Upcoming Deadlines

1
Upcoming Deadlines

• Fifth Homework (Video analysis of Path of
  Action)
• Due Friday, September 25th (This Friday)
• 15 points (10 points if late)
• Sixth Homework (Outline of First Term Paper)
• Due Wednesday, September 30th
• Campus-wide Furlough Day
• Monday, October 19th
• (Art/Phys 123 will meet on Wed., Oct. 21st)
• For full schedule, visit course website
• ArtPhysics123.pbworks.com

2
Activating your Clicker

• Turn on your clicker.
• Enter the number or letter that I give you for
  joining this class. Hit Enter/Send key.
• Clicker should read PHY123SCI2
• Type in your student ID hit Enter/Send.
• Clicker is now ready to use.

Hit any key to wake the clicker from sleep mode.
3
Homework Assignment 5

• Shoot reference of yourself doing a running jump.
  
• Use the Tracker software to mark your path of
  action while in the air (tracking center of
  torso).
• Upload image showing track and graphs for one of
  your jumps to your blog.
• Record a clip showing the tracked motion.
• Upload both original reference and the tracked
  Quicktime clip to your blog.
• For details, see course website for an example,
  see the course blog.
• This assignment is due by 8am on Friday,
  September 18th (This Friday). 15 points

4
Homework Assignment 6

• Write a one-page outline for your first term
  paper.
• The general topic for your first term paper is
• The Laws of Physics in an Animation Universe
• Modern science is based on the principles of
  experimental observation and theoretical
  analysis in this assignment you will apply these
  principles in a critical analysis of animation
  and special effects.

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Homework Assignment 6

• For the term paper, you will select an animation
  film (or a live-action film featuring CGI
  animation special effects).
• You will then formulate three distinct scientific
  hypothesis for the universe portrayed in that
  film (which may or may not obey the same physical
  laws as the real world).
• For example, in your animations universe the
  laws gravity may be different (e.g., heavy
  objects may fall faster than light objects).

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Homework Assignment 6

• Your hypotheses should be such that there is
  relevant observational evidence in the film you
  need to describe what that evidence is and how it
  supports your theories.
• You will also formulate competing theories and
  present evidence that the universe portrayed in
  your film does not follow those alternate
  hypotheses.

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Homework Assignment 6

• For this homework assignment you are to choose
  the film that you'll analyze and write a one-page
  outline for the term paper.
• Post this outline on your blog an example has
  been posted on the course blog.
• This assignment is due by 8am on Wednesday,
  September 30th. (10 points 5 points if late)
• Note The term paper will be due in mid-October.

8
Arcs in Animation
9
Disneys Principles of Animation
In their classic book, Disney Animation The
Illusion of Life, Frank Thomas and Olie Johnston
list a set of basic principles for animation.
10
Disneys Principles of Animation
In their classic book, Disney Animation The
Illusion of Life, Frank Thomas and Olie Johnston
list a set of basic principles for animation.
We have already discussed several of these
principles of animation, specifically
11
Disneys Principles of Animation
In their classic book, Disney Animation The
Illusion of Life, Frank Thomas and Olie Johnston
list a set of basic principles for animation.
Today we will discuss arcs and how they relate to
animated motion.
12
Arcs of Motion
Motion usually follows an arc, which may be
simple, like a circle. or very complex and
irregular.
13
Importance of Arcs
Disney animation legends Frank Thomas and Olie
Johnston write
One of the major problems for the inbetweeners is
that it is much more difficult to make a drawing
on an arc. Drawings made as straight inbetweens
completely kill the essence of the action.
14
Circular Arcs
Circular arcs are common since motion is often
around a fixed pivot point, such as a joint.
15
Speed in Circular Motion

• Rotational Speed Revolutions per second
• Tangential Speed Total distance per second

Same Rotational Speed Different Tangential Speeds
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Throwing Arm
The longer the throwing arm, the greater the
tangential speed so the farther it can throw.
Tangential Speed
Sling lengthens the arm at almost no cost in the
weight.
Doubling the arm lengthquadruples the range!
17
Timing on Circular Arcs
A circular arc is a simple path of action but the
timing may be complex and textured.

• In this golf swing the motion
• Slows out (accelerates) to hit the ball
• Uniform after the hit
• Slows in as the swing finishes follow-through

Slow in
Uniform
Slow out
18
Rolling Slipping
1
2
3
4
5
6
7
ROLLING
Rolling ball turns one revolution when it travels
a distance equal to three times its diameter
(actually 3.1416 diameter)
1
2
3
4
5
SLIPPING
Slipping and rolling are both uniform in spacing
and rotation.
19
Wagon Wheel Illusion
In this illusion, the wheel seems to spin
backwards.
Often seen in westerns
20
Nyquist Effect
In this illusion is due to persistence of vision.
Illusion
Actual Rotation
Wheel on frames1 and 2
2
1
The brain tracks the movement of the spokes by
looking for the nearest location on each frame.
This effect limits how fast a wheel can spin in
an animation and still be seen as turning.
21
Uniform Rotation in Perspective
The timing for uniform rotation has texture when
seen in perspective.
Rotation from key 1 to 5 in background takes
twice as long as from 6 to 8 in foreground.
22
Non-Uniform Circular Motion
Two common types of motion on circular arcs that
have non-uniform timing and spacing are
Exponential Spacing
Pendulum Spacing
ExampleTipping over
ExampleStride inwalking
23
Tipping Over
Tipping over is a common example of motion on a
circular arc. Two ways to tip over
X
X
Center tipped past point of contact
Center past an edge
24
Tipping Rotation
A brick rotates about a point as it tipsthat
point is the center of a circular arc.
X
X
X
X
Friction tends to keep the brick from sliding
until it loses contact with the table.
25
Exponential Spacing
1
Constant acceleration (Odd Rule)
1
3
Release
2
5
3
7
1
4
Release
2
3
Exponential Spacing
4
As the slope of the incline increases, the
acceleration itself accelerates.
26
Rolling off a Tipping Point
2
3
4
1
5
6
7
Peak
8
Slowing out from a tipping point is very slow
initially, but then accelerates rapidly.
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Anticipation Exponential Spacing
Play
Also notice motion blur near top of brick, which
has large tangential speed.
Texture of the timing as the brick tips over
creates anticipation, which you want at the start
of a scene
28
Pendulum Spacing
A pendulums path of action is also a circular
arc but the spacing is very different from the
exponential spacing of tipping over.
29
Spacing Timing in Swinging
A pendulum will slow in and out as it swings back
and forth, the same as a ball rolling in a
half-pipe.
Play
Most of the texture in the timing is at the
endpoints the timing is even in the center.
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Motion Graph
The motion graph (angle vs. frame) confirms that
the timing is mostly textured at the apexes.
7
1
Angle
4
Frame
31
Swinging in Perspective
Visually the timing has even more texture when
the swing occurs in perspective.
32
Who Framed Roger Rabbit? (1988)
The opening sequence in Who Framed Roger Rabbit?
makes great use of the textured timing of arcs in
perspective.
Animation byRichard Williams
33
Demo Dont Flinch

