Why vector quantities are not divisible?

Answer 1

I'll assume you are referring to the inverse of the most common process of vector multiplication, namely the formation of an inner product, also called a scalar product or dot product, between two vectors of the same size. In this operation, vectors with, for example, components (a,b,c,d) and (e,f,g,h) must be pairwise multiplied and summed, to arrive at the scalar result ae + bf + cg + dh. Any two ordinary vectors of matching size (number of components) can be "multiplied" to get an inner product. (There is another kind of multiplication of two 3-vectors called the cross-product, which is sometimes invertible, but because the cross-product only works with two vectors in 3-space, it does not seem useful to discuss the cross-product further in the context of general vector division. Similarly, one could individually multiply the components of the two vectors to get a sort of third vector. Although that operation would be invertible under some conditions, I am not aware of any meaning, or physical significance, for the use of that technique.

Since the result of taking the inner product of two vectors is a scalar, that is, a single real number, most of the information about the two vectors is lost during the computation. The only information retained by the inner product is the magnitude of the projection of one vector A onto the direction of another vector B, multiplied by the magnitude of B.

But division is the inverse operation of multiplication. In a sense, division undoes the work of a previous multiplication. Since all information about the direction of each vector is discarded during the calculation of an inner product, there is not enough information remaining to uniquely invert this operation and bring back, say, vector A, knowing vector B and the value of the scalar product.

Answer 2

Vector quantities cannot be divided into smaller parts because they have both magnitude and direction, which cannot be separated. Dividing a vector would break the link between its magnitude and direction, making it lose its meaning as a vector quantity.