Unit 4: Momentum and Energy

Transcription

1 Unit 4: Momentum and Energy Brent Royuk Phys-109 Concordia University Momentum Definition: momentum = mass x velocity p = mv Units: 1 kg m/s The Conservation Law In the absence of any external source, the momentum of a system of objects remains constant. 2 Conservation of Momentum The Classic Example: Two skaters 3 1

2 Conservation of Momentum Recoil: Jumping out of a boat, guns, etc. 4 Conservation of Momentum Football players 5 Conservation of Momentum Coupling boxcars 6 2

3 Conservation of Momentum 7 Conservation of Momentum Professor Goddard does not know the relation between action and reaction and the need to have something better than a vacuum against which to react. He seems to lack the basic knowledge ladled out daily in high schools New York Times editorial about Robert Goddard's revolutionary rocket work. "Correction: It is now definitely established that a rocket can function in a vacuum. The Times regrets the error." NY times, July Impulse To change momentum, a force must be applied for some time. Impulse = force x time = Δ p 12 3

4 The Impulse Bargain 13 The Impulse Bargain Bend legs when landing. Let glove recoil when catching a baseball. Follow through when batting to increase contact time. Airbags 14 Energy Introduction What do you know? Definition Forms Potential Kinetic The Unifying Idea Top-Down vs. Bottom-Up Note: No Machines or Efficiency 16 4

5 Work Definition The product of force and the distance moved due to that force W = F d Units 1 N m = 1 kg m 2 /s 2 = 1 joule (J) 17 Work Note: No motion = no work Holding up a wall Carrying a book Satellites What if you push a car at constant speed? Example: Lift a 1.0-kg weight a height of 2.0 m. How much work have you done? 18 Power Work Done Definition: Power = Time to Do It MKS Unit Another common power unit: 1 hp = 746 W Energy = P t kwh s Finding Energy Costs Just take kw times hours to get kwh s You can figure each kwh costs about $0.10, to get a quick estimate. Examples How much does it cost to leave a 100-W light bulb on for a month 19 5

6 Power Energy = Power x Time 20 Defining Energy The Work-Energy Definition Connection: Energy is the capacity Work is a process, and energy is a property of a system Aristotle coined the word energy from the Greek for at work. The Nearly Well-Know Dave Watson Definition Energy is a property or characteristic (or trait or aspect?) of matter that makes things happen, or, in the case of stored or potential energy, has the potential to make things happen. 21 Feynman on Energy It is important to realize that in physics today, we have no knowledge of what energy is. Richard Feynman There is a fact, or if you wish, a law, governing natural phenomena that are known to date. There is no known exception to this law it is exact so far we know. The law is called conservation of energy [it states that there is a certain quantity, which we call energy that does not change in manifold changes which nature undergoes]. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity, which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some number, and when we finish watching nature go through her tricks and calculate the number again, it is the same. Richard Feynman, Lectures on Physics, Vol

7 Forms of Energy Two types of energy: Kinetic and Potential Kinetic Energy: Energy of Potential Energy: Energy of Kinetic Energy Equation: K = Units? 1 2 mv2 Note v2 dependence. What s the other important type of kinetic energy? 23 Potential Energy Potential energy is energy of Any system that wants to move: something springy: springs, rubber bands, bent metal, item on a shelf, etc. For gravity: PE = weight x height = mgh 25 Potential Forms Elastic Energy Anything that stretches and snaps back: springs, waistbands, etc. Electromagnetic Energy Magnets, socks stick together, electricity that turns on lights Radiant Energy Any type of ER: radio waves, microwaves, light, x-rays, etc. Chemical Energy Comes from molecular arrangements- the energy you get when you burn things. Nuclear Energy This form only shows up when nuclei rearrange themselves, and this doesn t happen on a large scale very often in everyday life. 26 7

8 The Law of Energy AKA The Law of Conservation of Energy. Nature s Bookkeeping System Energy can change forms, but is never created or destroyed. Examples Drop a Rock Roller Coasters Path Independence 28 Energy Transformations 29 Pendulum Energy 30 8

9 Pendulum Energy 31 Graphical Analysis 32 Alternative Energy? Two types of Energy Therapy 1. Veritable (Measurable) Magnet, Light, Sound, etc. 2. Putative (Bioenergy, Qi, Prana; Has not been measured) Acupuncture, Qui Gong, Therapeutic Touch, Power Balance, etc. 35 9

10 Perpetual Motion 36 10