Transcript L08_Action - Virtual Homeschool Group

Upcoming Deadlines
Sixth Homework (Outline of First Term Paper)
Due Wednesday, September 30th
Seventh Homework – Cancelled due to furloughs
(Automatic 20 points of credit)
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
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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
Post this outline on your blog; an example outline and
term paper have 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.
Extra Credit Opportunity
Visit the Walt Disney Family Museum in
San Francisco (opens Oct. 1st).
Give me your ticket receipt for
ten points extra credit.
Hours: Wednesday-Monday: 10a.m.-6p.m
Admission: $15.00 for students
Location: 104 Montgomery Street
Inside The Presidio of San Francisco
Extra Credit Opportunity
Visit the Exploratorium in San Francisco.
Give me your ticket receipt for
ten points extra credit.
Hours: Tuesday-Sunday: 10a.m.-5p.m
Admission: $11.00 for students
Location: 3601 Lyon Street, SF 94123
Next to Palace of Fine Arts complex
Creating Action
Why Things Move
So far we’ve only looked at how things move
(slowing in/out, path of action, arcs, etc.).
Now it’s time to look at why things move,
that is, what causes action.
The short answer is forces.
To understand why things move the way they
do, you need to consider the forces at play.
Newton’s Laws of Forces
Newton established three basic laws to explain
how motion is caused by forces:
• Law of Inertia
• Law of Acceleration
• Action-Reaction Principle
Sir Isaac Newton
Disney and other early animators rediscovered
these laws of forces in their studies of motion.
Follow-Through
When a character stops, it
doesn’t suddenly freeze.
Some parts of the character
stop abruptly while others,
such as arms, long hair,
clothing, etc., continue
moving for a few frames.
In animation, this is known as
follow-through.
In physics, we know it as
Newton’s Law of Inertia.
Motion, with & without Forces
An object moves with constant, uniform
motion until acted on by a force.
No force
FORCE
An asteroid floats in
space with a constant
speed unless gravity
deflects its motion.
Balance of Forces
Rarely are there no forces but often forces
are balanced so they “cancel” each other out.
Important:
Balanced forces
does not mean
that there’s no
motion!
Floor
Gravity
Floor
Gravity
Tension
Gravity
Law of Inertia
Newton’s Law of Inertia says:
An object moves with constant, uniform
motion until acted on by an unbalanced force.
Floor
Gravity
The bowling ball moves with constant speed*
*In reality, there is a small unbalanced force,
friction, that does slow the ball’s speed.
Home Demo: Riding the Bus
When a moving bus halts, you continue
moving forward.
Play
The League of Extraordinary
Gentlemen (2003)
In this scene, Sean Connery
jumps out the side of a
speeding car and lands on his
feet. In reality, he would:
A) Roll forward from where
he lands, in the direction
of the moving car.
B) Roll backwards from
where he lands.
C) Land just as he does
in the movie; this was
actually done by a
stuntman.
Play
Jumping out of a Car
You are moving at the same speed as the car
when you jump out so you will roll forward.
Your path
You’ll start losing speed after you hit the
ground so, relative to the car, you’ll fall behind
as the car continues speeding along.
Centrifugal Force Revisited
Your
path
The centrifugal force you
experience on taking a sharp
curve is nothing more than
inertia keeping you moving
forward in a straight line.
It feels as if you’re pulled to
the outside bank of the curve.
Law of Inertia (cont.)
Newton’s Law of Inertia also says:
An object at rest (not moving) remains at
rest until acted on by an unbalanced force.
Floor
Gravity
A stationary bowling ball
remains stationary until
some unbalanced force
comes along.
This is nothing more than motion at constant
speed but with speed equal to zero.
Home Demo: Riding the Bus (cont.)
If the bus starts moving again, you remain
stationary, seemingly thrown backwards.
Play
Frame of Reference
Bus Moves
Background
As seen by observer
on the bus
As seen by observer
on the street
Class Demo: Tablecloth Pull
Flower
Vase
Tablecloth
Yank quickly
Due to the vase’s inertia it remains at rest since almost no force
acts on the vase if one pulls quickly & straight.
