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Transcript
Physics of Technology PHYS 1800 Lecture 13 Introduction Section 0 Energy Lecture 1 Slide 1 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 1 PHYSICS OF TECHNOLOGY Spring 2009 Assignment Sheet Date Day Lecture Chapter Jan 5 M Class Admin: Intro.Physics Phenomena 1 6 T Problem solving and math App. B, C 7 W Units, Scalars, Vectors, 1 9 F* Speed and Velocity 2 Jan 12 M Acceleration 2 14 W Free Falling Objects 3 16 F* Projectile Motion 3 Jan 19 M Martin Luther King No Class 21 W Newton’s Laws 4 23 F* Mass and Weight 4 Jan 26 M Motion with Friction 4 28 W Review 1-4 1-4 29 Th Test 1 30 F Circular Motion 5 Feb 2 M Planetary Motion and Gravity 5 4 W Energy 6 6 F* Harmonic Motion 6 Feb 9 M Momentum 7 11 W Impulse and Collisions 7 13Introduction F* Rotational 8 Section 0 Motion Lecture 1 Slide 2 Feb 16 M Presidents Day No Class 17 Tu Angular Momentum (Virtual Monday) 8 18 W Review 5-8 19 5-8 H Test 2 INTRODUCTION TO Modern Physics PHYX 2710 20 F* Static Fluids, Pressure 9 Fall 2004 Feb 23 M Flotation 9 25 W Fluids in Motion 9 27 F* Temperature and Heat 10 Mar 2 M First Law of Thermodynamics 10 Physics of Technology—PHYS 1800 4 W Spring 2009Heat flow and Greenhouse Effect 10 Energy *Homework Handout 6 F* Climate Change - Homework Due - 1 2 3 4 5 - 6 Lecture 13 Slide 2 7 Physics of Technology PHYS 1800 Lecture 13 Energy Introduction Section 0 Lecture 1 Slide 3 Introduction INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 3 Dennison’s Law of minimum effort: MAKE IT SIMPLE STUPID!!! Introduction Section 0 Lecture 1 Slide 4 INTRODUCTION TO Modern Physics PHYX 2710 Einstein on the Theory of General Relativity: “Everything should be made as simple as possible, but not simpler. ... “ Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 4 Can we build on what we’ve learned to make our life easier… ... Is there some sort ofIntroduction conservation Section 0 Lecture 1 law that is related to Newton’s Laws of motion… Slide 5 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 5 Describing Motion and Interactions Position—where you are in space (L or meter) Velocity—how fast position is changing with time (LT-1 or m/s) Acceleration—how fast velocity is changing with time (LT-2 or m/s2) Force— what is required to change to motion of a body (MLT-2 or kg-m/s2) Introduction Section 0 Lecture 1 Slide 6 In this chapter we will develop on of the most useful concepts in science…ENERGY…and learn what it means to conserve energy. INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 6 Newton’s Laws in Review • 1st Law —a special case of the 2nd Law for statics, with a=0 or Fnet=0 • An objects velocity remains unchanged, unless a force acts on the object. • 2nd Law (and 1st Law)—How motion of a object is effected by a force. – The acceleration of an object is directly proportional to the magnitude of the imposed force and inversely proportional to the mass of the object. The acceleration is the same direction as that of the imposed force. F ma units : 1 newton = 1 N = 1 kg m s2 • Introduction Section 0 Lecture 1 Slide 7 3rd Law —Forces come from interactions with other objects. • For every action (force), there is an equal but opposite reaction (force). INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 7 What can apply to all these sorts of problems… Forces are acting in each case, but force is not conserved really. Introduction Section 0 Lecture 1 Slide 8 F|| INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 8 Physics of Technology PHYS 1800 Lecture 13 Energy Introduction Section 0 Lecture 1 Slide 9 Work and Energy INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 9 Defining Work • Work is equal to the force applied times the distance moved. – Work = Force x Distance: – Work output = Work input W=Fd • units: 1 joule (J) = 1 Nm = 1 kg m2 / s2 [ML2T-2] Introduction Section 0 Lecture 1 Slide 10 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 10 Work and Power • Only forces parallel to the motion do work. • Power is the rate of doing work – Power = Work divided by Time: P=W/t units: 1 watt (W) = 1 J / s = 1 kg m2 / s3 [ML2T-3] Introduction Section 0 Lecture 1 Slide 11 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 11 A string is used to pull a wooden block across the floor without accelerating the block. The string makes an angle to the horizontal. Does the force applied via the string do work on the block? a) b) c) d) Yes, the force F does work. No, the force F does no work. Only part of the force F does work. YouIntroduction can’t tell from Section 0 Lecture this diagram. 