Upcoming Deadlines

Fifth Homework (Video Analysis of a Jump)Due Thursday, Sept. 29th (Next week)15 points (10 points if late)

Sixth Homework (Stop-motion Animation)Due Thursday, 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 #5Use 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

Thursday, September 29th (next week).15 points (10 points if late) *May be tricky

Homework Assignment #5

ParabolicPath of Action

Straight Line

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.

Creating Action

Why Things MoveSo 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 ForcesNewton 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-ThroughWhen 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 ForcesAn 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.

FORCE

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

GravityGravityTension

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 BusWhen 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.

GravityFloor

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.

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 MovesBackgroun

d

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 & DragAn 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 WavesAnimation drag is very noticeable when something like hair or cloth moves in an arc or in a wave-like motion.

Fukkireta

http://www.youtube.com/watch?v=NFep4vO4TRc

Click to play

Class Demo: Hula SkirtThe 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 ExercisesThe 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 TestAnimate 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 VacuumFeather 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 SheetA 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

Falling Coffee FilterTracked falling of a coffee filter.

Dist

ance

Falle

n

Time

Accelerates in first 1/3 second

ConstantSpeed

Click

Terminal SpeedSpeed 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 AnvilThe 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 AnvilThe 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 & ShapeTerminal 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 LectureCreating Action

Part IIBy Thursday of next week:Complete the 5th homework

(Video Analysis of Path of Action)

Please turn off and return the clickers!