The Dot Product Angles Between Vectors Orthogonal Vectors Section 6.2a HW: p. 520 13-27 odd.
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Transcript of The Dot Product Angles Between Vectors Orthogonal Vectors Section 6.2a HW: p. 520 13-27 odd.
The Dot ProductThe Dot ProductAngles Between VectorsAngles Between VectorsOrthogonal VectorsOrthogonal Vectors
Section 6.2aHW: p. 520 13-27 odd
Definition: Dot Product
The dot product or inner product of u = u , uand v = v , v is
1 21 2
u v = u v + u v1 21 2
The sum of two vectors is a… vector!vector!
The product of a scalar and a vector is a… vector!vector!
The dot product of two vectors is a… scalar!scalar!
Properties of the Dot Product
Let u, v, and w be vectors and let c be a scalar.
1. u v = v u
2. u u = |u| 2
3. 0 u = 0
4. u (v + w) = u v + u w
(u + v) w = u w + v w
5. (cu) v = u (cv) = c(u v)
Finding the Angle BetweenTwo Vectors (Law of cosines)
vu
v – u
0
2 2v v v u u v u u u v 2 u v cosθ
2 2v u v u u v 2 u v cosθ
2u v 2 u v cosθ
2 2 2 2v 2u v u u v 2 u v cosθ
2 2 2v u u v 2 u v cosθ
u vcosθ
u v
1 u v
θ cosu v
Theorem:Angle Between Two Vectors
vu
v – u
01 u v
θ cosu v
If 0 is the angle between nonzerovectors u and v, then
u vcosθ
u v
and
Definition: Orthogonal Vectors
The vectors u and v are orthogonal ifand only if u v = 0.
The terms “orthogonal” and “perpendicular”The terms “orthogonal” and “perpendicular”are nearly synonymous (with the exceptionare nearly synonymous (with the exception
of the zero vector)of the zero vector)
Guided Practice
Find each dot product
1. 3, 4 5, 2
2. 1, –2 –4, 3
3. (2i – j) (3i – 5j)
= 23= 23
= –10= –10
= 11= 11
Guided Practice
Use the dot product to find the length ofvector v = 4, –3 ((hint: use property 2!!!)hint: use property 2!!!)
Length = 5Length = 5
2v v v
v v v 4 4 3 3
25
Guided Practice
Find the angle between vectors u and v
0 = 55.4910 = 55.491
u = 2, 3 , v = –2, 5
u v 2 2 3 5 112 2u 2 3 13 2 2v 2 5 29
1 11θ cos
13 29
Guided Practice
Find the angle between vectors u and v
u cos i sin j3 3
5 5v 3cos i 3sin j
6 6
1 3,2 2
3 3 3,
2 2
01 3 3 3 3
u v2 2 2 2
3 3 3 3
4 4
Guided Practice
Find the angle between vectors u and v
u cos i sin j3 3
5 5v 3cos i 3sin j
6 6
1 3,2 2
3 3 3,
2 2
0 = 900 = 90
Is there an easier way to solve this???Is there an easier way to solve this???
1θ cos 0
Guided Practice
Prove that the vectors u = 2, 3 andv = –6, 4 are orthogonal
u v = 0!!!u v = 0!!!
u v 2 6 3 4 12 12 0 Check the dot product:
Graphical Support???Graphical Support???
First, let’s look at a brain exercise…Page 520, #30:
Find the interior angles of the triangle with vertices (–4,1),(1,–6), and (5,–1).
Start with a graph…
A(–4,1)
B(5,–1)
C(1,–6)
First, let’s look at a brain exercise…
A(–4,1)
B(5,–1)
C(1,–6)
4 1,1 6CA 55555555555555
5 1, 1 6CB 55555555555555
5,7
4,5
5 4 7 5CA CB 5555555555555555555555555555
15
2 25 7CA 55555555555555
74 2 24 5CB 55555555555555
41
First, let’s look at a brain exercise…
A(–4,1)
B(5,–1)
C(1,–6)
9,2BA 55555555555555
4, 5BC 55555555555555
26BA BC 5555555555555555555555555555
85BA 55555555555555
41BC 55555555555555
1 26cos
85 41B
63.869
180 74.197 63.869A 41.934
Definition: Vector Projection
The vector projection of u = PQ onto a nonzero vector v = PSis the vector PR determined by dropping a perpendicular fromQ to the line PS.
u
P
Q
SR
Thus, u can be broken into components PR and RQ:
u = PR + RQ
v
Definition: Vector Projection
u
P
Q
SR
Notation for PR, the vector projection of u onto v:
PR = proj uv
The formula:
proj u = vvu v|v| 2
Some Practice Problems
Find the vector projection of u = 6, 2 onto v = 5, –5 .Then write u as the sum of two orthogonal vectors, oneof which is proj u.v Start with a graph… v 2u proj u u
u v 6 5 2 5 20
22v 5 5 50
v 2
u vproj u v
v
205, 5
50 2, 2
Some Practice Problems…
Find the vector projection of u = 6, 2 onto v = 5, –5 .Then write u as the sum of two orthogonal vectors, oneof which is proj u.v
u = proj u + u = 2, –2 + 4, 4v 2
Start with a graph…v 2u proj u u
2 vu u proj u
6,2 2, 2
4,4
Some Practice Problems…
Find the vector projection of u = 3, –7 onto v = –2, –6 .Then write u as the sum of two orthogonal vectors, oneof which is proj u.v
u = proj u + u = –1.8,–5.4 + 4.8,–1.6v 2
Some Practice Problems…
Find the vector v that satisfies the given conditions:2u = –2,5 , u v = –11, |v| = 10
1 2v ,v v
1 22 5 11v v 2 21 2 10v v
A system to solve!!!
1 2
5 11
2 2v v
22
2 2
5 1110
2 2v v
2 22 2 2
25 110 12110
4 4 4v v v
Some Practice Problems…
Find the vector v that satisfies the given conditions:2u = –2,5 , u v = –11, |v| = 10
1 2v ,v v
22 2
29 110 810
4 4 4v v
2 22 2 2
25 110 12110
4 4 4v v v
22 229 110 81 0v v
2 21 29 81 0v v 2
811,29
v
v 3, 1 OR 43 81
,29 29