Upcoming Deadlines

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

ParabolicPath of Action

Straight Line

Parabolic Curve

Homework Assignment #6In this assignment you will create a stop-

motion 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 #6If 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 #6Your 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 #6While 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)NeverB)Once, maybe twiceC)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

Disney and other early animators rediscovered these laws of forces in their studies of motion.

Sir Isaac Newton

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

An asteroid floats in space with a constant speed unless gravity deflects its motion.

FOR

CE

Balance of ForcesRarely 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!

Gravity

Gravity

Gravity

Tension

Floor

Floor

Law of InertiaNewton’s Law of Inertia says:An object moves with constant, uniform motion until acted on by an unbalanced force.

Gravity

Floor

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

Frame 439

Frame 440

Frame 441

Stuntman flies out the window at about 10 m.p.h.

This is a bit slow but at a realistic speed the audience wouldn’t see the action.

Shoot ‘Em Up (2007)Frame 459

Frame 460

Frame 461

Frame 462

Stuntman flies into the van at about 5 m.p.h.

Noticeably much too slow but the sequence is outrageous anyway, so it works.

The League of Extraordinary Gentlemen (2003)

In this scene, Sean Connery jumps out the side of a speeding car and lands on his feet.

http://www.youtube.com/watch?v=n8SDdkKSqns

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.

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 forwardYou are moving at the same speed as the car when you jump out so you will roll forward.

Jumping out of a Car

Your path

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.

car

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.

Centrifugal Force Revisited

Your path

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.

Gravity

FloorA 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.

If the bus starts moving again, you remain stationary, seemingly thrown backwards.

Home Demo: Riding the Bus (cont.)

As seen by observer sitting in the bus

Frame of Reference

As seen by observer on the street

Bus Moves

Background

Space Balls (1987)

Jackass (2002)

http://www.youtube.com/watch?v=V-dFVdhgSsc

Class Demo: Tablecloth Pull

Yank quickly

Due to the vase’s inertia it remains at rest since almost no force acts on the vase if one pulls quickly andstraight.

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 withthe 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 ResistanceAir resistance is a force created when an

object moves through air.

Depends on:

•Size (area) of the object•Speed of the object

Larger the size or speed, larger the resistance.

Air 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?

AirResistance

Weight

Book and sheet fall together

Falling on the MoonThere’s no atmosphere and thus no air resistance on the Moon.

http://www.youtube.com/watch?v=5C5_dOEyAfk

Falling with Air Resistance1

3

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.

5

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)SlowerB)FasterC)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 SpeedAir Resistance

Gravity

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 Speed1/100 inch 1/10 5 m.p.h.

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

T

Terminal Speed & Thickness

Piece of paper falls much faster when you drop it sideways instead of face-down.

Air Resistance

Gravity

Gravity

Air Resistance

Small thickness;Slow terminal speed Big thickness;

Fast terminal speed

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 isabout 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

Tracked falling of a coffee filter.

Dis

tan

ce F

alle

n

Time

Accelerates in first 1/3 second

ConstantSpeed

Click

Air Resistance Threshold

Distance fallen from apex

Speed(miles per hour)

1 foot 5

4 feet 10

9 feet 15

16 feet 20

25 feet 25

49 feet 35

100 feet 50

400 feet 100

900 feet 150

Air resistance is only noticeable once an object’s speed gets close to its terminal speed.

This table gives the speed of an object from the distance it’s fallen it there is no air resistance.

For example, since a brick’s terminal velocity is 100 m.p.h. then air resistance is not noticeable for a 100 foot drop.

Home Demo: Drop the CatCats 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.

From: www.animalhealthcare.ca

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!

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.

Don’t try this

demo!

Squirrels cannot die from a fall.

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

iflysfbay.com

With a big fan (blowing 120-150 mph), you can experience terminal speed and skydive indoors.

Next LectureCreating Action

Part IIBy Wednesday of this week:Complete the 5th homework

(Video Analysis of Path of Action)

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