Transcript Uniform Motion

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Sixth Homework (Stop-motion Animation of Falling)
Due Tuesday, February 28th (Next week)
20 points (if late, 10 points)
Bonus prize of 20 extra points to top three.
Seventh Homework (Outline of First Term Paper)
Due Tuesday, March 6th (In two weeks)
10 points (5 points if late)
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
See Chico and Rita, now playing at
Camera 12 in downtown San Jose.
Turn in your ticket
stub for 5 points
extra credit.
Extra Credit Opportunity
Attend any Cinequest Film Festival
event in downtown San Jose.
Turn in proof of your attendance (e.g., access
pass, ticket receipt) for 5 points extra credit.
For info: www.cinequest.org
Homework Assignment #6
Stop-motion animation of a falling object.
* Plan your scene, especially the timing, spacing,
path of action, anticipation, etc.
* Photograph your object in a sequence of images
suitable for combining into an animation.
* Create a video clip with at least a dozen unique
frames.
* Adjust the timing by adding or removing frames;
if needed, re-shoot your animation.
Homework Assignment #6
* Post your animation to your blog in an entry
entitled "Stop Motion Animation of Falling".
* In your posting describe in one paragraph how
you created your animation.
Due by 8am on Tuesday, February 28th
20 points (if late, 10 points)
Score based on believability and creativity.
The top three animation clips in the class will
receive a bonus of 20 extra points.
Homework Assignment #6
Here is a good example from another semester.
Not only does the falling motion look believable but
the action is simple yet entertaining.
http://www.youtube.com/watch?v=uOTncsocsUE
Survey Question
You have already started
working on your stop-motion
animation due next week.
True or False
Review Question
A spool is pulled by a string,
wrapped around the
center, as shown.
The spool will move:
A) Left
B) Right
Pull
Force and Direction
A) Left
Objects always change
their velocity in the
direction of the
applied force.
Pull
Motion
Motion
Pull
Review Question
Blow hard through a funnel
with a ping pong ball in
the funnel’s bowl.
Instead of being blown
away, the ball is held
tightly in the bowl.
This is because:
BLOW
Ping
Pong
Ball
A) Compression produces low pressure
B) Moving air produces low pressure
C) The ball spins, producing low pressure
D) None of the above
Class Demo: Blow the Funnel
B) Moving air produces
low pressure.
This demonstrates the
Bernoulli effect,
which says that the
higher the air speed,
the lower the
pressure.
BLOW
L
Ping L
Pong
Ball
A
Squash and
Stretch
Character Animation
In the first part of the
course we’ve covered
some of the basics of
animation.
We now move to more
advanced topics,
specifically those
relevant to character
animation.
Snow White
Disney’s Snow White was the
first full-length feature film using
traditional (cel) animation.
Snow White (1933, 7 min.)
Snow White and the Seven
Dwarfs (1937, 83 min.)
Snow White (1933)
http://www.youtube.com/watch?v=IUsp2XxgZTY
Snow White (1937)
http://www.youtube.com/watch?v=nN1BegE3QR0
Disney’s Principles of Animation
From life studies, Disney animators
discovered that #1 principle that was key
to adding realism in character animation
was “Squash & Stretch.”
1. Squash & Stretch
2.
3.
4.
5.
Timing
Anticipation
Staging
Follow Through
& Overlapping Action
6. Straight Ahead &
Pose-to-Pose Action
7. Slow In and Slow Out
8. Arcs
9. Exaggeration
10.Secondary Action
11. Appeal
Basic Squash & Stretch
Stretch shows speed
due to motion blur.
Basics of squash
and stretch are
present in even the
simple ball bounce
exercise.
Actual
Shape
Actual
Shape
As Seen by
Human Eye
Cartoon
Squash shows force,
such as on impact.
Character Animation
Basic squash
and stretch are
easy to learn
from a bouncing
ball but their
importance is in
how squash and
stretch appear in
the motion of
characters.
From Preston Blair’s, Cartoon Animation
Luxo Jr.’s Squash & Stretch
“An object need not deform in order to squash and
stretch. For instance, a hinged object like Luxo Jr.
squashes by folding over on itself, and stretches by
extending out fully.” John Lasseter
Luxo Jr. (1986)
Luxo Jr. was the first CG film nominated
for an Academy Award.
http://www.youtube.com/watch?v=PvCWPZfK8pI
Water Balloon Drop
Water balloon is a good animation exercise
because it moves like an animate character.
By Mai Vu
http://www.youtube.com/watch?v=ajC1oCZlkQI
By Ken Calvert
http://www.youtube.com/watch?v=0yWTJpaoJXI
Water Balloon Reference
http://www.youtube.com/watch?v=FI-Mq6BDtMQ
It’s always useful to
animators to study
reference, both live
and video.
The motion of the
water balloon is
different with every
take, yet has a
consistent feel.
http://www.youtube.com/watch?v=XbGVBV3-F48
Elements of a Water Balloon Drop
Let’s look more carefully at the physics in
each element of a water balloon drop.
Initial Drop
Bounce
Squash
Settle
Part 1: The Initial Drop
The initial part of the water balloon drop is
similar to other drop tests yet the fluid nature
of the balloon adds new elements.
