• Introduce Interactive Learning Segments and try a few
• The Language of Vectors:
– Understand conventions used in denoting vectors.
– Visualize vector operations both from the geometric and algebraic point of view.
• Dot Products:
– Understand geometric and algebraic formulation of the vector dot product.
– Understand some properties of the dot product
Phy 221 2005S Lecture 2

• A scalar quantity is one that is represented by a single number (e.g. Mass Length time temperature volume…)
• A vector is a quantity which has both magnitude and direction (e.g. displacement, velocity, force)
• Magnitude: How long is the vector
• Direction: angle counterclockwise from x-axis or other sensible description.
• Geometrically: we represent a vector as an arrow
Phy 221 2005S Lecture 2

• Equal Vectors: vectors are equal if they have the same magnitude and direction regardless of where the vector
“starts”

A

A


B 
B
• Opposite Vectors:
Vector are opposite if the magnitude is the same but direction is opposite
• Unit Vectors:

A 
A

A
 

B 
B

A + B = C
Geometrically: Parallel transport the tail of B to the head of A. The sum goes from the tail of A to the head of B.
Algebraically: Add the components
C(3,5)
B(-2,4) A 5 1
B -2 4
C 3 5
A (5,1) Vector addition is commutative and associative
Phy 221 2005S Lecture 2

• Get your clickers ready
• I will post a question, you should talk to your neighbor and “vote” on the correct answer with your clicker
• You can change your answer if you change your mind.
• When you click, a box with your clicker number will appear
• At the end of the time, a bar graph will show the summary of results.
Phy 221 2005S Lecture 2

ACT: Vector addition
• All the vectors below have the same magnitude. Which of the following arrangements will produce the largest resultant when the two vectors are added?
1. 2. 3.
Phy 221 2005S Lecture 2

• To keep straight which variables are scalar quantities, it is conventional to draw a little arrow over vector variables. A B C
• If A is a vector, we use A (without arrow) to denote the magnitude.
• Sometimes boldface is used for vector and normal font for magnitude. Thus vector=A; magnitude=A
• More formally, we can indicate magnitude of a vector by vertical bars.
• Thus A=|A|.
Phy 221 2005S Lecture 2

• Geometric:
C
A
• Algebraic: A x
+B x
A y
+B y
A z
+B z
=C x
=C y
=C z
B
A+B=C
• Subtraction:
B
D
A-B=D
A
-B
Phy 221 2005S Lecture 2
Note two ways to think:
 D is A plus –B
 D goes from tip of B to tip of A when A and B are rooted at a common point

• The components of a vector can be thought of as the projections along the coordinate axes. These are sometimes called the Cartesian coordinates:
• We can denote A in terms of its components as: A=(A x
,A y
,A z
) y
A y
A
A x

• A unit vector is a vector of length 1. To indicate a vector is a unit vector, we put a hat on it:
A denotes a unit vector parallel to
• Some special unit vectors

A
– The unit vector that points along the x axis is denoted i ^ z y j
^
A i
^
Any vector can be written in terms of these basic unit vectors
If A=(A x
,A y
,A 
A z
) then

A x i
ˆ 
A y j
ˆ 
A z k
ˆ x
Phy 221 2005S Lecture 2

• In 2 dimensions, one can also describe a vector by its magnitude and direction.
• The direction is the angle  taken counterclockwise from the x axis
| A |

A x
2 
A y
2
A x
A y

|

|
A | cos
A | sin


  arctan


A y
A x


|A|
A
A x
A

How do I remember which is sin and which is cos?
 arccos
Phy 221 2005S Lecture 2

• Dot Products are the workhorse of vector analysis
Definition
Algebraic Geometric

A
•In terms of components:


B

A x
B x

A y
B y

A z
B z
•If  is the angle between A and B:

A


B

| A || B | cos

Where does this come from?
B

A
Phy 221 2005S Lecture 2

• The dot product takes two vectors as inputs and produces a scalar as output.
• The dot product is commutative: A•B=B•A
• Dot product distributes over vector addition:

A

(


B


C )


A


B

• In particular, the components
A


A


C i, j and k
A x


A
 i

A y


A

 j A z


A
 k
ˆ
• The length of a vector can be expressed in terms of the dot product: |A|²=A•A
Phy 221 2005S Lecture 2

• Vectors that are going in the same direction

A


B

AB
• Vectors that are going in opposite directions

A


B
 
AB
Remember
• Perpendicular vectors

A


B

0 vector #1 vector #2
Note: Later in the course we will learn about another kind of product called the “cross product” which takes in two vectors and spits out another vector. Do not get them confused.
Phy 221 2005S Lecture 2
Dot Product
Scalar

• We can use the geometric definition of the dot product to determine the angle between two vectors:

A


B


A

| || B | cos
 cos
 
|

A
A

||

B
B |
• This tells the angle between A and B but not the direction of the angle
B

A
Phy 221 2005S Lecture 2

Example of Vector Algebra:
Law of cosines
Consider a triangle, The sum of the vectors representing the sides is 0
C

B p
A
C=-(A+B) C²=(A+B)²=(A+B)·(A+B)
=A²+B²+2A·B
=A²+B²+2AB cos( p
)
=A²+B²-2AB cos( 
)
Binomial expansion of dot product: a useful trick-learn it!
Phy 221 2005S Lecture 2