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Vectors and Polar Coordinates

Lecture 2 (04 Nov 2006)

Enrichment Programme for Physics Talent 2006/07Module I

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2.1 Vectors and scalars2.2 Matrix operations of

rotations2.3 Polar coordinates

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Vector: quantity having both magnitude and direction, e.g., displacement, velocity, force, acceleration, …

Scalar: quantity having magnitude only, e.g., mass, length, time, temperature, …

2.1 Vectors and scalars

A

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Fundamental definitions:

Two vectors and are equal if they have the same magnitude and direction regardless of the initial points

Having direction opposite to but having the same magnitude

2.1 Vectors and scalars

A

B

A

B

A

A

A

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Addition:

subtraction:

2.1 Vectors and scalars

C A B

A

B

A B

A

C

A

B

C A B

B

C

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Laws of vector

2.1 Vectors and scalars

1.

2.

3.

4.

( ) ( )

( ) ( )

(5.

)

(6. )

A B B A

A B C A B C

A A

A A

A A A

A B A B

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Null vector: vector with magnitude zero

Unit vector: vector with unit magnitude, i.e., .

Rectangular unit vectors , and .

, (x, y, z) are different components of the vector .Magnitude of : 2 2 2A x y z

2.1 Vectors and scalars

aA A

unit vector

i j k

i

j

kˆˆ ˆA xi yj zk

A

A

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Example: Find the magnitude and the unit vector of a vector ˆˆ ˆ2A i j k

Magnitude: 2 2 2( 1) 2 ( 1) 6A

Unit vector:

1 2 1 ˆˆ ˆˆ6 6 6

Aa i j k

A

ˆA Aa

Write: , where

2.1 Vectors and scalars

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Dot and cross productDot product: , where is the angle between vectors and .

Laws of dot product:

2.1 Vectors and scalars

cosA B AB

A

B

1 2 3 1 2 3

1 1 2 2 3 3

( )

ˆ ˆ ˆ ˆˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ1 0

ˆ ˆˆ ˆ ˆ ˆ

;

and

1.

2.

3.

4.

A B B A

A B C A B A C

i i j j k k i j j k k i

A Ai A j A k B B i B j B k

A B A B A B A B

A

B

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2. and

Example: Evaluate the dot product of vectors ˆˆ ˆ2A i j k

1. and ˆˆ ˆ2 3B i j k

ˆˆ ˆ 3A i j k ˆˆ ˆ3 2B i j k

2 6 1 5A B

1.

2. 1 3 6 4A B

2.1 Vectors and scalars

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Dot and cross product

cross product: , where is the angle between vectors and . is a unit vector such that , and form a right-handed system.

2.1 Vectors and scalars

ˆ sinA B cAB

A

B

cA

B

c

A

B

ˆ sincAB

A

B

area of the parallelogram

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Dot and cross product

Laws of cross product:

2.1 Vectors and scalars

( )

ˆ ˆˆ ˆ ˆ ˆ 0

ˆ ˆ ˆˆ ˆ ˆ ˆ ˆ ˆ

A B B A

A B C A B A C

i i j j k k

i j k j k i k i j

;

, ,

1.

2.

3.

i

j

k

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2. and

Example: Evaluate the cross product of vectors

ˆˆ ˆ2A i j k

1. and ˆˆ ˆ2 3B i j k

ˆˆ ˆ 3A i j k ˆˆ ˆ3 2B i j k

ˆˆ ˆ3 7A B i j k

1.

2. ˆˆ ˆ11 4A B i j k

2.1 Vectors and scalars

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2.2 Matrix operations of rotations

a vector in a 2-dimensional plane can be written as ,

and are called the basis vector, since any vector can be written as a linear combination of the basis vector

1 2ˆ ˆ

v v i v j

i j

i

j(v1, v2)

Vectors in 2-dimensions

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2.2 Matrix operations of rotations

any vector in R2 can be written as

and are called the base vectors, since any vector can be written as a linear combination of the base vectors, namely

Is base vectors unique?

1 2ˆ ˆ

v v i v j

i j

Vectors in 2-dimensions

1 2ˆ ˆ

v v i v j

base vectors are not unique!

i

ˆ 'j

(v1’, v2

’)ˆ 'i

(v1, v2)j

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Hence, and are example of orthonormal base vectors.

Generally, let and are base vectors, i.e. 1 1 2 2ˆ ˆ v v e v e

1e 2e

Base vectors are said to be orthonormal if

1 1 2 2

1 2

ˆ ˆ ˆ ˆ 1

ˆ ˆ 0

e e e e

e e

i j

2.2 Matrix operations of rotationsVectors in 2-dimensions

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1e

2ˆe

(v1’, v2

’)1e

(v1, v2)

2eLet both and are orthonormal base vectors, i.e.,

1 2ˆ ˆ( , )e e 1 2ˆ ˆ( , ) e e

11 1 2 2 1 22ˆ ˆ ˆˆ v v e e vev ev

using different coordinate system to represent is possible.since 1 1 1 2 2

2 1 1

1 1 1

2 222

ˆ ˆ

ˆ ˆ

ˆ ˆ

ˆ ˆ

v v e e

e e

e v e

v v e v eHow to express them in matrix

form?

