equations of motion: normal and tangential coordinates ... files/fall 2007... · normal and...
TRANSCRIPT
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EQUATIONS OF MOTION:
NORMAL AND TANGENTIAL COORDINATES
Today’s Objectives:
Students will be able to:
1. Apply the equation of
motion using normal and
tangential coordinates.
2. Analyze the kinetics of a
particle using cylindrical
coordinates.
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APPLICATIONS
Race tracks are often banked in the turns to reduce the frictional
forces required to keep the cars from sliding at high speeds.
If the car’s maximum velocity and a minimum coefficient of
friction between the tires and track are specified, how can we
determine the minimum banking angle (θ) required to prevent
the car from sliding?
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APPLICATIONS
The forces acting on the 100-lb boy can be analyzed using the
cylindrical coordinate system.
If the boy slides down at a constant speed of 2 m/s, can we
find the frictional force acting on him?
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APPLICATIONS
(continued)
Satellites are held in orbit around the earth by using the earth’s
gravitational pull as the centripetal force – the force acting to
change the direction of the satellite’s velocity.
Knowing the radius of orbit of the satellite, how can we
determine the required speed of the satellite to maintain this
orbit?
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APPLICATIONS
(continued)
When an airplane executes the vertical loop shown above, the
centrifugal force causes the normal force (apparent weight)
on the pilot to be smaller than her actual weight.
If the pilot experiences weightlessness at A, what is the
airplane’s velocity at A?
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n – t Coordinates
Normal and Tangential Components
Cylindrical Coordinates
Use when the particle moves in a
curved path which is known.
Use when all the forces acting on a
particle can be resolved into
cylindrical components.
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n – t Coordinates
Normal and Tangential Components
Cylindrical Coordinates
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n – t Coordinates
Normal and Tangential Components
Cylindrical Coordinates
Can be in x - y or n – t coordinates.
Coordinates are in r, θ, and z.
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n – t Coordinates
Normal and Tangential Components
bntbbnntt amamamuFuFuF
amFrrrrrr
rr
++=++
=
∑∑∑∑
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n – t Coordinates
Normal and Tangential Components
bntbbnntt amamamuFuFuF
amFrrrrrr
rr
++=++
=
∑∑∑∑
Force) l(Tangentia tt maF =∑dv/dt or v dv/ds: Time rate of change in the magnitude of the
velocity.
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n – t Coordinates
Normal and Tangential Components
bntbbnntt amamamuFuFuF
amFrrrrrr
rr
++=++
=
∑∑∑∑
Force) l(Tangentia tt maF =∑When acting in the direction of the motion, the particle speeds
up.
If the Tangential Force acts in
the direction opposite the
motion, the particle slows down.
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n – t Coordinates
Normal and Tangential Components
bntbbnntt amamamuFuFuF
amFrrrrrr
rr
++=++
=
∑∑∑∑
Force) l(Tangentia tt maF =∑
Force) (Normal nn maF =∑ν²/ρ: Time rate of change in the velocity’s direction.
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n – t Coordinates
Normal and Tangential Components
bntbbnntt amamamuFuFuF
amFrrrrrr
rr
++=++
=
∑∑∑∑
Force) l(Tangentia tt maF =∑
Force) (Normal nn maF =∑This vector always acts towards the center of curvature.
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n – t Coordinates
Normal and Tangential Components
bntbbnntt amamamuFuFuF
amFrrrrrr
rr
++=++
=
∑∑∑∑
Force) l(Tangentia tt maF =∑
Force) (Normal nn maF =∑This is also called the centripetal force.
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n – t Coordinates
Normal and Tangential Components
bntbbnntt amamamuFuFuF
amFrrrrrr
rr
++=++
=
∑∑∑∑
Force) l(Tangentia tt maF =∑
Force) (Normal nn maF =∑
Force) (Binormal bb maF =∑
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n – t Coordinates
Normal and Tangential Components
bntbbnntt amamamuFuFuF
amFrrrrrr
rr
++=++
=
∑∑∑∑
Force) l(Tangentia tt maF =∑
Force) (Normal nn maF =∑
Force) (Binormal bb maF =∑
In this class, it’s always zero!
