06.07.2015 Views

Textbook pdf's

Textbook pdf's

Textbook pdf's

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

Perhaps the most important observation to be made about the dot product is that when<br />

two nonzero vectors are perpendicular, their dot product is always 0. This will have<br />

many important applications in Chapter 8, when we discuss lines and planes.<br />

EXAMPLE 1<br />

Calculating the dot product of two geometric vectors<br />

Two vectors, a ! and b ! , are placed tail to tail and have magnitudes 3 and 5,<br />

respectively. There is an angle of between the vectors. Calculate a ! !<br />

120°<br />

# b .<br />

Solution<br />

Since 0 a ! 0 3, @ b ! @ 5, and cos 120° 0.5,<br />

a ! # b<br />

!<br />

13215210.52<br />

7.5<br />

Notice that, in this example, it is stated that the vectors are tail to tail when taking<br />

the dot product. This is the convention that is always used, since this is the way of<br />

defining the angle between any two vectors.<br />

INVESTIGATION A.Given vectors a ! and b !<br />

where , and the angle between the<br />

vectors is calculate a ! b ! 0a ! 0 5 b ! 8<br />

60°, # .<br />

B. For the vectors given in part A calculate b ! # a ! . What do you notice?<br />

a<br />

Will this relationship always hold regardless of the two vectors used and the<br />

60°<br />

measure of the angle between them? Explain.<br />

C. For the vectors given in part A calculate a ! # a ! and . Based on your<br />

observations, what can you conclude about u ! u ! b ! # b !<br />

# for any vector u ! ?<br />

D.Using the vectors given in part A and a third vector c ! , 0c ! a<br />

0 4, as shown in the<br />

diagram, calculate each of the following without rounding:<br />

! !<br />

i. iii. a ! !<br />

#<br />

!<br />

60°<br />

b c 1b c 2<br />

20°<br />

ii. the angle between iv.<br />

b ! c ! and a !<br />

a ! !<br />

b<br />

# b a! #<br />

! c<br />

c<br />

.<br />

E. Compare your results from part iii and iv in part D. What property does this<br />

demonstrate? Write an equivalent expression for c ! #<br />

! !<br />

1a b 2 and confirm it<br />

a<br />

using the appropriate calculations with the vectors given in part D.<br />

F. Using the 3 vectors given above, explain why .<br />

G.A fourth vector d ! , d ! 1a ! !<br />

# b 2 #<br />

! c a! # ! 1b #<br />

!<br />

60°<br />

c 2<br />

d 20°<br />

b<br />

3, is given as shown in the diagram. Explain why<br />

c<br />

a ! # d<br />

!<br />

a! # 1d<br />

!<br />

2 1a<br />

! 2 # d<br />

!<br />

1a<br />

! 2 # 1d<br />

!<br />

2<br />

H.Using the vectors given, calculate , and c ! !<br />

a ! # 0 !<br />

b ! # 0 ! # 0 . What does this imply?<br />

372 7.3 THE DOT PRODUCT OF TWO GEOMETRIC VECTORS<br />

NEL

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!