Transcript Uniform Motion - artphysics123

Fukkireta
http://www.youtube.com/watch?v=NFep4vO4TRc
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Fifth Homework (Video Analysis of a Path of Action)
Due Tuesday, February 21st (Next week)
15 points (10 points if late)
Sixth Homework (Stop-motion Animation of Falling)
Due Tuesday, February 28th (In two weeks)
20 points (if late, 10 points)
Bonus prize of 20 extra points to top three.
For full schedule, visit course website:
ArtPhysics123.pbworks.com
Extra Credit Opportunity
Academy Award
Nominated Animated
Short Films
Camera 3 theater in
downtown San Jose
Turn in your ticket stub
for 5 points extra credit.
Extra Credit Opportunity
Art of Dreamworks’ Puss in Boots at the
Cartoon Art Museum in San Francisco.
Turn in proof of your
attendance to this
exhibition (e.g., ticket
receipt) for 10 points
extra credit.
For info: cartoonart.org
Homework Assignment #5
Use Tracker to analyze the motion of
yourself doing a running jump.
Shoot reference with at least 5 takes.
Track the center of your body (center of torso at
about the beltline) in the air.
Upload original video, screen shot with graphs,
and the video with tracking*.
This assignment is due by 8am on
Tuesday, Feb. 21st (next week).
15 points (10 points if late)
*May be tricky
Homework Assignment #5
Straight Line
Parabolic
Path of Action
Parabolic Curve
Homework Assignment #5
Video Reference with Tracking
Survey Question
How is the pace of the course so far?
A)
B)
C)
D)
E)
Much too Fast
A little Fast
About Right
A little Slow
Much too Slow
Note: You score 1 point of credit for answering
survey questions, regardless of your answer.
Review Question
The timing and spacing for a brick tipping over
is very similar to that of a leg swinging forward
in a walk.
True or False?
Review Question
False. Tipping over has exponential spacing (increasing
acceleration and speed) but a leg swinging forward has
pendulum spacing (increasing speed, decreasing
acceleration).
Pendulum Spacing
Exponential Spacing
Example:
Tipping over
Example:
Stride in
walking
Review Question
In reality, it is impossible to
travel upside-down, as Wile
E. Coyote does in this
scene. True or False?
“Beep Beep” (1952)
Wile E. Coyote & Loop-D-Loop
False.
If his speed is high enough then he stays
in contact with the arch, just like the
water in the spinning bucket.
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.
Shoot ‘Em Up (2007)
If the crash occurs at 35 miles per hour then the
hero flies off at a speed of about 2 feet per frame.
Shoot ‘Em Up (2007)
Frame 438
Stuntman flies out the
window at about 10 m.p.h.
Frame 439
Frame 440
Frame 441
This is a bit slow but at
a realistic speed the
audience wouldn’t see
the action.
Shoot ‘Em Up (2007)
Frame 459
Stuntman flies into the van
at about 5 m.p.h.
Frame 460
Frame 461
Frame 462
Noticeably much too
slow but the sequence
is outrageous anyway,
so it works.
The League of Extraordinary
Gentlemen (2003)
http://www.youtube.com/watch?v=n8SDdkKSqns
In this scene, Sean Connery jumps out the side of a
speeding car and lands on his feet.
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.
Jumping out of a Car
A) Roll forward
You are moving at the same speed as the car
when you jump out so you will roll forward.
Your path
car
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.
Frame of Reference
Bus Moves
Background
As seen by observer
sitting in the bus
As seen by observer
on the street
Space Balls (1987)
Jackass (2002)
http://www.youtube.com/watch?v=V-dFVdhgSsc
Class Demo: Tablecloth Pull
Due to the vase’s
inertia it remains at
rest since almost no
force acts on the
vase if one pulls
quickly and
straight.
Yank quickly
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.
Drag in Arcs and Waves
Animation drag is very
noticeable when
something like hair or
cloth moves in an arc
or in a wave-like
motion.
