Transcript Uniform Motion

Upcoming
Deadlines
Fifth Homework (Video Analysis of a Jump)
Due Wednesday, Sept. 29th (Next week)
15 points (10 points if late)
Sixth Homework (Stop-motion Animation)
Due Wednesday, Oct. 6th (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
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,
video with tracking*.
This assignment is due by 8am on Wednesday,
September 29th (next week).
15 points (10 points if late)
*May be tricky
Homework Assignment #5
Straight Line
Parabolic
Path of Action
Parabolic Curve
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.
Jackass 2 (2006)
http://www.youtube.com/watch?v=36fD7KSUjkw
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
Noticeably much too
slow but the sequence
is outrageous anyway,
so it works.
Frame 462
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
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
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
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.
Next Lecture
Creating Action
Part II
By Wednesday of next week:
Complete the 5th homework
(Video Analysis of Path of Action)
Please turn off and return the clickers!