The Dot Product: Angles and Projections
One product of two vectors returns a number that encodes the angle between them. It tests perpendicularity and splits one vector into parts along and across another.
What a learner can do afterwards
- Find the angle between two vectors from their dot product
- Decide perpendicularity from a dot product of zero
- Resolve a vector into components parallel and perpendicular to another
1 · Read
The dot product turns two vectors into one number: multiply matching components and add. For (2, 5) and (4, 1) you get 8 + 5 = 13. The same number equals the length of the first vector times the length of the second times the cosine of the angle between them. So divide the dot by both lengths to get the cosine, then take the inverse cosine. A positive dot means an acute angle, zero means a right angle, and negative means obtuse.
A zero dot product from two nonzero vectors means the vectors are perpendicular. Check (3, 1) and (-1, 3): 3 times -1 plus 1 times 3 is 0, so they meet at a right angle. By contrast, (3, 1) and (1, 3) dot to 3 + 3 = 6, so their angle is acute.
To find how much of a vector u runs along a direction v, divide v by its length, then dot u with that unit vector. The direction (3, 4) has length 5, so its unit vector is (3/5, 4/5), and (2, 1) has component 6/5 + 4/5 = 2 along it. The projection vector is the component times the unit vector: (6/5, 8/5). The shortcut (u dot v over v dot v) times v gives the same vector: 10/25 times (3, 4).
The part across the direction is what is left. For (2, 1) and the direction (3, 4), subtract the projection: (2, 1) minus (6/5, 8/5) is (4/5, -3/5). Check it: (4/5, -3/5) dotted with (3, 4) is 12/5 - 12/5 = 0, so this part is perpendicular.
For angles, find the dot from components, divide by both lengths to get the cosine, then take the inverse cosine. If the dot is zero and neither vector is zero, the angle is 90 degrees.
Multiply matching parts and add, read the angle from the cosine, and split vectors with projection.
The lesson and quiz items were written for this dataset. The lesson sections and quiz item q5 were later revised.
Extra items x1 to x8 were written for this dataset. The rewritten readings were written for this dataset. Videos are from Khan Academy.
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