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## Chapter 3 outline: Main Ideas

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**Chapter 3 outline: Main Ideas**• Work and Power • Energy • Kinetic Energy • Potential Energy • Conservation of Energy • Momentum • Linear Momentum • Angular Momentum • Energy and Civilization 3-1**Work**• We have seen that FORCE is the physical phenomenon behind motion and changes in motion • WORK is a way to determine the measure of CHANGE caused by a force 3-2**Not the usual definition**• Definition of WORK: Work=Force x Distance • This means that something has to move • The force must be the force along the direction of motion • UNIT: Joule = Nm • Other Unit: foot-pound (English System) • Energy: same as work • UNIT: Joule • Power: Energy / time • UNIT: WATT 3-3**Mathematically**• Work is expressed in a simple equation: • Where the Force and the distance through which the object is moved are along the same direction 3-4**Special Case: Work against gravity**• Whenever any object is lifted, work must be done against the gravitational force in order to move the object • To determine the magnitude of this work, we begin with the definition of the force due to gravity on an object of mass m, also called the WEIGHT of the object: • And represent the distance moved (upward or downward) with the letter h in the WORK equation: 3-5**Reminder: Newton 2**• According to Newton’s second law: • FORCE is defined as mass x acceleration • Because of the relationship between WORK and ENERGY, Whenever energy is being expended, a force is being exerted. 3-6**POWER**• Power is a measure of the RATE at which work is accomplished • Defined by • The unit of power: WATT (W) • 1 W = 1 J/s 3-7**Definition of ENERGY**ENERGY CAN BE THOUGHT OF AS “THE ABILITY TO DO WORK” 3-8**Two Main Kinds of Energy**kinetic energy • The energy of motion • ONLY MOVING OBJECTS have kinetic energy potential energy The energy of position Many different types of potential energy 3-9**KINETIC ENERGY (KE)**• The energy of MOTION • SIMPLE definition NO VELOCITY NO KE LARGER MASS LARGER KE LARGER VELOCITY LARGER KE 3-10**Discussion Example**Which object has the greatest amount of kinetic energy? A. 200 g baseball traveling at 50 m/s B. 4 kg bowling ball traveling at 7 m/s C. 2000 kg truck sitting in the parking lot 3-11**Do the Math, using our definition of KE**A. 200 g baseball traveling at 50 m/s KE = (1/2) x 0.200 (kg) x [50 (m/s)]2 = (1/2) x (0.200) x (2500) JOULES = 250 Joules C. 2000 kg truck sitting in the parking lot KE = (1/2) x 2000 (kg) x [0 (m/s)]2 = (1/2) x (4) x (0) JOULES = 0 Joules B. 4 kg bowling ball traveling at 7 m/s KE = (1/2) x 4 (kg) x [7 (m/s)]2 = (1/2) x (4) x (49) JOULES = 98 Joules 3-12**Potential Energy due to Gravity**• An object possesses Potential energy due to its position in the gravitational field. • On Earthwe define gravitational potential energy in terms of the distance of the object from the ground. • Potential energy of an object due to gravity is the gravitational potential energy = m x g x h 3-13**Discussion Example**Your old car has stalled again. • Your friends push it to the top of a hill and it rolls down the other side. • Half way down the other side the engine starts. At what point did your car have greatest potential energy? Gravitational PE mass height off the ground So, the car has the greatest gravitational PE when it is at the TOP OF THE HILL 3-14**Discussion Example**Two cars of equal mass are on the same street. The street is level. One car is driving with a velocity of 20 km/hr due north while the other car is parked. Which car has more gravitational potential energy? A. Both cars have the same gravitational potential energy B. The parked car has more gravitational potential energy C. The moving car has more gravitational potential energy D. The heavier car has the most gravitational potential They are the same! Gravitational Potential energy depends only on the distance from the ground, not on the object’s motion. 