2.2 Matrix operations of rotationsVectors in 2-dimensions

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1 1 2 1

2 1 2 2

1 1 1

2

1 1

2 2

1 1

2 2

2

ˆ ˆ

ˆ ˆ

cos sin

sin cos

cos sin

sin cos

ˆ ˆ

ˆ ˆ

v e e v

v e e v

v v

v v

v v

v v

e e

e e

or in matrix form:

Note are orthogonal.

2.2 Matrix operations of rotationsVectors in 2-dimensions

2ˆe

(v1’, v2

’)1e

(v1, v2)

2e

1e

cos sin cos sin

sin cos sin cos

and

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1 1

2 2

v vR

v v

Hence, an orthogonal matrix R acts as transformation to transforms a vector from one coordinates to another, i.e.,

2.2 Matrix operations of rotationsVectors in 2-dimensions

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2.3 Polar coordinates

The position of the “Red Point” can be represented by (r, ) instead of (x, y) in Cartesian Coordinates.

r = magnitude of the position vector r

= angle of the position vector and the x-axis x

y

O

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2.3 Polar coordinates

r

In Polar Coordinates, we define two new base vectors instead of in Cartesian Coordinates.

ˆˆ,r ˆ ˆ,i j

: a unit vector in the direction of increasing r (i.e. -direction)

r

r

: a unit vector in the direction of increasing

y

xO

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2.3 Polar coordinates

r

Any vector on the 2D plane can be expressed in terms of and :

y

x

r

ˆˆrV V r V

In particular, the position vector is given by

r

ˆr rr

O

V

rV

V

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2.3 Polar coordinates

Conversions between Polar Coordinates (r, ) and Cartesian Coordinates (x, y):

Cartesian Coordinates:

,x y , ,x y

Cylindrical Coordinates:

,r 0, 0 2r

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2.3 Polar coordinates

Conversions between Polar Coordinates (r, ) and Cartesian Coordinates (x, y):

:, ,r x y cos ,x r siny r

:

2 2r x y tany

x

:, ,r x y

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2.3 Polar coordinates

Conversions between Polar Coordinates (r, ) and Cartesian Coordinates (x, y):

:

ˆ ˆcos sinˆ ir j

ˆ ˆsin cosˆ i j

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2.3 Polar coordinates

ˆˆrV V r V

Differentiating a vector in Polar Coordinates (r, ):

:

ˆˆ ˆˆrr

dVdVdV dr dr V V

dt dt dt dt dt

ˆ ˆˆ ˆsin cosdr d d d

i jdt dt dt dt

ˆˆ rr

dVdVdV d dr V V

dt dt dt dt dt

ˆˆ ˆ ˆcos sin

d d d di j r

dt dt dt dt

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2.3 Polar coordinates

Central Force Field Problem:

ˆ( ) ( )F r F r r

ˆ( ) ( ) 0dL

N r F r r rF rdt

External Torque = 0:

Conservation of Angular Momentum L

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Recall: momentum , where m is the mass, is a measure of the linear motion of an object.

The angular momentum of an object is defined as: a measure of the rotational motion of an object.

Box 2.1 Angular momentum

L r p

p mv

r

p

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As linear momentum, an object keeps its motion unless an external force is acted;

An object has a tendency to keep rotating unless external torque is acted. It is the conservation of angular momentum.

Box 2.1 Angular momentum

The conservation of angular momentum explains why the Earth always rotates once every 24 hours.

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Area swept out in a very small time interval:

m

L

t

A

tvmrm

A

tvrA

2

2

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1

2.3 Polar coordinates

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2.3 Polar coordinates In general, planets’ orbits are elliptical To describe its motion,

r

v

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is constant if angular momentum

is conserved and m is unchanged.m

L

t

A

2

2.3 Polar coordinates

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This is in fact one of his famous three laws of planetary motion, which are deduced from Tycho’s 20 years observation data.

Johannes Kepler ( 開普勒 ) 1571 - 1630

2.3 Polar coordinates

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The second law of planetary motion: equal time sweeps equal area

closer to the sun, planet moves faster

farther away from the sun, planet moves slower

2.3 Polar coordinates

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Coordinates Systems in 3D Space

Cartesian Coordinates:

, , ,x y z

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Coordinates Systems in 3D Space

Cylindrical Coordinates:

0, 0 2 , ,z

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Coordinates Systems in 3D Space

Spherical Coordinates:

0, 0 2 , 0r