• Pendulum swings back and forth yet it doesnt hit
  your face.

34
Bowling Ball Pendulum
Play
This clip lets you experience what its like to
do this demo.
35
Spirals
A spiral is just a circular arc with a radius
thats either increasing (spiral out) or
decreasing (spiral in).
Concept art fromPirates of the Caribbean 3
36
Rotational Speed in Spirals
If the radius decreases without pulling the
object inward then the rotational speed increases
(due to shrinking radius) but the tangential
speed stays constant.
Spacings along the curve stay constant.
Spiral In
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Demo Interrupted Pendulum
An interrupt bar changes the radius of the arc
for a pendulum.
Tangential speed does not increase due to the
pendulum whipping around the interrupt
bar. Energy is not increased by the interrupt
bar so ball swings back to the same spot.
Bar
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Rotation in Spirals (cont.)
If the radius decreases by pulling the object
inward then the rotational speed increases due to
shrinking radius and due to an increase in the
tangential speed.
Spacings along the curve get bigger and bigger.
Spiral In
39
Demo Skaters Spin
FAST Rotation
Slow Rotation
Exert a force to pull hand weights toward my
body, causing a big increase in rotational and
tangential speeds
40
Rotation in Spirals (cont.)
If the tangential speed decreases (say by
friction) but inward force constant then the
rotational speed still increases.
Spacings along the curve get shorter yet
itstillspinsfasterand faster.
Spiral In
Coin Vortex
41
How Does the Brick Fall?
1
Does the brick rotate and then fall down the side
of the table?
X
X
2
X
3
No! The brick does not fall this way.
X
4
Play
42
Forces on the Tipping Brick
The table pushes on the brick upward and towards
the right. Gravity pulls downward
X
X
Center of the brick shifts down and towards the
right.
If no table
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Pushing Off by the Table
The table pushes away on the brick, which causes
the brick to move away from the table as it falls.
X
X
X
Once it loses contact with the table, only the
force of gravity accelerates the brick.
44
Centrifugal Force
Insect inside a can rotating in a circle

• When we move on an arc, it seems to us as if
  there is an outward force, pushing us away from
  the center of the circle.
• Physicists call this apparent force the
  centrifugal force.

What we see
What the insect feels
45
Class Demo Bucket Overhead

• I will put a bucket full of water over my head
  without getting wet. How?

Centrifugal Force
By rotating it fast enough. The water stays in
the bucked as if pressed into it by a centrifugal
force.
You experience centrifugal force on taking a
sharp turn
46
Wile E. Coyote Loop-D-Loop
Play
In reality, it is impossible to travel
upside-down, as Wile E. Coyote does in this
scene. True or False?
Beep Beep (1952)
47
Demo Loop-the-Loop
If the speed of the ball is large then not only
does it stay on the track, the ball even pushes
outward and against the rail.
Release
Velocity
48
Demo Loop-the-Loop
Ball could even circle a loop with a gap, if the
speed was just right so gravity was equal to the
centrifugal force.
GAP
49
Simulated Gravity

• Centrifugal force could be used to simulate
  gravity in a space station.
• With the right rate of rotation a person on the
  outer rim would feel as if they stood on the
  surface of Earth.
• Scientifically accurate in the movie 2001 A
  Space Odyssey (1968)

Rotation
50
Simple Spinning
1
A falling brick may turn by simple spinning
around its center.
1
2
Arbitrary
3
3
In simple spinning, the angle rotates at a
constant rate. A brick tipped 45º as it loses
contact with the table will fall spinning about
30º every two frames.
5
4
7
51
Tumbling
1
A falling brick may turn by a more complicated
tumbling motion.
1
2
Arbitrary
3
3
There is no simple way to describe tumbling.
However, the bricks center still follows the
same path of action.
5
4
7
52
Tennis Racket Theorem
SPIN
When an object turns about its long or its short
axis, it tends to spin. When an object turns
about its middle axis, it tends to tumble.
TUMBLE
SPIN
53
Irregular Objects
SPIN
SPIN
TUMBLE
TUMBLE
SPIN
SPIN
54
Spin or Tumble?
A

• Rotation around two of these axes is
  spinning.The axis of rotation that tumbles is
• Head-to-toe axis
• Side-to-side axis
• Front-to-back axis

B
C
55
Next LectureCreating Action

• By Friday of this week
• Complete the 5th homework
• (Video analysis of Path of Action)

Please return the clickers!