Fukkireta
Click to play
http://www.youtube.com/watch?v=NFep4vO4TRc
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.
Dancing with
the Sacks
Importance of Follow-through & Drag
“Now we could use Follow-through on the fleshy parts to
give us the solidity and dimension, we could drag the
parts to give the added feeling of weight and reality. It all
added up to more life in the scene. The magic was
beginning to appear.”
From The Illusion of Life - Disney Animation
Notice the subtle
follow-through in the
hands, skirt, and
pant legs for the last
drawing of the
Moving Hold.
By Ham Luske
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.
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 by
Gloria Cho and
Katie Corna.
Leaf Drop Test
http://www.youtube.com/watch?v=mbMo4HFJC1Y
Paper Drop Test
http://www.youtube.com/watch?v=vKf-vIDSIik
Air Resistance
Air resistance is a force created when an
object moves through air.
Depends on:
Air Resistance
•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.
Demo: Falling in a Vacuum
Feather falls slowly due to
air resistance force.
If we remove the air (create
a vacuum) then feather
and coin fall with same
acceleration.
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.
http://www.youtube.com/watch?v=5C5_dOEyAfk
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.
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.
Heavier parachutist has
higher terminal speed
Wile E Coyote with Anvil
The accident-prone
Wile E Coyote walks
off a cliff carrying an
anvil.
If he lets go 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
Air Resistance
T
Gravity
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
Terminal Speed & Thickness
Piece of paper falls much faster when you
drop it sideways instead of face-down.
Air Resistance
Small thickness;
Slow terminal
speed
Air Resistance
Big thickness;
Fast terminal
speed
Gravity
Gravity
Terminal Speed & Shape
Terminal speed of aerodynamic
shapes, like a sphere, are about
50% faster than for a rectangle.
For example, the terminal speed of a raindrop with a radius
of 1/8th inch is about 20 m.p.h.
Large raindrops are flattened
due to air resistance and
very large drops are split
into smaller drops by the force
of air resistance.
The Incredibles (2004)
http://www.youtube.com/watch?v=j2SmaI6iPxA
What is unrealistic about the way
objects fall in this scene?
The Incredibles (2004)
They land in the water…
… chat for 10 seconds…
… and then fuselage lands!
Fuselage should have landed
before they reached the water.
Terminal Speed & Density
The denser the material, the
higher the terminal speed.
The table gives the terminal speed for density of water.
The terminal speed for wood is about the same as for water
since the density of wood is close to that of water.
The terminal speed for rocks is about 50%-75% larger since
rocks are 2-3 times denser than water.
Metals, like iron and copper, are 8-9 times denser than water
so the terminal speed is about three times larger.
For example, a brick’s terminal speed is about 100 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 Coffee Filter
Distance Fallen
Tracked falling of a
coffee filter.
Click
Accelerates in
first 1/3 second
Constant
Speed
Time
Cat Drop Video Reference
http://www.youtube.com/watch?v=YJy17-BHQXg
Cat Drop Motion Graph
Good parabolic arc; no noticeable air resistance
Cat Drop from Building
Dropping the cat from a height of 100 ft
(about 8th floor) it reaches terminal velocity
at about half-way down.
100
90
80
Height (feet)
For cats, falling
four stories is
same as forty.
Terminal
velocity
40 m.p.h.
70
60
50
No Air
Resistance
40
30
20
10
Don’t try
this demo!
0
0
10
20
30
40
Frames
50
60
70
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 (splaylegged), 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!
Home Demo: Keep It Up
You can estimate the terminal speed as the wind
speed needed to support the object.
Indoor Skydiving
With a big fan (blowing 120-150 mph), you can
experience terminal speed and skydive indoors.
iflysfbay.com
Next Lecture
Creating Action
Part II
For Tuesday of next week:
Homework #5
(Video Analysis of Path of Action)
Please turn off and return the clickers!