Chapter 3-4 Energy Work Power

Transcription

1 Chapter 3-4 Energy 3-1. The Meaning of Work 3-2. Power 3-3. Kinetic Energy 3-4. Potential Energy 3-5. Energy Transformations 3-6. Conservation of Energy 3-7. The Nature of Heat 3-8. Linear Momentum 3-9. Rockets Angular Momentum Special Relativity Rest Energy General Relativity The Energy Problem The Future 3-1. Work Work equals force times distance. W = Fd The SI unit of work is the joule. 1 joule (J) = 1 newton-meter (N m) W=Fd=(100N)(8m)=800N m=800j 3-2. Power Power is the rate at which work is being done: P = W/t SI unit of power is the watt. 1 watt (W) = 1 joule/second (J/s) The kilowatt (kw) is a convenient unit of power for many applications. 1

2 Horsepower James Watt Perfected the steam engine 200 years ago Had to provide a comparison to the work output of a horse. He found that: Typical horse could perform 497 W of work for as much as 10 hours a day Watt increased the standard to 746 W 1 horsepower (hp) = 746 W = kw 1 kilowatt (kw) = 1.34 hp Early steam engines ranged from hp 3-3. Kinetic Energy Energy is that property something has that enables it to do work. The energy of a moving object is called kinetic energy (KE): KE = ½mv 2 where m = mass and v = speed. KE increases very rapidly with speed because of the v 2 factor Potential Energy Potential energy (PE) is the energy an object has by virtue of its position. Gravitational Potential Energy: PE = mgh 2

3 3-5. Energy Transformations Energy can be transformed or converted from one form to another Nature of Heat Count Rumford supported the British in the Revolutionary War and supervised the making of cannons. He observed that during the boring process heat was given off (frictional heat) that could be used to boil water and could be produced over and over again from the same piece of metal. Heat must be energy Energy Transformations Types of Energy 1. Kinetic energy 2. Potential energy 3. Chemical energy 4. Heat energy 5. Electric energy 6. Radiant energy 3

4 3-6. Conservation of Energy The law of conservation of energy states that energy cannot be created or destroyed, although it can be changed from one form to another. Matter can be considered as a form of energy; matter can be transformed into energy and energy into matter according to the law of conservation of energy. E o = m o c 2 where E o = rest energy, m o = rest mass, and c = speed of light (3x10 8 m/s or 186,000 miles/sec) Linear Momentum Linear Momentum is a measure of the tendency of a moving object to continue in motion along a straight line: p = mv 3-8. Linear Momentum The law of conservation of momentum states: In the absence of outside forces, the total momentum of a set of objects remains the same no matter how the objects interact with one another. 4

5 3-8. Linear Momentum Newton s Cradle-an example of the conservation of linear momentum Rockets The momentum of the exhaust gases is balanced by the rocket's upward momentum. Multistage rockets are more efficient than single-stage, and so are widely used Rockets Rockets are a version of Newton s third law of motion as well as the conservation of linear momentum. 5

6 3-10. Angular Momentum Angular momentum is a measure of the tendency of a rotating object to continue spinning about a fixed axis L=mvr L= angular Momentum m=mass circling v=velocity of rotation r=distance from center The smaller the r the faster the v. Angular momentum is conserved Angular Momentum Definition: The more angular momentum an object has, the greater its tendency to continue to spin (and be stable) Toy tops Footballs The earth Bullets Defining angular momentum is complicated; depends on How fast the object is turning Mass of the object How the mass is distributed (the further the mass is from the center of the object, the greater the angular momentum) Angular Momentum Gyroscopes Due to angular momentum, when a force is applied in one direction, the combined forces, including the angular momentum, will be in a perpendicular direction. watch?v=8h98bgrzpom Torpedo Gyroscope 6

7 3-10. Angular Momentum Naval Gyroscopes used to stabilize ships and guns Angular Momentum Naval Gyroscopes used to stabilize ships and guns Special Relativity Albert Einstein ( ) published the special theory of relativity in Special relativity is based on two postulates: 1. The laws of physics are the same in all frames of reference moving at constant velocity. 2. The speed of light (c ) in free space has the same value for all observers (c = 3 x 10 8 m/s) 7

8 3-11. Special Relativity m o = mγ heavier t o l o = t / γ slower = l / γ shorter Twin Paradox Muon Experiment 3.12 Rest Energy 3.12 Rest Energy E = mc 2 or Energy and Mass are the same! Example 3.8 p 91 How much mass is converted to energy in a 100MW nuclear power plant? T=(60)(60)(24)= 86,400 s/day E=Pt=10 8 W(86,400 s/day)=8.64 x J m = E/c 2 = 8.64 x J/(3 x 10 8 m/s) 2 m = 9.6 x 10-5 kg or about oz 8

9 3-13. General Relativity General theory of relativity was developed by Einstein in 1916, which related gravitation to the structure of space and time and showed that even light was subject to gravity The Energy Problem 1. Oil and natural gas reserves will last about another century.. 2. Although coal reserves will last several hundred more years, mining coal is dangerous, and burning coal creates environmental problems such as acid rain, air pollution, and enhanced global warming. 3. The potential for a large-scale nuclear accident is present. 4. Discharge of radioactive wastes into the environment from badly run nuclear power plants has occurred. 5. An unsolved disposal problem of radioactive nuclear waste exists. Fig

10 Fig Fig

11 Fig Fig The Energy Problem Chernobyl Nuclear Accident 11

12 3-14. The Energy Problem Chernobyl Nuclear Accident The Future The Future Energy consumption

13 Lecture Quiz What is the equation that defines work? Power? 2. How many horsepower is there in one kilowatt? 3. What kinds of energy are involved in a windmill electric generator? 4. Which theory of relativity involves gravity? 5. What is the major fuel used for energy? 13