#### Transcript L08_Action - Virtual Homeschool Group

Upcoming Deadlines Sixth Homework (Outline of First Term Paper) Due Wednesday, September 30th Seventh Homework – Cancelled due to furloughs (Automatic 20 points of credit) Campus-wide Furlough Day Monday, October 19th (Art/Phys 123 will meet on Wed., Oct. 21st) For full schedule, visit course website: ArtPhysics123.pbworks.com Activating your Clicker * Turn on your clicker. * Enter the number or letter that I give you for joining this class. Hit Enter/Send key. * Clicker should read PHY123SCI2 * Type in your student ID; hit Enter/Send. Clicker is now ready to use. Hit any key to wake the clicker from sleep mode. Homework Assignment #6 Write a one-page outline for your first term paper. The general topic for your first term paper is: The Laws of Physics in an Animation Universe Post this outline on your blog; an example outline and term paper have been posted on the course blog. This assignment is due by 8am on Wednesday, September 30th. (10 points; 5 points if late) Note: The term paper will be due in mid-October. Extra Credit Opportunity Visit the Walt Disney Family Museum in San Francisco (opens Oct. 1st). Give me your ticket receipt for ten points extra credit. Hours: Wednesday-Monday: 10a.m.-6p.m Admission: $15.00 for students Location: 104 Montgomery Street Inside The Presidio of San Francisco Extra Credit Opportunity Visit the Exploratorium in San Francisco. Give me your ticket receipt for ten points extra credit. Hours: Tuesday-Sunday: 10a.m.-5p.m Admission: $11.00 for students Location: 3601 Lyon Street, SF 94123 Next to Palace of Fine Arts complex 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. Play 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. Play Jumping out of a Car You are moving at the same speed as the car when you jump out so you will roll forward. 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. 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. Play Frame of Reference Bus Moves Background As seen by observer on the bus As seen by observer on the street Class Demo: Tablecloth Pull Flower Vase Tablecloth Yank quickly Due to the vase’s inertia it remains at rest since almost no force acts on the vase if one pulls quickly & straight. 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. 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. Play Play Dancing with the Sacks 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 The Law of Inertia explains motion without forces (or with only balanced forces). The Law of Acceleration explains motion with unbalanced forces. Forces & Slowing In/Out B A L L N G F A L L I When a force pulls in the direction that an object is already moving, the object slows out (accelerates) Moving this way Force If a force pushes opposite to the direction of motion then the object slows in (decelerates) B A L L Force R I S I N G Moving this way Forces & Path of Action When a force is perpendicular to the path of action then it deflects the motion into an arc. If force is at an arbitrary angle then both timing and path of action are affected. Force (up & left) Moving this way More Force, More Acceleration The greater the net force on an object, the greater the acceleration of that object. More Mass, Less Acceleration The greater the mass of an object, the less it accelerates when acted on by a force. Simple Addition of Forces When two forces pull in parallel directions it’s simple to add them to get the total force. Gravity (Weight) Total Force For example, if a bubble weighs 3 oz and has a buoyancy of 2 oz then the total force is 1 oz. Buoyancy Law of Acceleration If the total force equals the object’s weight then the spacing is the same as when falling. If the total force is greater than the weight then the spacings and increments are larger. If the total force is less than the weight then the spacings and increments are shorter. Law of Acceleration Example The total force equals 1/3 of the weight so the spacings are 1/3 of what they are for falling. Gravity (Weight) Total Force Normally an object that falls for 6 frames goes a distance of 12 inches so this bubble falls only 4 inches after 6 frames Golf Ball Bubble Buoyancy 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. Play (by Gloria Cho) Play (by Kanako Shibanuma) Air Resistance Air resistance is caused by molecules of air striking a moving object. Air Resistance Depends on: •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. 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. Play 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. Falling Coffee Filter Tracked falling of a coffee filter. Height Play Play Accelerates in first 1/3 second Constant Speed Time 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. Home Demo: Keep It Up You can estimate the terminal speed as the wind speed needed to support the object. With a big fan (blowing 120-150 mph), you can even “skydive” indoors. iflysfbay.com Play 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! Wile E Coyote with Anvil Wile E Coyote is falling, with an anvil in his backpack. If he gets rid 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 Estimating Terminal Speed (cont.) Terminal speed of aerodynamic shapes, such as a sphere, are about 50% faster than for the rectangle. For example, the terminal speed of a raindrop with a radius of 1/8th inch is about 20 m.p.h. Terminal speeds for objects made of denser materials is also higher. For stone the terminal speed is about twice as large; for metal it’s about three times larger. For example, a brick’s terminal speed is about 140 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 Speed Without Air Resistance 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 140 m.p.h. then air resistance is not noticeable for a 400 foot drop. 100 feet 50 400 feet 100 900 feet 150 Next Lecture Collisions, Crashes, and Combat By Wednesday of this week: Complete the 6th homework (Outline for First Term Paper) Please return the clickers!