點解F=ma?

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http://h.imagehost.org/0358/ScreenHunter_01_May_19_23_19.gif

Why F = ma ?
眾所周知, 這是牛頓第二定律.
但這不是牛頓作出來的,
這是他從實驗中得來的結果.
他發現越大力拉/推一件物體, 物體會改變速度(即變速),
於是, 對於同一件物體, 他用儀器量度不同的淨力和產生的加速率.
他發現 F 和 a 成正比例
然後, 他改變物件的質量, 使用相同的淨力, 得出不同的加速率.
他發現 m 和 a 成反比例
合併兩個實驗結果後,
便得出
F = ma 的公式.

簡單來說, 牛頓把實驗結果以數學公式表達出來.

The equation force (F) = mass (m) x acceleration (a) is an experimental relationship, i.e. it cannot be derived from other physics lawes and principles.

In the fact, the equation comes from Newton's Second Law of Motion, which states that the rate of change of momentum of a body is proportional to the force applied to it, and takes place in the direction in which the force acts.
Newton's Laws can only be verified by experiments.

Based on common experience, it is not difficult to visualize that the higher the mass of an object, the harder to make the object move (i.e. a larger force needs to be applied). Also, the stronger the push on an object, the faster it moves, i.e. a larger acceleration is resulted.

It is only through experiments to establish the dependence of mass and acceleration with force is linear. Hence, we have F = ma.