Initial Drop
Incompressibility
Most solids and liquids are almost
incompressible; it takes enormous force
to change their volume.
Elastic materials may stretch easily but
their volume stays constant.
Young’s Modulus (Stiffness)
Young’s modulus indicates amount of force
required to compress or expand a material.
Material
Young’s Modulus (GPa)
Rubber
0.01 to 0.1
Water
7
Wood
9 to 11
Concrete
30
Steel
200
Rubber is compressible but water is
almost as incompressible as wood!
Demo: Incompressibility of Water
Place a brick on top of a
syringe filled with air.
Air compresses (a bit).
Place a brick on top of a
syringe filled with water.
Water doesn’t compress.
Demo: Bed of Nails
Pressure depends on
weight and on the area
supporting that weight.
One may safely lay or sit on a
bed of nails, as long as there
are enough nails so that the
pressure, measured as force
per nail, is small.
Weight of 150 pounds is
distributed over 300 nails.
Force per nail is ½ lb. Need 5
lb per nail to pierce skin.
Pressure in Liquids
Pressure in a liquid depends on depth.
As with a stack of bricks, weight of what’s
above determines pressure.
Low
Low
Medium
High
Medium
High
Water Balloon at Rest
Water pressure pushes on the rubber,
stretching it into a flattened shape.
Low
Water
Pressure
Medium
High
Tension of the rubber also affects the shape.
Repose Angle and Contact Angle
Water balloon has a repose angle and contact
angle that depend on the rubber’s stiffness.
Repose
Stiff
Flaccid
Contact
Beads of liquid have similar
shapes, depending on
surface tension.
Mercury
Water
Class Demo: Pressure & Weight
A can full of water has
holes in the sides
through which water
comes out.
What happens when you
drop the can?
?
?
Class Demo: Pressure & Weight
The can is now in freefall and
thus weightless.
Water stops flowing as the can
falls since the pressure is due
to the water’s weight.
Falling Water Balloon
Slides off
and falls
Because free fall is a weightless
state, the water balloon will be
roughly spherical as it falls.
Faucet drip
May have some initial
vibrations depending on
how it is released.
Part 2: Squash on Impact
The fluid nature of the balloon makes the
squash on impact very pronounced.
Squash
http://www.youtube.com/watch?v=UMeJ4ZfRd3w
Force of Impact for Squash
To understand the force of impact you need to
understand momentum and impulse.
http://www.youtube.com/watch?v=-p5zoufjOwc
Momentum
Momentum of an object is,
(Momentum) =
(Mass) x (Velocity)
Examples of objects with large
momentums are:
• Supertanker (large mass)
• Bullet (large velocity)
Momentum and Force
To stop an object with a large momentum
requires either:
• Large force (stopping the object quickly).
• Small force applied for a long time.
Notice that changing object’s momentum
depends on force and time interval.
Impulse
Define impulse acting on an object as,
(Impulse) =
(Force on object) X (Time interval)
Objects have momentum.
Impulse acts on an object.
Impulse & Momentum
Momentum is related to impulse by,
(Change in momentum) = (Impulse)
When mass stays constant then,
(Mass) X (Change in velocity) =
(Force) X (Time interval)
Demo: Egg Throw
Throw a raw egg
as fast as
possible at a
bed sheet that’s
held loosely.
X
X
X
X
X (Hold here)
Egg Throw Analyzed
Throw egg at a bed sheet; it stops
but doesn’t break.
Throw egg at the wall with same
speed, it stops but breaks.
In which case is the impulse
on the egg the greatest?
A) Hitting the bed sheet
B) Hitting the wall
C) Same in the two cases
Egg Throw Analyzed
C) The impulse is the same in
the two cases.
The change in velocity is the same
in the two cases so the change in
momentum is the same.
Since the impulse equals the
change in momentum, the
impulse is the same in the two
cases.
But the forces are not the same!
Egg Throw Analyzed
Throw egg at sheet or wall
with same speed.
Impulse is the same in the two
cases.
Which case has:
Largest time of impact?
Throw at the sheet.
Largest force on the egg?
Throw at the wall.
LONG TIME
small force
short time
LARGE FORCE
Automobile Safety
Maximizing time of impact on the driver
minimizes the force of impact.
This principle used in design of:
Seatbelts
Air Bags
Crumple
Zones
Demo: Vampire Stake
Place a very heavy stake on my chest
and strike with a hammer.
Why am I not killed?
Demo: Vampire Stake
Safest when slow moving stake placed on
a soft, fleshy spot on the chest.
(force) x (TIME)
(FORCE) x (time)
XX
Ouch!
Not safe if stake
strikes hard skull
Ironman
Ironman falls off the roof of a
10 story building and lands
on the sidewalk.
Even though he is an
ordinary man, he’s unhurt
as long as his metal suit is
undamaged and intact.
This is physically realistic.
True or False
Ironman
False. The suit does not
increase the time of
impact so the force of
impact is the same.
The suit is no safer
than being inside of
a falling safe.
Next Lecture
More Squash & Stretch
Homework #6
Stop-motion Animation of Falling
Due Tuesday, Feb. 28th (Next week)
Please turn off and return the clickers!