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n – t Coordinates
Normal and Tangential Components
bntbbnntt amamamuFuFuF
amFrrrrrr
rr
++=++
=
∑∑∑∑
Force) l(Tangentia tt maF =∑
Force) (Normal nn maF =∑
If the path is defined as y = f(x),
the radius of curvature at the point where the particle is located can be obtained from:
( )[ ]22
2/32
/
/1
dxyd
dxdy+=ρ
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n – t Coordinates
Normal and Tangential Components
Cylindrical Coordinates
ntnntt amamuFuF
amFrrrr
rr
+=+
=
∑∑∑
zzrr
zzrr
umaumauma
uFuFuF
amF
rrr
rrr
rr
++
=++
=
∑∑∑∑
θθ
θθ
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n – t Coordinates
Normal and Tangential Components
bntbbnntt amamamuFuFuF
amFrrrrrr
rr
++=++
=
∑∑∑∑
Force) l(Tangentia tt maF =∑
Force) (Normal nn maF =∑
Force) (Binormal bb maF =∑
Cylindrical Coordinates
zzrr
zzrr
umaumauma
uFuFuF
amF
rrr
rrr
rr
++
=++
=
∑∑∑∑
θθ
θθ
zz maF =∑
)( 2θ&&& rrmmaF rr −==∑
)2( θθθθ&&&& rrmmaF −==∑
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n – t Coordinates
Normal and Tangential Components
Cylindrical Coordinates
bntbbnntt amamamuFuFuF
amFrrrrrr
rr
++=++
=
∑∑∑∑
Force) l(Tangentia tt maF =∑
Force) (Normal nn maF =∑
Force) (Binormal bb maF =∑
zzrr
zzrr
umaumauma
uFuFuF
amF
rrr
rrr
rr
++
=++
=
∑∑∑∑
θθ
θθ
)( 2θ&&& rrmmaF rr −==∑)2( θθθθ&&&& rrmmaF −==∑
zz maF =∑Normal Force N is always perpendicular to
the tangent of the path.
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n – t Coordinates
Normal and Tangential Components
Cylindrical Coordinates
bntbbnntt amamamuFuFuF
amFrrrrrr
rr
++=++
=
∑∑∑∑
Force) l(Tangentia tt maF =∑
Force) (Normal nn maF =∑
Force) (Binormal bb maF =∑
zzrr
zzrr
umaumauma
uFuFuF
amF
rrr
rrr
rr
++
=++
=
∑∑∑∑
θθ
θθ
Frictional Force F always acts opposite the
motion, tangentially.
)( 2θ&&& rrmmaF rr −==∑)2( θθθθ&&&& rrmmaF −==∑
zz maF =∑
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n – t Coordinates
Normal and Tangential Components
Cylindrical Coordinates
bntbbnntt amamamuFuFuF
amFrrrrrr
rr
++=++
=
∑∑∑∑
Force) l(Tangentia tt maF =∑
Force) (Normal nn maF =∑
Force) (Binormal bb maF =∑
zzrr
zzrr
umaumauma
uFuFuF
amF
rrr
rrr
rr
++
=++
=
∑∑∑∑
θθ
θθ
Ψ specifies the directions of N and F relative to the radial line.
)( 2θ&&& rrmmaF rr −==∑)2( θθθθ&&&& rrmmaF −==∑
zz maF =∑
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DETERMINATION OF ANGLE ψ
The angle ψ, defined as the angle
between the extended radial line
and the tangent to the curve, can be
required to solve some problems. It
can be determined from the
following relationship.
θ
θψddr
r
dr
r d ==tan
If ψ is positive, it is measured counterclockwise from the radial
line to the tangent. If it is negative, it is measured clockwise.
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EXAMPLE
Given:At the instant θ = 60°, the boy’s
center of mass G is momentarily at
rest. The boy has a weight of 60 lb.
Neglect his size and the mass of the
seat and cords.
Find: The boy’s speed and the tension
in each of the two supporting
cords of the swing when θ = 90°.
1) Since the problem involves a curved path and finding the
force perpendicular to the path, use n-t coordinates. Draw
the boy’s free-body and kinetic diagrams.
2) Apply the equation of motion in the n-t directions.
3) Use kinematics to relate the boy’s acceleration to his speed.
Plan:
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EXAMPLE
(continued)Solution:
1) The n-t coordinate system can be established on the
boy at some arbitrary angle θ. Approximating the boy
and seat together as a particle, the free-body and
kinetic diagrams can be drawn.
