關於功的兩個小問題

Q: 可是,功有分正負,而且力也是一個向量.為甚麼功是一個純量而不是向量?

Work done is defined as the "dot-product" of force and displacement. Here both force and displacement are vectors (向量). Their dot-product is a scalar (純量).

Q: 可以解釋為什麼熱可以作功嗎

Energy can transformed from one form to another. Heat can do work when it is transformed to mechanical energy (機械能). For example, when a hot gas expands in a cylinder, it does mechanical work, say, by pushing a piston.




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2012-08-22 14:20:33 補充：
"dot-product" is one form of vector multiplication (the other form is "cross-product). It is defined, in mathematics, as the product of the magnitudes of the 2 vectors, times the cosine of angle between them. The result will be a scalar.

2012-08-22 14:23:04 補充：
In the case of work done, W = F.s.cos(a)
where F is the force magnitude, s is the displacement magnitude, and angle a is the angle between vector F and vector s.