Why is sine not used for dot product?

In summary: Both definitions produce the same result, but the coordinate definition is more explicit. It's more concise, and it allows us to work with vectors in a more general manner.So I guess a better question would of been: why is cosine the beginning of the unit circle? (which I’m sure there’s many good reasons for).Why is cosine the "start" of the unit circle?It is because the dot product is the projection of one vector onto another. If you draw a diagram and calculate the projected length of the shorter onto the longer vector, it goes as cosine of the angle by elementary trigonometry.
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Kirkkh
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Re-examine an old question on here from 2012 “Why sine is used for cross product and cosine for dot product?
There’s a old 2012 post on here “Why sine is used for cross product and cosine for dot product?” —there are a lot of great answers (which is how I came about this forum). After reading over the replies, it occurred to me: really it’s just because cosine is the “start” of a unit circle.

Which is to say we set up a “dot product” to be a single number, it’s a simple idea —how do two similar vectors relate? Given that —we use the simplest metric (cosine). If we used sine (again, not being at the bringing of the circle) it would add additional information that would need to be subtracted.

So I guess a better question would of been: why is cosine the beginning of the unit circle? (which I’m sure there’s many good reasons for).
 
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It is because the dot product is the projection of one vector onto another. If you draw a diagram and calculate the projected length of the shorter onto the longer vector, it goes as cosine of the angle by elementary trigonometry. Invoking a unit circle would restrict dot products to unit vectors.
 
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I look at it in a vector sense. I have a two vectors A and B and I want to know what is the projection of A on B and what portion of A is NOT projected on B ie is perpendicular to B.

The projection can be found by the dot product A.B = |A||B|cos(angle_between_A_and_B).

This comes from thinking of A as the hypotenuse of a right triangle and B as a side and |B|/|A|= cos(angle_between_A_and_B) . Notice |A|sin(angle_between_A_and_B) is the other side of the triangle.

Hence, for the projection of A on B we get |A|cos(angle_between_A_and_B)

Next for the portion of A that is NOT projected onto B ie that is perpendicular to B.

We use the cross product and sin(angle_between_A_and_B) and for symmetry we define the result as a vector perpendicular to both A and B:

whose length is: |AxB| = |A||B|sin(angle_between_A_and_B)
 
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Kirkkh said:
“Why sine is used for cross product and cosine for dot product?” —there are a lot of great answers (which is how I came about this forum). After reading over the replies, it occurred to me: really it’s just because cosine is the “start” of a unit circle.
??
Why do you think that cosine is the "start" of the unit circle?
The "start" of the unit circle would be the point (1, 0) for an angle of 0 (radians). At this point ##\cos(0) = 1## and ##\sin(0) = 0##. So both trig functions are involved, as they are at all points of the unit circle.
Kirkkh said:
Given that —we use the simplest metric (cosine). If we used sine (again, not being at the bringing of the circle) it would add additional information that would need to be subtracted.
Why do you think that cosine is the simpler metric?
In relation to two vectors with components among the reals, there are two definitions for the dot product: a coordinate definition, and a coordinate-free definition.
The coordinate definition is ##u \cdot v = u_1v_1 + u_2v_2 + \dots + u_nv_n##.
The coordinate-free definition is ##u \cdot v = |u||v|\cos(\theta)##, where ##\theta## is the angle between the two vectors.
 
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FAQ: Why is sine not used for dot product?

Why is sine not used for dot product?

The dot product is used to find the projection of one vector onto another. Sine, on the other hand, is used to find the ratio of the opposite side to the hypotenuse in a right triangle. These two operations serve different purposes and therefore, sine is not used for dot product.

Can sine be used for dot product?

Technically, sine can be used for dot product, but it would not give the desired result. The dot product is used to find the scalar projection of one vector onto another, while sine only gives the ratio of the opposite side to the hypotenuse. Therefore, using sine for dot product would not accurately reflect the magnitude of the projection.

What is the difference between sine and dot product?

The main difference between sine and dot product is their purpose. Sine is used to find the ratio of the opposite side to the hypotenuse in a right triangle, while dot product is used to find the scalar projection of one vector onto another. Additionally, sine is a trigonometric function, while dot product is a mathematical operation.

Why is cosine used for dot product instead of sine?

Cosine is used for dot product because it gives the scalar projection of one vector onto another, while sine only gives the ratio of the opposite side to the hypotenuse. This makes cosine a more suitable function for dot product as it accurately reflects the magnitude of the projection.

Can sine and cosine be used interchangeably for dot product?

No, sine and cosine cannot be used interchangeably for dot product. While they both involve trigonometric functions, they serve different purposes. Sine gives the ratio of the opposite side to the hypotenuse, while cosine gives the ratio of the adjacent side to the hypotenuse. Therefore, using them interchangeably would result in different values for the dot product.

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