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First Term Paper
The Laws of Physics in an Animation Universe
Due Tuesday, March 12th (Next Week)
100 points (50 points if late)
Eighth Homework (Reverse Video Reference)
Due Tuesday, March 19th (In 2 weeks)
20 points (10 points if late)
For full schedule, visit course website:
www.Animation123.com
Extra Credit Opportunity
Attend any screening or event at the
Cinequest Film Festival, San Jose.
Festival runs Feb. 26 – Mar. 10
Turn in proof of attendance (ticket receipt)
for 10 points extra credit.
For info: www.cinequest.org
First Term Paper
The required length of the term paper is 1500 words.
Use a word count to verify that you are not significantly
below this required length.
Upload your term paper to your blog in a post entitled
"The Laws of Physics in an Animation Universe."
If you find that you significantly deviate from the outline,
please add a brief explanation at the end of your term
paper (which does not count towards the 1500 words).
Term paper is due by 8am on Tues., March 12th
100 points (50 points if late)
SPECIAL OPTION
If you score 80 points or higher on your
first term paper then you have the option
of skipping the second term paper.
If you are eligible and select this option then you
receive the same points for the second term
paper as you did on the first paper.
If you are eligible but decide to turn in a second
term paper anyway then your score will be no
less that that of the first term paper.
Survey Question
“This course has ___ changed the
way that I view animation.”
A)
B)
C)
D)
Very Significantly
Significantly
Somewhat
Been interesting but not really
B) Left Leg
In this classic
contrapposto pose,
the weight of the
body is primarily on
which leg?
A) Right Leg
Review Question
Hips and Shoulders
A) Right Leg
Notice the angles of the hips
and shoulders and that they
converge on the side that
bears most of the weight.
Weight shifts from foot to foot
are reflected by
corresponding shifts in the
hips and shoulders.
Donatello's bronze statue of David (circa 1440s)
Review Quiz
In which of these
situations does the
character gain
weight?
A) #1 and #2
B) #1 and #3
C) #2 and #3
D) #2 and #4
E) #3 and #4
#1: Falling &
Speeding Up
#2: Rising &
Speeding Up
#3: Rising &
Slowing Down
#4: Falling &
Slowing Down
Dynamic Weight
D) #2 and #4
With gravity
Against gravity
With gravity
Against gravity
Corey Tom
Falling &
Speeding Up:
Lose Weight
Rising &
Speeding Up:
Gain Weight
Rising &
Slowing Down:
Lose Weight
Falling &
Slowing Down:
Gain Weight
Review Question
In this scene from Bullitt, the car is making
a high speed turn in which direction?
Centrifugal Force
Centrifugal
force pulls the
CG towards
the outside of
the curve and,
combined with
the weight, it
tilts the car.
A) ScreenLeft Turn
Centrifugal
Force
X
CG
Weight
This
side
goes
down.
Action &
Reaction
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
Today we look at the Action-Reaction Principle,
also known as Newton’s Third Law.
Action-Reaction Principle
For every action force there is
an equal reaction force in the
opposite direction.
Action and Reaction
Common expression for the principle is,
To every action there’s an equal and
opposite reaction.
What’s an “action”?
A force exerted by one object on second object.
What’s a “reaction”?
A force exerted by second object back on the first
object that is causing the action.
How can reaction be “equal” and “opposite”?
Equal in magnitude but opposite in direction.
Action-Reaction Pairs
Action-reaction always occurs in pairs.
Action: (Thing A) exerts a force on (Thing B).
Reaction: (Thing B) exerts a force on (Thing A).
Timing of Action and Reaction
The action force and reaction force
always act simultaneously.
Action force on jaw occurs
simultaneous with reaction
force on the fist.
This is not Newton’s 3rd Law
Slaps are not simultaneous.
Class Demo: Mutual Repulsion
Mr. A pushes on Mr. B, who just holds his arm steady.
The action force accelerates Mr. B towards screen-right.
The reaction force accelerates Mr. A towards screen-left.
Action
Mr. B holds
Mr. A
pushes
Reaction
Put them on
skateboards to
minimize the
effect of friction.
Equal Forces, Not Accelerations
Action and reaction forces have equal magnitude
but the resulting accelerations are not equal.
Small weight,
Big recoil speed
Action
Big weight,
Small recoil speed
Reaction
Small
Acceleration
Large
Acceleration
Mr. A pushes Mr. B
Law of Acceleration (Reminder)
Object A has less weight (lighter) than Object B.
Push each object with the same force.
Force
Object A
Accelerations
Force
Object B
By Law of Acceleration, Object A accelerates
more than the (heavier) Object B.
Mutual Repulsion Summary
If A pushes B then both accelerate by equal forces.
By Law of Acceleration, Object A, having less weight,
will accelerate more than the heavier Object B.
