1 國立交通大學應用數學系 莊重教授
§13.3 Arc Length and Curvature
1. Arc length:r(t) x(t),y(t) or x(t),y(t),z(t)
b a b a b a
dt t r
dt dt dy dt
dx dy dx
L
) ( '
2 2
2 2
b a b a b a
dt t r
dt dt dz dt
dy dt
dx
dz dy
dx L
) ( '
2 2
2
2 2
2
(i). 從物理角度看:
r' (t)代表物體在 t 時間的瞬間速度
|r' (t)|代表物體在 t 時間的瞬間速率
|r' (t)|Δt = 小範圍的距離
. )
(
' t dt 代表此物體從t a到t b所走過的距離
br
a
2. Arc length function ( or Distance Function )
. )
( '
. ) ( ' ) (
速度
距離的變化率
t dt r ds
du u r t
s t
a
Example 1:
Find the length of the curve
( ) sin 2 , cos 2 , 2
2, 0 1 .
3
t t t tt r
.
9 (D)16 27
6 13 (C)13 9 (B)13 8 13 27 13
)2 A
(
Solution:
(A)
Example 2:
Let C be a curve described by x = f (t), y = g (t), α≦ t ≦β, where f ' and g ' are continuous on [ α, β ] and C is traversed exactly once as t runs from α to β. Which one of the following is always true?
2 國立交通大學應用數學系 莊重教授
2 2
2 2
2 2
)) ( ) ( ( )) ( ) ( ( )
D (
) C (
) B (
) A (
x x
y y
dt dt dy dt
dx dt dt dy dt dx
dt dt dy dt dx
dt dt dy dt
dx
Solution:
(D) Example 3:
Let the distance traveled by a particle with position
2 4 (D) 2 6 (C) 0 (B) 2 ) A (
? Then
. be 3 to 0 from varies as
) cos , (sin )) ( ), (
( 2 2
d
d t
t t
t t t
y t
x
Solution:
(C)
3. 將一個 curve 用 arc length(s) 來作參數式是一個非常有用的想法和技巧.
(如此的表達方式不隨著不同座標系統而改變)
4. Unit tangent vector:
.
| ) ( '
| ) ( ) '
(
r tt t r
T
3 國立交通大學應用數學系 莊重教授
5. Curvature(曲率):a measure of how quickly the curve changes direction at a given point.
Definition:
ds
dT
曲率
Example 4:
Show that the curvature of a circle with radius a is 1. a Theorem:
(i). 3
| ) ( '
|
| ) (
"
) ( '
| ) ( '
) ( '
t r
t r t r t r
t T dsdtdt dT ds
dT
(ii). Given a plane curve y = f (x), then its curvature κ at a given point x is
1 | " ' ( ( ) )
2|
32) (
x f
x x f
Proof:
(i).
' | |'
T(1)
dtT ds r
r
(2) T'
"
22
dt T ds dt
s
r
d
3 2 2
2 2
2
|'
|
|
"
'
|
|'
|
|'
|
|'
|
"
'
"
|' '
|
|'
|
|'
||
|
"
'
. '
"
' ) 2 ( ) 1 (
r r r r T
r r r dsdt
r T r
dt T T ds dt T
r ds r
T dt T
r ds r
6. Principal unit normal vector N (t).
| . ) ( '
| ) ( ) '
(
T tt t T
N
4 國立交通大學應用數學系 莊重教授
7. Binormal vector B (t).
) ( ) ( )
(
t T t N tB