Inertia & Drag
An object won’t move until a force acts on it so
long hair trails behind as head turns.
Although this is due to the hair’s inertia, in
animation it’s usually called drag.
An object at rest remains at rest until acted on by a force.
Inertia & Drag (cont.)
Hair remains in motion even after the head stops
turning, which is follow-through due to inertia.
Object in motion remains in motion until
acted on by an unbalanced force.
Class Demo: Hula Skirt
The motion of a hula
skirt is an excellent
example of animation
“drag.”
Also notice how the
skirt moves outward
as it turns due to
centrifugal force.
Flour Sack Exercises
The sack drop and sack
pantomime are common
animation exercises.
A flour sack is a good proxy
for learning character
animation since it shows
follow-through and drag.
Play
Play
Dancing with
the Sacks
Newton’s Laws of Forces
Newton established three basic laws to explain
how motion is caused by forces:
• Law of Inertia
• Law of Acceleration
• Action-Reaction Principle
Sir Isaac Newton
The Law of Inertia explains motion without
forces (or with only balanced forces).
The Law of Acceleration explains motion with
unbalanced forces.
Forces & Slowing In/Out
B
A
L
L
N
G
F
A
L
L
I
When a force pulls in the direction that an object is
already moving, the object slows out (accelerates)
Moving
this way
Force
If a force pushes opposite to the direction of
motion then the object slows in (decelerates)
B
A
L
L
Force
R
I
S
I
N
G
Moving
this way
Forces & Path of Action
When a force is perpendicular to the path of
action then it deflects the motion into an arc.
If force is at an arbitrary angle then
both timing and path of action are
affected.
Force
(up & left)
Moving
this way
More Force, More Acceleration
The greater the net force on an object, the
greater the acceleration of that object.
More Mass, Less Acceleration
The greater the mass of an object, the less it
accelerates when acted on by a force.
Simple Addition of Forces
When two forces pull in parallel directions it’s
simple to add them to get the total force.
Gravity
(Weight)
Total
Force
For example, if a
bubble weighs 3 oz
and has a buoyancy
of 2 oz then the
total force is 1 oz.
Buoyancy
Law of Acceleration
If the total force equals the
object’s weight then the spacing
is the same as when falling.
If the total force is greater than
the weight then the spacings and
increments are larger.
If the total force is less than the
weight then the spacings and
increments are shorter.
Law of Acceleration Example
The total force equals 1/3 of the weight so the
spacings are 1/3 of what they are for falling.
Gravity
(Weight)
Total
Force
Normally an object that
falls for 6 frames goes a
distance of 12 inches so
this bubble falls only 4
inches after 6 frames
Golf
Ball
Bubble
Buoyancy
Leaf/Paper Drop Test
Animate a leaf (or piece of paper) drifting
slowly to the ground.
That was not a
good leaf drop
Let’s see some
good ones.
Play
(by Gloria Cho)
Play
(by Kanako Shibanuma)
Air Resistance
Air resistance is caused by molecules of
air striking a moving object.
Air Resistance
Depends on:
•Size (area) of the object
•Speed of the object
Larger the size or speed,
larger the resistance.
Gravity
Demo: Hand out the Window
Experience the force of air resistance by
holding your hand out a car window.
Resistance increases as speed increases.
Resistance increases as area increases.
Home Demo: Drop the Sheet
A flat sheet of paper falls slowly because of air
resistance.
What happens if we place it on top of a book,
blocking the air from reaching it?
Air
Resistance
Weight
Book and sheet fall
together
Falling on the Moon
There’s no atmosphere and thus no air
resistance on the Moon.
Play
Falling with Air Resistance
1
3
5
5
5
5
Accelerating
Motion
Uniform
Motion
Light objects, such as
a beach ball, initially
fall with accelerating
motion.
Due to air resistance,
the motion transitions
to uniform motion
after falling a certain
distance.
Falling Coffee Filter
Tracked falling of a
coffee filter.
Height
Play
Play
Accelerates in
first 1/3 second
Constant
Speed
Time
Terminal Speed
Speed of falling objects increases until air
resistance force balances gravity force.