Reaction
Action
Acceleration
Acceleration
Object A
Object B
Class Demo: Mutual Repulsion (cont.)
If we replace Mr. B with a solid wall then Mr. A pushes on the wall
(that’s the action force) but due to the enormous mass of the
building, the wall does not move.
Wall exerts a reaction force, pushing Mr. A away from the wall.
Mr. A
Action
Reaction
Acceleration
Wile E. Coyote & Action/Reaction
“Beep Beep” (1952)
Action/Reaction is
often violated for
comic effect.
http://www.youtube.com/watch?v=csIIFGFr1wU
http://www.youtube.com/watch?v=vobhUvhO8N0
“Gee Whiz-z-z-z” (1956)
Jumping Action/Reaction
Jumping is done by
pushing downward on
the ground (action) so
the ground pushes
upward on you
(reaction).
Reaction
Action
Force, Time, and Impulse
When any force causes a change in
velocity, the more time that the force can
act, the greater the change in the velocity.
Pushing apart,
these guys
only exert
forces on each
other as long
as their hands
are together.
Crouching for a Jump
Crouching is useful when
jumping because it extends
the time of contact for
pushing off the ground.
X
X
Apex
Crouch
Take-off
Walking Action/Reaction
In walking a character
exerts an action force on
the floor, which results in
a reaction force on the
character.
If the character is unable to
exert a side-ways action
force (e.g., slippery ice)
then the character cannot
walk.
Reaction
Action
Reaction Force on the Foot
The floor exerts a reaction force
that either slows us down (from
squash to passing position) or
speeds us up (from passing
position to stretch).
Squash
Passing
Position
“Butterfly diagram” of
force vectors.
Stretch
Squash
Passing
Position
Stretch
Floor’s Reaction Force
Walking this way
Pushing an Object
The man pushes on
the rock but he is also
pushing on
the ground.
Man moves forward when
reaction from the ground is
more than from the rock.
Quiz Question
The woman
pushing the car
exerts more force:
A) With her hands
B) With her feet
C) About the
same with her
hands as with
her feet
http://www.youtube.com/watch?v=bREfcVPssiE
Quiz Question
C) About the same with her
hands as with her feet.
Woman doesn’t
move so the
two reaction
forces balance
thus the two
action forces
must be equal.
Reaction: Car
pushes Woman
Reaction: Ground
pushes Woman
Action:
Woman
pushes
Ground
Action:
Woman
pushes
Car
Fight Scenes
Fight scenes are more realistic when reaction
punches and kicks show the reaction force.
From Russia with Love (1963)
This early James Bond film has a very
brutal, realistic fight scene.
http://www.youtube.com/watch?v=Ea7JxAvzUMg
A recent Bond film (Casion Royale, 2006) starts with a
similarly realistic fight scene in a bathroom.
Kung fu Hustle (2004)
In a comedy it’s best to make a fight scene
less realistic, which makes it funny.
http://www.youtube.com/watch?v=glsUlacDN7Q
Spiderman 3 (2007)
Most elements of this fight scene do not have
realistic reaction forces to match the actions.
The Incredible Hulk (2008)
Using motion capture has the disadvantage that
some stunts can only be done using wire rigs.
The action will look fake without the right reaction.
Kung-Fu Panda
End credits of KFP play with
the 3rd law, but consistently.
Strong characters have mass;
the weak ones are light.
Weak
Strong
Action/Reaction in KFP
http://www.youtube.com/watch?v=YGg6QiNOZ28
Watch the reaction for each character as they strike
the inanimate punching bag.
Manipulating Action-Reaction
Action/Reaction
principle is often
manipulated to
give “weight” to
a dramatic
character or to
make a comedic
character “light.”
Mutual Attraction
Action / Reaction applies when a character pulls.
In this example Mr. A is pulling and Mr. B holds
the rope but they both move towards the center.
Mr. B
Reaction
Mr. A
Action
Demo: Mutual Attraction
http://www.youtube.com/watch?v=gPDjaMsG_rg
Mutual Attraction
If only Mr. A pulls on Mr. B then Mr. B accelerates.
Reaction force of equal magnitude so Mr. A also moves.
Who moves faster? Mr. A, Mr. B, or the same?
Mr. B
Reaction
Action
Mr. A
Mr. A weighs
less than Mr. B
Accelerations
Mr. A goes faster
(greater acceleration)
since his mass is less.
Mutual Attraction Summary
If A pulls B then both accelerate by equal forces.
By Law of Acceleration, Object A, having less weight,
will accelerate more than the heavier Object B.
Object A
Reaction
Action
Accelerations
Object B
Class Demo: Mutual Attraction (cont.)
When both persons pull then there are two action forces
and two reaction forces.