3-15**Transformation of Energy**CONSERVATION OF ENERGY Energy may change form from one type to another, BUT THE TOTAL AMOUNT OF ENERGY IN THE UNIVERSE DOES NOT CHANGE 3-16**SIMPLE PENDULUM**3-17**The simple PENDULUM**• During the motion of a swinging pendulum, the energy is constantly changing from KE (kinetic energy) to PE (potential energy) • KE is MAX at lowest point • KE is MIN at the top of the path (v - 0) • PE is MAX at top of path 3-18**EXAMPLE: Energy Transfer**A roller coaster (unmotorized) car • starts coasting at the top of one hill, • rolls down into a valley and then • comes to a stop at the top of a neighboring hill. The top of the neighboring hill is at a lower elevation than the starting point. The transfer of energy in this case is potential to kinetic and heat and back to potential and heat. 3-19**Other FORMS of ENERGY**• Thermal Energy • Elastic Potential ENERGY (Springs) • Chemical Potential Energy • Wave Energy • Mass Energy 3-20**Thermal Energy**• James P. JOULE discovered that HEAT is a form of ENERGY • Two objects with the same temperature have the same amount of thermal energy. • Common unit of HEAT ENERGY calorie OR CALORIE • HEAT energy is EVER PRESENT!! • Whenever energy is being expended, heat is always produced. 3-21**The Heat Unit: Calorie or Calorie**• CALORIE (with a big C): amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius. 3-22**Other types of potential energy**• Elastic potential energy • Springs • rubber bands • CHEMICAL Potential Energy • Energy in a battery • Energy in any combustible material 3-23**But Wait there’s more…**• Wave Energy • Energy can take the forms of waves, including sound waves and light waves. • Sound Waves • Sound wave energy causes kinetic energy of molecules in the air or some other medium. • Sound waves are longitudinal waves. • Sound waves are transmitted by molecular motion. • Sound waves are created by a vibrating object. 3-24**What is HEAT??**Heat is the transfer of energy NOTE: • we cannot say that matter “contains” heat • rather, matter contains energy • The amount of energy matter contains • can be change by transfer of heat 3-25**POWER = ENERGY PER TIME**• OR • Power Unit? • unit for energy = Joule • Unit for time = second • So, unit for power = Joule / second Joule/second = WATT 3-26**This is the**UNIT OF ENERGY on your ELECTRICITY BILL (kWh) ENERGY and POWER • New Definition: Power = Energy / Time • This means Energy = Power x Time • so, another valid unit for ENERGY is • Watt x second OR • Watt x hour OR SOMETHING EVEN MORE FAMILIAR • Kilowatt x hour 3-27**Watt about Horsepower???**• Horsepower is a unit invented by JAMES WATT • so that he could impress people about the strength of his steam engine • one horsepower = 550 ft-lb per second = 746 watts 3-28**More about describing motion**MOMENTUM A quantity that helps to describe the effect of an object’s motion 3-29**Momentum**• Momentum is a quantity which tells us more about the motion than speed or position • Usually, linear momentum is denoted by the letter “p” p = m x v MOMENTUM 3-30**Discussion/ Thought**• A kid rolling down the street on a skateboard... or • a large truck at rest? 3-31**Discussion Question**A student on a bike is traveling down a street with a velocity of 10 m/s toward the south. She passes a delivery van parked on the side of the road. Compare the momentum of the biker with that of the van. A) The van has more momentum than the biker due to its much larger mass. B) The biker has more momentum than the van because the biker has a non-zero velocity. c) The van and the biker have about the same momentum D) None of the above v-> v=0 3-32**Collisions-a way to use momentum**• Collisions provide a way to better understand motion. • Two main classes of collisions • Elastic collision • Inelastic collision Elastic collision: objects not deformed bouncing balls, bumper cars... Inelastic collision: objects become permanently deformed auto accidents, throwing pie in someone’s face 3-33**Conservation of Momentum**• When outside forces are not present, the combined total momentum of objects in motion does not change • This is true for all motion • Momentum can be EXCHANGED during the interaction of particles in motion, but the system as a whole can not gain or lose momentum • So, for any interaction: MOMENTUM BEFORE = MOMENTUM AFTER 3-34**HUMAN ENERGY USE**• Humans convert CHEMICAL ENERGY, stored in food substances, into KINETIC ENERGY, present whenever our muscles are at work. • If we take in more calories than we expend, our body stores energy as fat to make up the difference. • Estimate the amount of power given off each day by a person consuming 1500 KCal per day on a diet which stores no food as fat (all energy is expended) 3-35**FIGURE IT OUT!!