鐵球,放開雙手,兩個鐵球同時落到了地上。來觀看的學者和民眾信服了。
2006-11-20 2:27 am
正常來球說應該是鐵球先。
因為鐵球較重,地心吸力會把較重的物件先拉(吸引)到地面。
參考: 100% 自己 XD


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