1 Slide 12 Only the part of the force that is parallel to the distance moved does work on the block. This is the horizontal part of the force F. INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 12 If there is a frictional force opposing the motion of the block, does this frictional force do work on the block? a) b) c) d) Yes, the frictional force does work. No, the frictional force does no work. Only part of the frictional force does work. YouIntroduction can’t tell from Section 0 Lecture this diagram. 1 Slide 13 Since the frictional force is antiparallel to the distance moved, it does negative work on the block. INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 13 Does the normal force of the floor pushing upward on the block do any work? a) b) c) d) Yes, the normal force does work. No, the normal force does no work. Only part of the normal force does work. You can’t tell from this diagram. Introduction Section 0 Lecture 1 Slide 14 Since the normal force is perpendicular to the distance moved, it does no work on the block. INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 14 A force of 50 N is used to drag a crate 4 m across a floor. The force is directed at an angle upward from the crate as shown. What is the work done by the horizontal component of the force? a) b) c) d) e) 120 J 160 J 200 J 280 J 0J The horizontal component of force is 40 N and is in the direction of motion: Introduction Section 0 Lecture 1 Slide 15 W=F·d = (40 N) · (4 m) = 160 J. INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 15 What is the work done by the vertical component of the force? a) 120 J b) 160 J c) 200 J d) 280 J e) 0 J The vertical component of force is 30 N but isn’t in the direction of motion: W = Introduction F · d Section 0 Lecture 1 = (30 N) · (0 m) = 0 J. Slide 16 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 16 What is the total work done by the 50-N force? a) 120 J b) 160 J c) 200 J d) 280 J e) 0 J Only the component of force in the direction of motion does work: W = Introduction F · d Section 0 Lecture 1 = (40 N) · (4 m) = 160 J. Slide 17 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 17 Physics of Technology PHYS 1800 Lecture 13 Energy Introduction Lecture 1 Slide 18 Energy and Simple Machines Section 0 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 18 Simple Machines, Work, and Power • A simple machine multiplies the effect of an applied force. – For example, a lever : • A small force applied to one end delivers a large force to the rock. • The small force acting through aIntroduction large distance Section 0 Lecture 1 moves the rock a small distance. Slide 19 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 F2 d2 = F1 d1 Energy Lecture 13 Slide 19 Simple Machines, Work, and Power • A simple machine multiplies the effect of an applied force. – For example, a pulley : • A small tension applied to one end delivers twice as much tension to lift the box. • The small tension acting through Introduction a large distance Section 0 Lecture moves the box a small distance. 1 Slide 20 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 20 Mechanical Advantage and Simple Machines • The mechanical advantage of a simple machine is the ratio of the output force to the input force. – For the pulley example, the mechanical advantage is 2. • Work is equal to the force applied times the distance moved. – Work = Force x Distance: W=Fd – Work output = Work input • units: 1 joule (J) = 1 Nm= 1 kg m2 / s2 [ML2T-2] Introduction Section 0 Lecture 1 Slide 21 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 21 Physics of Technology Next Lab/Demo: Energy & Oscillations Collisions and Momentum Thursday 1:30-2:45 ESLC 53 Ch 6 Next Class: Friday 10:30-11:20 Slide 22 BUS 318 room Review Ch 6 Introduction Section 0 Lecture 1 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Energy Lecture 13 Slide 22