Transcript Uniform Motion

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Fifth Homework (Video Analysis of a Jump)
Due Wednesday, Feb. 24th (This week)
15 points (10 points if late)
Sixth Homework (Stop-motion Animation)
Due Wednesday, March 3rd (Next week)
20 points (if late, 10 points)
Bonus prize of 20 extra points to top three.
For full schedule, visit course website:
ArtPhysics123.pbworks.com
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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,
Feb. 24th (this week).
15 points (10 points if late)
*May be tricky
Homework Assignment #5
Straight Line
Parabolic
Path of Action
Parabolic Curve
Homework Assignment #6
In this assignment you will create a stopmotion animation of a falling object.
It can be a simple as a ball bounce cycle or
(preferably) something more interesting, like
a flour sack or a water balloon.
Because you will have to pose your object at
different heights, you'll either need to
suspend it from a string or have the motion
occur on the ground with the camera
positioned directly overhead.
Homework Assignment #6
http://www.youtube.com/watch?v=uQXXmGUwn4E
Homework Assignment #6
If you have a webcam, an easy way to film a
stop-motion animation is to use SAM
Animation (http://www.samanimation.com/).
It's free software that runs on both Macs and
PCs; it was developed at Tufts University as
part of a National Science Foundation project
for teaching using animation.
The software is designed so that it can be used
by even elementary school children so you
shouldn't have much trouble figuring it out.
Homework Assignment #6
An alternative is to shot a sequence of photos
(not a movie!) with your digital camera and to
combine the photos into a movie file or an
animated GIF.
This is as simple as using cut/paste if you have
Quicktime Pro; if not then you could use any
one of a number of freeware or trialware
video editing programs (the software that
came with your digital camera may have this
capability).
Homework Assignment #6
You can also create an animated GIF in
Photoshop. To do that, place each photo
in a different layer, use the Animation
tool (in Window menu) to combine them,
finally use "Save for Web & Devices" to
save the animation as a GIF (there are
many tutorials on-line explaining how to
do this).
Homework Assignment #6
Your animation should be composed of at least a
dozen different images, though you may want
to use the same photo in one or more frames
to get the timing right.
Try to make the motion as physically correct as
possible; because it is difficult to go back and
make corrections in stop motion animation
you'll want to plan out your object's
positioning ahead of time.
Homework Assignment #6
Your assignment will be scored on both the
believability of the object's motion and on
the creativity of your animation.
You can adjust the timing by putting the same
image on more than one frame (e.g., "shooting
on twos") or deleting some frames.
Don't hesitate to simply reshoot your
animation; often that's easier than trying to
make corrections.
Homework Assignment #6
While I encourage you to help each other out,
for this assignment each person needs to
create their own animation.
Finally, describe in a brief paragraph how you
created your animation. Post your animation
clip to your blog in an entry entitled "Stop
Motion Animation of Falling.“
Due by 8am on Wednesday, March 3rd.
20 points (if late, 10 points)
The top three animation clips in the class will
receive a bonus of 20 extra points.
Survey Question
The lecture slides are posted
on the course website as both
PowerPoint and PDF files.
How many times have you
looked at these slides?
A) Never
B) Once, maybe twice
C) A few times
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.
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.
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
Air Resistance Threshold
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 100 m.p.h.
then air resistance is not
noticeable for a 100 foot drop.
100 feet
50
400 feet
100
900 feet
150
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!
Cat Drop Video Reference
http://www.youtube.com/watch?v=YJy17-BHQXg
Cat Drop Motion Graph
Good parabolic arc; no noticeable air resistance
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
By Wednesday of this week:
Complete the 5th homework
(Video Analysis of Path of Action)
Please turn off and return the clickers!