T = tension in each cord
W = weight of the boy
n
t
man
mat
Kinetic diagram
W
n
t
2T θ
Free-body diagram
=
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EXAMPLE
(continued)2) Apply the equations of motion in the n-t directions.
Note that there are 2 equations and 3 unknowns (T, v, at).
One more equation is needed.
Using an = v2/ρ = v2/10, w = 60 lb, and m = w/g = (60/32.2),
we get: 2T – 60 sin θ = (60/32.2)(v2/10) (1)
(b) ∑Ft = mat => W cos θ = mat
=> 60 cos θ = (60/32.2) at
Solving for at: at = 32.2 cos θ (2)
(a) ∑Fn = man => 2T – W sin θ = man
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EXAMPLE
(continued)
v dv = at ds where ds = ρ dθ = 10 dθ
3) Apply kinematics to relate at and v.
This v is the speed of the boy at θ = 90°. This value can be
substituted into equation (1) to solve for T.
2T – 60 sin(90°) = (60/32.2)(9.29)2/10
T = 38.0 lb (the tension in each cord)
=> v dv = 32.2 cosθ ds = 32.2 cosθ (10 dθ )
=> v dv = 32.2 cosθ dθ
=> = 32.2 sinθ => v = 9.29 ft/s
∫∫90
600
v
90
60
v2
2
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Example
Given: A 200 kg snowmobile
with rider is traveling down the
hill. When it is at point A, it is
traveling at 4 m/s and increasing
its speed at 2 m/s2.
Find: The resultant normal
force and resultant frictional
force exerted on the tracks at
point A.
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GROUP PROBLEM SOLVING
Given: A 200 kg snowmobile with
rider is traveling down the
hill. When it is at point A, it
is traveling at 4 m/s and
increasing its speed at 2 m/s2.
Find: The resultant normal force and resultant frictional force
exerted on the tracks at point A.
Plan: 1) Treat the snowmobile as a particle. Draw the free-
body and kinetic diagrams.
2) Apply the equations of motion in the n-t directions.
3) Use calculus to determine the slope and radius of
curvature of the path at point A.
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Given: A 200 kg snowmobile with rider is
traveling down the hill. When it is
at point A, it is traveling at 4 m/s
and increasing its speed at 2 m/s2.
Find: The resultant normal force and
resultant frictional force exerted on
the tracks at point A.
L6
The n-t coordinate system can be established on the snowmobile at point A. Treat the
snowmobile and rider as a particle and draw the free-body and kinetic diagrams:
W = mg = weight of snowmobile and passenger
N = resultant normal force on tracks
F = resultant friction force on tracks
tn
matman
t
θ
θn
N
F
W
=
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GROUP PROBLEM SOLVING
(continued)
2) Apply the equations of motion in the n-t directions:
∑ Ft = mat => W sinθ – F = mat
∑ Fn = man => W cos θ – N = man
Using W = mg and an = v2/ρ = (4)2/ρ
=> (200)(9.81) cos θ – N = (200)(16/ρ)
=> N = 1962 cosθ – 3200/ρ (1)
Using W = mg and at = 2 m/s2 (given)
=> (200)(9.81) sin θ – F = (200)(2)
=> F = 1962 sinθ – 400 (2)
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3) Determine ρ by differentiating y = f(x) at x = 10 m:
Determine θ from the slope of the curve at A:
From Eq.(1): N = 1962 cos(56.31) – 3200/19.53 = 924 N
From Eq.(2): F = 1962 sin(56.31) – 400 = 1232 N
y = -5(10-3)x3 => dy/dx = (-15)(10-3)x2 => d2y/dx2 = -30(10-3)x
tan θ = dy/dx
θ = tan-1 (dy/dx) = tan-1 (-1.5) = 56.31°
x = 10 mθ
dy
dx
ρ = =
[1 + ( )2]3/2dy
dx
d2y
dx2
[1 + (-1.5)2]3/2
-0.3x = 10 m
GROUP PROBLEM SOLVING
(continued)
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EXAMPLE
Plan: Draw a FBD. Then develop the kinematic equations
and finally solve the kinetics problem using
cylindrical coordinates.
Solution: Notice that r = rc cosθ, therefore:
r = -2rc sinθ θ
r = -2rc cosθ θ2 – 2rc sinθ θ
.
.. ..
.
.
Given: The ball (P) is guided along
the vertical circular path.