If both pull with same force, how much greater is the
acceleration than when only one pulls?
Mr. B
Reaction A
Action A
Mr. A
Action B
Twice the force,
twice the
acceleration
Reaction B
Accelerations
Class Demo: Mutual Attraction (cont.)
We replace Mr. B with a solid wall and Mr. A pulls on the wall (that’s
the action force) .
Due to the enormous mass of the building, the wall does not move.
Wall exerts a reaction force, which pulls Mr. A towards the wall.
Mr. A
Reaction Action
Acceleration
Action / Reaction for Gravity
The reaction force
due to the
gravitational pull
on a character
has a negligible
effect since Earth
is massive.
Action Force:
Earth pulls
down on Wile
Reaction Force:
Wile pulls up
on Earth
Pulling a Character
Forces on Gia
Gravity
Forces on Alex
Gravity
Action: Alex
exerts force
on ground
Reaction: Ground
exerts force on Alex
IMPORTANT!!!
Action force & reaction
force NEVER cancel
because they act on
different objects!
Repeat this to yourself over and over again
Balance of Forces?
Miss A pushes the cart (action); cart pushes back on her (reaction). Do
these forces cancel?
No, the two forces act on different objects.
Force on Miss A is to the left; how
Miss A
can she move forward (to the
right)?
Miss A pushes back on the
Action
ground with her feet (action)
reaction of ground on her is to
Reaction
the right.
What if ground had zero friction
Reaction
(like ice)?
ActionThen Miss A can’t move forward.
Action
Reaction
Pairs
Balance of Forces?
Mr. B also pushes from the inside of cart but obviously he can’t
move the cart alone. In terms of Newton’s laws, why not?
Because the total force exerted by Mr. B on the cart is zero.
What other force does Mr.
B exert on the cart besides
his hands?
His butt pushes back on
the cart and the floor of the
cart pushes back on him.
The two action forces
balance each other and
the two reaction forces
balance each other.
Mr. B
Reaction
Action
Action
Reaction
ActionReaction
Pairs
Wile E., Propelled
http://www.youtube.com/watch?v=rv7PWDjjjqk
From “Ready, Set, Zoom”
Would using an outboard motor in a tub for propulsion,
as done by Wile E. Coyote, actually work? A) Yes or B) No
Internal Propulsion
B) No. Internal propulsion is
not possible because the
impulse gained from one
reaction is lost due to
another internal action.
Water
pushes
propeller
Propeller
pushes
water
Action/Reaction Pairs
Tub
pushes
water
Water
pushes
Tub
Wile E., Propelled II
http://www.youtube.com/watch?v=wkMOfeXItUU
From “Scrambled Arches”
Would using a strong fan and a big sail for propulsion,
as done by Wile E. Coyote, actually work? A) Yes or B) No
Internal Propulsion
Doesn’t
work!
B) No. Internal propulsion is
not possible because the
impulse gained from one
reaction is lost due to
another internal action.
Air pushes
propeller
Propeller
pushes air
Action/Reaction Pairs
This
would
work!
Sail
pushes
air
Air
pushes
sail
Recoil
Action/Reaction principle applies to recoil.
The action force that
accelerates the bullet
results in a matched
reaction force in
opposite direction,
recoiling the gun.
Heavy gun
Fast bullet
Recoil Speed =
(Bullet/Gun Weight Ratio) x (Bullet Speed)
Recoil from a Gun
High recoil speed is cause by either:
* Large Bullet/Gun Weight Ratio
* High Bullet Speed
Shooting an elephant gun
http://www.youtube.com/watch?v=MlFlXMHaSVQ
http://www.youtube.com/watch?v=bpFDHO-tqUY
Predator (1987)
Recoil & Wile E. Coyote
http://www.youtube.com/watch?v=Wxm9EdVFxk8
Unexpected recoil is
another common gag
in animated cartoons.
http://www.youtube.com/watch?v=7H6Pwzdw7oQ
Guided Muscle (1955)
Wall-E Fire, Propelled
http://www.youtube.com/watch?v=2lkffSsImXc
Would using a fire-extinguisher for propulsion, as done
by Wall-E, would actually work, A) Yes or B) No?
Class Demo: Extinguisher Rocket
http://www.youtube.com/watch?v=9F_VjHZATbY
Using a fire-extinguisher for rocket-like propulsion.
Fire Extinguisher Bike
Two-stage Rockets
The first stage accelerates the rocket until it runs out of fuel.
The first stage is discarded, reducing the rocket’s mass.
Rocket mass reduced so recoil acceleration is greater.
Mythbusters Two-Stage Rocket Sled
http://www.youtube.com/watch?v=gK7ffrN1zcI
Next Lecture
Jumping
Next Assignment:
First Term Paper
Due Tuesday (Next week)