Dot product for Vector equation

AI Thread Summary
The discussion centers on the dot product of vectors, specifically the equation A • B = |A| |B| cosθ. It clarifies that the alternative equation proposed, A • B = A^2 • B^2 - 2AB cosθ, is incorrect because the square of a vector results in a scalar, making the dot product definition invalid. The dot product can be defined in multiple ways, including using components, and its utility in vector operations is affirmed. Additionally, the relationship between A|| and A cosθ is explained, noting that both represent the projection of vector A along the unit vector u. Understanding these concepts is essential for grasping vector mathematics.
ricky_fusion
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Hi,

I have a question about dot product for vector.

For detail : https://ecourses.ou.edu/cgi-bin/ebook.cgi?doc=&topic=st&chap_sec=02.4&page=theory

Is there anyone understand about it and explain to me the basic concept, why :
1. A • B = |A| |B| cosθ, not A • B =A^2 • B^2 - 2AB cosθ (Cosinus equation)
2. A|| = A cosθ u = (A • u) u (for vector component)
A|| = A • u (for direction component)

Thanks,
Ricky
 
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ricky_fusion said:
Hi,

Is there anyone understand about it and explain to me the basic concept, why :
1. A • B = |A| |B| cosθ, not A • B =A^2 • B^2 - 2AB cosθ (Cosinus equation)

2. A|| = A cosθ u = (A • u) u (for vector component)
A|| = A • u (for direction component)

Thanks,
Ricky

1. For one thing, the square of a vector is a scalar. Does not make sense to have the dot product of two scalars. In the cosinus equation, the left hand side is not A*B but the third side (C). Your "alternative" does not make any sense, sorry.

The dot product can be DEFINED in several equivalent ways. One of them is the above, another way is to use components. There is not much point in asking why something is defined this way and not the other... You can ask if this is an useful operation between vectors and the answer is definitely YES.

2. A*u and A cos(theta) are the same thing if theta is the angle between A and u.
(A*u) is the projection (a scalar) of A along the unit vector u. If you multiply the projection with the vector u you get a vector, the component of A along u.
 
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