When forces balance, zero
acceleration so constant speed.
This is the terminal speed, the
maximum speed when falling.
Home Demo: Keep It Up
You can estimate the terminal speed as the wind
speed needed to support the object.
With a big fan (blowing 120-150 mph), you
can even “skydive” indoors.
iflysfbay.com
Play
Home Demo: Drop the Cat
Cats seem to have an uncanny ability to
survive falls from high places. For
example, cats have been known to survive
falls of up to 32 stories. By contrast,
dogs rarely survive falls of more than six
stories. Humans usually die when they fall
from such heights.
In a study of cats that had fallen from up to 32 stories, an
interesting finding emerged: while the rate of injury in cats
seemed to increase linearly depending on the length of the
fall, after seven stories, the rate of injury seemed to level
off! In other words, the survival rate and severity of
injuries were no more severe in a cat that fell seven stories
than in one that fell 32 and in some cases, injuries were
even less!
From: www.animalhealthcare.ca
Home Demo: Drop the Cat (cont.)
After further study, the reasons for this
discrepancy became clear. When a person falls
from a building, maximum speed or "terminal
velocity" is reached after 32 stories. Cats, on
the other hand, achieve terminal velocity at
after falling only five stories!
Until a cat reaches terminal velocity, it will
experience acceleration and tend to reflexively
extend its limbs, making it more susceptible to
injuries. However, when a cat reaches terminal
velocity, its vestibular system (i.e. the organs
of balance) become less stimulated, causing the
cat to relax. It will then orient its limbs more
horizontally (splay-legged), thereby increasing
air drag in much the same way a parachute
does. In this posture, the force of impact also
appears to become more evenly distributed.
Squirrels cannot
die from a fall.
Don’t try
this demo!
Wile E Coyote with Anvil
Wile E Coyote is
falling, with an anvil
in his backpack.
If he gets rid of
the anvil, he’ll fall
A) Slower
B) Faster
C) At the same
speed
Wile E Coyote with Anvil
The answer is:
A)Slower
You reach terminal
speed when the force
of air resistance
balances your weight.
The less you weight, the less air resistance is
needed so the terminal speed is also lower
(lower speed <-> lower air resistance).
Estimating Terminal Speed
Terminal speed of a rectangular
object (with the density of water)
falling flat is approximately:
(Speed) = (50 m.p.h.) x T
where T is thickness in inches.
Thickness, T
T
Terminal Speed
1/
1/
5 m.p.h.
100
inch
10
¼ inch
½
25 m.p.h.
1 inch
1
50 m.p.h
4 inch
2
100 m.p.h.
9 inch
3
150 m.p.h
Air Resistance
T
Gravity
Estimating Terminal Speed (cont.)
Terminal speed of aerodynamic
shapes, such as a sphere, are about
50% faster than for the rectangle.
For example, the terminal speed of a raindrop with a
radius of 1/8th inch is about 20 m.p.h.
Terminal speeds for objects made of
denser materials is also higher.
For stone the terminal speed is about twice as large;
for metal it’s about three times larger.
For example, a brick’s terminal speed is about 140 m.p.h.
(Falling flat so thickness is 2 inches)
Leaf/Paper Terminal Speed
The terminal speed of a leaf
or sheet of paper is about
5 feet per second, which is
about 3½ miles per hour
(or 2-3 inches per frame).
Terminal speed is reached
after falling about 4 frames
(flat orientation).
Air Resistance
Gravity
Falling Speed Without Air Resistance
Air resistance is only
noticeable once an object’s
speed gets close to its
terminal speed.
Distance
fallen
from apex
Speed
(miles per
hour)
1 foot
5
4 feet
10
This table gives the speed of
an object from the distance
it’s fallen it there is no air
resistance.
9 feet
15
16 feet
20
25 feet
25
49 feet
35
For example, since a brick’s
terminal velocity is 140 m.p.h.
then air resistance is not
noticeable for a 400 foot drop.
100 feet
50
400 feet
100
900 feet
150
Next Lecture
Collisions, Crashes,
and Combat
By Wednesday of this week:
Complete the 6th homework
(Outline for First Term Paper)
Please return the clickers!