VIEWS: 7 PAGES: 65 POSTED ON: 10/1/2011
Concepts that drive the world Energy: Physics Energy cuts across all fields Physics E = mc2 How stars shine Electricity Heat Roller Coasters Energy: Climate & Weather… Climate/Weather Hurricanes Several thousand atomic bombs Lightning Global warming – redistribution of energy across the globe Energy: Chemistry Energy cuts across all fields Chemistry Activation energy Energy needed for a reaction to occur Heat and temperature is important Think of Ozone... Energy: Food Food Average American 2700 Calories (recommended 2200) Note the big C! kilocalories Really means 270,000 calories Nutrition Labels Nutrition labels tell you about the energy content of food Conversions: Fat: 9 Cal/g Carbs: 4 Cal/g Protein: 4 Cal/g This product has 72 Cal from fat, 48 Cal from carbohydrates, and 32 Cal from protein sum is 152 Calories: compare to label 152 Cal = 636 kJ: enough to climb about 1000 meters (64 kg person) 1 Cal = 4.134 Jules Energy and Food… Average American 2700 calories (recommended 2200) 7 acres to support one American 1.5 acres to support one African The world has enough capacity to support around 10 billion people So what supports me? How did the science that we used to get to the Moon evolve? Aristotle: A long lasting paradigm Natural motion depended on nature of the object. Examples: A rocks falls Smoke rises The falling speed of an object was supposed to be proportional to its weight So heavier objects fall faster Natural motion could be circular (perfect objects in perfect motion with no end) So planets orbit the Sun in perfect circles Aristotle: Why do things move? Pushing or pulling forces imposed motion. Some motions were difficult to understand. Example: the flight of an arrow Must be some force that pulls it forward after it leaves the bow air rushed around behind the arrow and pushed it forward. Other Contributions Aristotle also believed: Earth was immovable at the center of the Celestial Sphere, and - All other celestial bodies: moved with uniform speed. In pure circular motion Are "perfect" and unchangeable So how do things move anyway: The Greek perspective Earth was special and at the center Series of Crystal Spheres (Aristotle) The prime mover Outer sphere Just inside the prime- mover sphere was the fixed non-moving stars Inner spheres were the planets the Sun and the Moon The Greeks: Contributions One of the most important - and infamous - contributions of the Greeks was the geocentric model of the universe. The geocentric model means that the Earth, "geo," is at the center, "centric," of the universe, and that all other bodies move around it with the Earth at a fixed location. Why Earth should be the center? This view held so much sway because of many of the philosophies of the ancient Greeks. They believed that the circle is the perfect form, and that the simplest model that made sense must be the correct one. since the simplest model was that the Earth stood still and everything moved around it, then that must also be true. After all, we can't feel the Earth moving, so why should be believe that it does without any extraordinary evidence? Aristotle: The man in science!!!!!!!! Aristotle was unquestioned for 2000 years. Most thought that the Earth was the center of everything for it was in its normal state No one could imagine a force that could move it. It was huge!!!!!!!!!!!!!!! COPERNICUS: A new paradigm Sun was center, not Earth. He was hesitant to publish because he didn't really believe it either. De Revolutionibus reached him on the day he died, May 24, 1543. Copernicus – gradually moving Nicolaus Copernicus (1473-1543) articulated a cosmological theory discounting the geocentric Ptolemaic universe. did not abandon the Ptolemaic cosmogony all together. his observations resulted in a universe limited by the sphere of the fixed stars. Copernicus not alone… Giordano Bruno (1548-1600) criticized this limited scheme. His theories of the infinite universe rejected the traditional geocentric (or Earth-centered) astronomy the sphere of the fixed stars is only a trick of the eye because the fixed stars are too far away to recognize their movement Lets see if Bruno was right… Parallax, or more accurately motion parallax (Greek: παραλλαγή (parallagé) = alteration) apparent shift of an object against a background due to a change in observer position Easy to test with your finger!! Earth and parallax GALILEO: The emergence of a new paradigm 17th Century scientist who supported Copernicus. He refuted many of Aristotle's ideas. Worked on falling object problem - used experimentation!!!. Galileo and myth Although historical evidence does not appear to support the contention, Galileo is alleged to have demonstrated by dropping balls of different weights from the Leaning Tower of Pisa that they fell with constant acceleration. Quick Quiz Imagine the following situation: You have a coffee filter and a lead ball. You drop them at the same time which one hits first? Wad up the coffee filter to the same shape as the lead ball which one hits first? Well…. Lets find out…. Do some tests... Galileo experiment on the Moon A tennis ball and a golf ball dropped side-by-side in air. The tennis ball is affected more by the air’s resistance than the golf ball. The larger the object is, and the faster it is falling, the greater the air’s resistance to its motion, as skydivers all know… When most of the air is removed from a container, feathers and apples fall almost side- by-side, their speeds changing at almost the same rate. If all the air was removed, they would accelerate downward at exactly the same rate. Galileo: Summary Knocked down Aristotle's push or pull ideas. Rest was not a natural state. The concept of inertia was introduced Galileo is sometimes referred to as the “father of experimentation.” Objects fall at the same rate, independent of weight! (acceleration, though Galileo didn’t know the word yet) Important for the navy!!! Implications: Dropped Ball: Falling Downward In free fall objects accelerate constantly toward Earth at the rate of g . Objects moving upward slow down until their direction is reversed, and then they accelerate downward. At the top of their path the upward speed is zero. How long? Only instantaneously. A constant acceleration means the speed is changing all the time, so the