**POWER = ENERGY / TIME • Find the energy (Joules) 1500 CALORIES = 1500 kilocalories # Joules(J) = 4.184 x Calories Energy (J) = 4.184 x 1500 x 103 calories Energy = 6,276,000 Joules • TIME = # SECONDS IN ONE DAY 24 hours x 3600 seconds/hour = 86,400 s • POWER = ENERGY / TIME = 6,276,000 Joules / 86,400 s = 73 WATTS 3-36**Energy and Civilization**• The source of all fuels used for energy on our planet is the SUN • All food sources and fuel sources were at one time given energy via the radiation that falls on our planet from the sun • Today, the majority of our energy is provided by fossil fuel sources • These sources are ‘finite’ 3-37**Fossil Fuels**• PETROLEUM PRODUCTS • COAL • NATURAL GAS • All are carbon based • All are rich in chemical potential energy. • The are classified as nonrenewable resources. • The natural processes that form fossil fuels still happen on Earth- but at a much slower rate than previously 3-38**Fossil Fuels USE**• For industrial nations fossil fuels account for Ninety percent of the total fuel consumption • Where did the energy stored in fossil fuels come from originally? • The energy stored in fossil fuels had to come from somewhere…. • originally in The Sun (think about it...) 3-39**SOLAR ENERGY**• An example of a RENEWABLE energy resource (sort of) • BIG problem: STORAGE • Battery technology still has a long way to go • With poor means of storage, Solar Energy is available ONLY during times when the sun is shining 3-40**Energy: Summary**• Work is defined as a measure of the change caused by a force • The unit of work is JOULE (N m) • Power is the rate of work done • The unit of power is WATT (W) • Energy is the property of something to do work • The unit of energy is JOULE • Energy occurs in several forms: kinetic, potential, chemical and rest energy • Linear momentum is a measure of the tendency of a moving object to stay in motion: p = mv 3-41**Energy: Summary(2)**• Angular momentum is measure of the tendency of a rotating body to keep rotating • All momentum is conserved • The special theory of relativity defines the rules for measurements when there is relative motion between the observer and the object to be measured • General relativity relates gravity to the structure of spacetime 3-42**In-Lecture Quiz Chapter 3**If you touch the desktop, and then touch the metal legs of the desk, one will “feel” colder. Which object has a higher temperature? A. Metal Leg B. Desktop C. They are the same D. Not enough information to tell 3-43**In-Lecture Quiz Chapter 3**Name a unit that could be used to measure HEAT. A. Newton B. Joule C. Calorie D. B and C E. A, B and C 3-44**In-Lecture Quiz Chapter 3**In which of the following cases is WORK DONE? A. You try to pull your car out of a ditch, but it does not move. B. Your room-mate kicks his dirty clothes into the corner of his room. C. Your mother holds a sleeping baby. D. You sit at your desk and study physics all night. 3-45**In-Lecture Quiz Chapter 3**• Suppose you climb a flight of stairs to an elevation of 4 meters. How much work did you do? A. No Work (0 Joules) B. It depends on how much you weigh C. It depends on how wide the stairs are D. It depends on whether you are wearing your favorite shoes 3-46**In-Lecture Quiz Chapter 3**A driver increases his velocity from 50 km/hr to 150 km/hr. How much does he increase the kinetic energy of the car with this increase in speed? • A. Kinetic energy is 3 times greater • B. Kinetic energy is 6 times greater • C. Kinetic energy is 9 times greater • D. Kinetic energy is 12 times greater 3-47**In-Lecture Quiz Chapter 3**• You realize that you have left your shoes in your car. You must walk outside with bare feet. It is a HOT sunny afternoon. You have the choice of (A) walking on the grass or (B) walking on the sidewalk. Think about which one you choose? 3-48**A**B C In-Lecture Quiz Chapter 3 In a simple pendulum system, we know that energy is continuously converted from potential to kinetic. At which point in the path of the pendulum is the kinetic energy greatest? A. Top of path (highest point) B. Bottom of path (lowest point) C. Somewhere in between A & B,like point C D. It depends on the mass 3-49**In-Lecture Quiz Chapter 3**• An object which is AT REST can have: A. A non-zero Kinetic energy B. A non-zero Gravitational Potential Energy C. A non-zero Momentum D. non-zero Mass E. non-zero Veloctiy NOTE: There may be more than one choice… 3-50