W = 0.5 lb, θ = 0.4 rad/s,
θ = 0.8 rad/s2, rc = 0.4 ft
Find: Force of the arm OA on the
ball when θ = 30°.
.
..
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EXAMPLE
Given: The ball (P) is guided along the
vertical circular path.
W = 0.5 lb,
= 0.4 rad/s,
= 0.8 rad/s2,
rc = 0.4 ft
Find: Force of the arm OA on the ball when
θ = 30°.
.
θ&
θ&&
°=30θ
t r
θ
n
=
θ
ψθ
θ
θ
θ
mg
Ns NOA
θma
rma
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EXAMPLE
(continued)
Free Body Diagram: Establish the inertial coordinate
system and draw the particle’s free body diagram. Notice that
the radial acceleration is negative.
θ,r
°=30θ
t r
θ
n
=
θ
ψθ
θ
θ
θ
mg
Ns NOA
θma
rma
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Kinematics: at θ = 30°
r = 2(0.4) cos(30°) = 0.693 ft
r = -2(0.4) sin(30°)(0.4) = -0.16 ft/s
r = -2(0.4) cos(30°)(0.4)2 – 2(0.4) sin(30°)(0.8) = -0.431 ft/s2..
.
Acceleration components are
ar = r – rθ2 = -0.431 – (0.693)(0.4)2 = -0.542 ft/s2
aθ = rθ + 2rθ = (0.693)(0.8) + 2(-0.16)(0.4) = 0.426 ft/s2
.. .
....
tan ψ = r/(dr/dθ) where dr/dθ = -2rc sinθ
tan ψ = (2rc cosθ)/(-2rc sinθ) = -1/tanθ ∴ ψ = 120°
EXAMPLE
(continued)
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Kinetics:
∑ Fr = mar
Ns cos(30°) – 0.5 sin(30°) = (-0.542)
Ns = 0.279 lb
0.5
32.2
∑ Fθ = maθ
NOA + 0.279 sin(30°) – 0.5 cos(30°) = (0.426)
NOA = 0.3 lb
0.5
32.2
EXAMPLE
(continued)
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GROUP PROBLEM SOLVING
Given: A plane flies in a vertical loop
as shown.
vA = 80 ft/s (constant)
W = 130 lb
Find: Normal force on the pilot at A.
)2cos(600 θ−=rKinematics:
r θθ·· )2sin(1200=
r·
θθθθ···· )2sin(1200)2cos(2400
2 +=
At A )90( °=θ 0=r·
Solution:
Plan: Determine θ and θ from the velocity at A and by
differentiating r. Solve for the accelerations, and
apply the equation of motion to find the force.
. ..
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GROUP PROBLEM SOLVING
(continued)
· ··2
2267.42)133.0(2400)180sin(1200)180cos(2400
sft
r −=−=°+°= θθ··
222
33.53)133.0(60067.42s
ftrra r
−=−−=−= θ·· ·
Therefore r θθ··· rrv A
=+= 22)()(
Since ftr 600= at A,s
rad133.0600
80==θ
·
Since vA is constant, aθ = rθ + 2rθ = 0 => θ = 0· ·.. ..
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GROUP PROBLEM SOLVING
(continued)
Notice that the pilot would experience weightlessness when his
radial acceleration is equal to g.
Kinetics: ∑ Fr = mar => -mg – N = mar
N = -130 – (53.3) => N = 85.2 lb130
32.2
Free Body Diagram & Kinetic Diagram
θ
rN
mg
rma
=
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n – t Coordinates
Normal and Tangential Components
bntbbnntt amamamuFuFuF
amFrrrrrr
rr
++=++
=
∑∑∑∑
Force) l(Tangentia tt maF =∑
Force) (Normal nn maF =∑
Force) (Binormal bb maF =∑
dv/dt: Time rate of change in the magnitude of the velocity.
ν²/ρ: Time rate of change in the velocity’s direction.
In this class, it’s always zero!
Cylindrical Coordinates
zzrr
zzrr
umaumauma
uFuFuF
amF
rrr
rrr
rr
++
=++
=
∑∑∑∑
θθ
θθ
zz
rr
maF
maF
maF
=
=
=
∑∑∑
θθ
Normal Force N is always perpendicular to
the tangent of the path.
Frictional Force F always acts opposite the
motion, tangentially.
Ψ specifies the directions of N and F
relative to the radial line.