speed only passes through the value of zero at the top of the path. Tossed Ball: Falling Upward Here two heavy balls begin “free fall” at the same time. The red one is dropped, so it moves straight downward. The yellow ball is given some speed in the horizontal direction as it is released. The horizontal lines show that they keep pace with each other in the vertical direction. Why? They have the same acceleration, g, downward, and they both started with zero speed in the downward direction. The yellow ball’s horizontal speed is not affected by gravity, which acts only in the vertical direction So they hit the ground at the same time! Cannonballs shot horizontally with different speeds from the ship travel different distances. But each cannonball drops the same distance in the same amount of time, since the vertical acceleration is the same for each. So no matter how fast you fire the cannonball they are all in the air the same amount of time!!!! Sir Isaac Newton (1642 – 1727) Published the Principia Laid out his laws of motion and universal gravitation laws Newton showed that Kepler’s Laws are specific examples of broader implications of very general laws of motion and gravity Not a very nice man Brilliant, knew it, didn’t hide it, destroyed rivals Newton’s Three Laws 1. A body continues at rest or in uniform motion in a straight line unless acted on by some net force. 2. The acceleration of a body is inversely proportional to its mass, directly proportional to the net force, and in the same direction as the net force F = mass * acceleration 3. To every action, there is an equal and opposite reaction Newton’s First Law A body at rest tends to remain at rest A body in motion tends to remain in motion Newton’s 1st Law: Why you wear a seatbelt Newton’s Second Law The force exerted on an object is equal to the product of that object’s mass times its acceleration. The acceleration is in the same direction as the force. Force = mass x acceleration F=ma (force and acceleration are vectors) Newton’s 2nd Law What is Acceleration? Acceleration is: (change in velocity)/(change in time) How fast you change your velocity in a certain amount of time Velocity is a vector – has direction and magnitude So change your direction or speed you experience acceleration Where does the bug have: Constant velocity ? Acceleration ? Quiz time again Imagine you are driving a car. The car is moving at a constant velocity of 50 mph. You cover a distance of 50 miles. What is its acceleration? 1 m/hour2 50 m/hour2 2500 m/hour2 0 m/hour2 Newton’s 3 rd Law The Law of Action and Reaction: When any two objects interact, the force exerted by the first object on the second is equal and opposite to the force exerted by the second on the first. Or, for every action (force), there is an equal and opposite reaction; that is forces are mutual and act in pairs. Quiz Time again: Newton’s 3rd Law Sometimes movies get it right Watch the fire extinguisher… Red Planet Action-Reaction Pairs Book pulling on Earth Table pushing on book Physics Physics is Fun! Earth pulling on Book Book pushing on table Newton’s Law of Gravitation G m1m2 F = - r2 where G is the “gravitational constant” 6.67 x 10-11 Nm2/kg2 m1 is the mass of one body m2 is the mass of another body r is the separation between them (frequently labeled d) Measured from the center of each object Inverse Square Law Quiz Time!! Imagine you have two objects of equal mass (say 10 kilograms). The two objects are setting 2 meters apart and you move them till they are 4 meters apart. How much has the gravitational force changed? Forces Initial After G m1m2 G m1m2 Fa = - 2 Fb = - r r2 G m1m2 G m1m2 Fa = - Fb = - 2 2 42 G m1m2 G m1m2 Fa = - Fb = - 4 16 Double the distance decrease the force by Four Newton Gravitation and the Earth From the second law of motion (F = ma), we know that a body of mass m subjected to the Earth's gravitational force of attraction F undergoes an acceleration at the Earth's surface of g = F/m. From the law of gravitation, this force is F = Gm1m2/r2, where m1 is the mass of the Earth, and m2 is your weight and r is the separation between the centers of the two bodies, or the Earth's radius. Figuring out acceleration due to gravity Assuming that we know G, we have m2g = Gm1m2/r2, or g = Gm1/r2 = 9.8 m/s2 where your mass has canceled out. The acceleration of the attracted body (you) does not depend on its own mass Mathematical proof that Galileo was right!!!! Weight and Mass Weight is a type of force: It is the earth’s gravitational force on an object An object’s weight is proportional to its mass weight mass weight = constant · mass On the Earth’s surface, that constant, g, is 9.8 Newtons/kilogram = 9.8 meters/second2 (9.8 is approximately 10) 32 feet/second2 g is called the acceleration due to gravity Forces are measured in… • Force = mass x acceleration, or F=ma (Newton’s 2nd law) • Measured in Newtons (unit) • weight = mass x acceleration, and on earth, W=mg • 1 Newton 1 kilogram·meter/second2 (definition) • A Newton is about ¼ pound, about the weight of a medium apple! So, how does this effect me??? Helps us in many ways Cars, rockets, merry-go-rounds Why objects fall down… Newton felt that the force of gravity was focused at the center of an object Intuition at first Needed to invent calculus to prove it Center of mass •Much of physics involves looking for ways to simplify complicated interactions. •An example is the motion of a baseball bat thrown into the air. •If one looks carefully, there is a special point of the bat that moves in a simple parabolic path. •That special point is the center of mass of the bat. The center of mass of a body or a system of bodies is the point that moves as though all of the mass were concentrated there, and all external forces were applied at that point. Why things fall over…. Every object has a special point called the center of gravity (CG). The CG is usually right smack in the center of the object. if the center of gravity is supported, the object will not fall over. Application ! You generally want a running back with a low CG then it’s harder to knock him down. The lower the CG the more stable an object is. Next Going to use what we have learned about energy and forces to get us to the Moon and how roller coasters work!
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