PS100, Section 3, Winter 2000 Chapter 7, Page 1

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1 Vocab quiz and TEST 1!! Main Ideas Vocab quiz can be done through midnight Mon. Exam 1 deadline is Tues. Start doing practice questions now. Conservation Laws energy mass charge linear momentum angular momentum Forms of Energy Energy Transfer and Transformation 1 2 Conservation Laws The law of conservation of stupidity. What does it mean to conserve something? A conserved quantity: can neither be created or destroyed, but can be changed from one form to another or transferred from one object to another. Everyone is born with a certain amount of stupidity. The smarter they become about one thing, the stupider they become about something else. This is why some science/computer geeks are so socially inept. 3 4 Conservation of Linear Momentum Which of these explains why the exhaust being ejected causes the rocket to accelerate? Momentum is (mass) x (velocity) Conserved in the absence of external net force For a system with constant mass, this is just of restatement of Newton s First Law 5 (A) The rocket pushes backward against the exhaust, and so the exhaust pushes forward against the rocket. (B) An object in motion tends to stay in motion. (C) The exhaust pulls the air away from the front of the rocket creating a region of lower pressure. The rocket is then sucked forward into the low pressure region. (D) As the rocket moves upward, the force of gravity becomes smaller so that it is easier to push the rocket faster. (E) The motion of the rocket causes higher pressures in front of the rocket which act on the rocket to move it forward. 6 PS100, Section 3, Winter 2000 Chapter 7, Page 1

2 Conservation of Linear Momentum When a firecracker explodes in midair, what is true about it after the explosion? A. the path of the center of the explosion is exactly the same as the path the firecracker would have followed if it had never exploded. B. The path of the center of the explosion is higher than it would be if the firecracker hadn t exploded C. The path is lower D. The path is impossible to predict. Conservation of Linear Momentum at Very High Speeds From the point of view of the astronauts there is no momentum before the explosion, and equal and opposite momentum after. 7 8 Conservation of Linear Momentum at Very High Speeds From The distance the point measured of view on ground of ground control control is shorter the than explosion the one measured stops ½ on of the object space shuttle, but the so it other doesn t half travel is NOT as far moving each second. twice as fast! At face value this violates conservation of momentum What gives? Mass Increases at High Speed! p = m v Either momentum is not conserved at high speeds or mass is not conserved. Conservation of momentum follows from position symmetry (Noether s theorem). This makes it very solid. Mass, as it turns out, is not conserved by itself. Instead it is conserved together with energy by the relation E = mc Conservation of Angular Momentum Conservation of Angular Momentum Angular Momentum: (mass) x (speed) x (distance to rotation axis) Torque: product of part of force perpendicular to rotation axis times the distance to the rotation axis ( twisting force ) Law: angular momentum is conserved in the absence of an external torque 11 Angular momentum = m v r Because of the conservation of angular momentum, divers can control their rotational velocity by tucking their limbs. Demos: The Chair Video: skater 12 PS100, Section 3, Winter 2000 Chapter 7, Page 2

3 Conservation of Energy Conservation of Energy Energy can be neither created nor destroyed. The total amount of energy in the universe never changes. However, energy can change from one form to another, or be transferred from one object to another. (Note: we will have to modify this later.) Every human being is born with a certain amount of energy Most of us squander ~ 90% of our energy in the first 10 years of life. This explains why parents never have enough energy to keep up with their kids What is Energy? Forms of Energy Energy is a measure of the destructive capacity something has. Some things have energy because of where they are sitting. Some things have energy because of how fast they are moving. Kinetic Energy energy of motion Potential Energy energy of position Kinetic Energy Gravitational Potential Energy energy of motion What would you rather catch, a 100gram baseball moving at 10m/s or a 10 gram bullet moving at 100m/s? energy associated with the height of an object gravitational potential energy = (weight)(height) kinetic energy = (1/2)(mass)(speed 2 ) mass matters, but speed matters more PS100, Section 3, Winter 2000 Chapter 7, Page 3

4 Electrical Potential Energy Internal Energy energy associated with the electrical force Case 1 Case 2 A A B B video video 19 Energy of individual atoms and Thermal energy random kinetic energy of atoms and Electric potential energy of atoms and Physical state Elastic potential energy, Chemical potential energy 20 Examples of conservation Energy Transfer and Transformation (in book) Jumping Bouncing a ball Bowling ball on a string Water behind a dam Pole vault Conduction (Heat Flow ): a process in which internal energy is transferred because of a difference in temperature. (electric stove, soldering iron) Radiation: energy is transmitted by visible light, infrared radiation, ultraviolet radiation, X-rays, or radio waves. (sun, space heater) Convection: internal energy is transferred from because matter moves from one place to another. (hot air furnace, gulf stream, Santa Ana wind) Work: energy is transferred or transformed by forces acting on an object. (friction, muscles, electric motor) skater Where does the skater s extra energy come from? Friction with the ice Elastic potential energy in the rubber band Gravitational potential energy Popcorn Name 3 methods for popping popcorn Microwave -- Radiation Hot air -- Convection Stove top -- Conduction Identify the method of heat transfer for each PS100, Section 3, Winter 2000 Chapter 7, Page 4

5 Which of the three methods is analogous The three to convection? stooges put out a fire B C A Particle Accelerators Nuclear Reactor Small particles are regularly accelerated to speeds near the speed of light. The faster they go, the more force is required to accelerate them. What is the ratio of force to acceleration? Nuclear bombs and nuclear reactors work change energy enough to measure changes in mass. What you start with has a little more mass than what you end up with. Mass dropped because the products have less energy. The energy is released as light, sound, heat, blown up buildings etc PS100, Section 3, Winter 2000 Chapter 7, Page 5