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Transcript
Physics of Technology PHYS 1800 Lecture 3 Introduction Section 0 Motion Lecture 1 Slide 1 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 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 Motion 10 *Homework Handout 6 F* Climate Change - Homework Due - 1 2 3 4 5 - 6 Lecture 3 Slide 2 7 Physics of Technology PHYS 1800 Lecture 3 Motion Introduction Section 0 Lecture 1 Slide 3 Units of Motion INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 3 What Do We Need To Measure? What is the minimum about things we need to know? Where things are—a length, L When things are there—a time, T How thing interact with gravity—a mass, M How things interact with E&M—a charge, Q Introduction Section 0 Lecture 1 Slide 4 How thing inter act with weak nuclear force How things interact with strong nuclear force INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 4 Describing Motion Position—where you are in space (L-meter) Speed—how fast position is changing with time (LT-1 or m/s) Acceleration—how fast speed is changing with time (LT-2 or m/s2) Introduction Section 0 Lecture 1 Slide 5 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 5 Units of Motion Need a distance unit: m, cm, mm, km ft, in, mi light years, furlongs Need a time unit: sec, min, hr, day, year ms, ns, fs fortnights Speed: A distance divided by time (DT-1) m/s, mi/hr, mm/yr, furlongs/fortnight Acceleration: A distance divided by time squared Introduction Section 0 Lecture 1 Slide 6 (DT-2) A speed divided by time (DT-2) m/s2, mi/hr2, mm/yr2, furlongs/fortnight2 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 6 Examples of Distance Units Consider the lowly penny: Value 0.75 in 2 cm Scientific notation 7.5 · 10-1 in 2 cm 0.02 m Notes on units “natural unit” “natural” unit, metric unit (note prefix) SI unit, metric unit 0.00002 km metric unit (note prefix) 2 · 10-5 km 2 · 10-2 m 1.2 · 10-5 mile 0.000012 mile 0.01 fathom “odd” unit 1 · 10-2 fathom 0.000000000000000000002 light year “odd” unit 2 · 10-20 light year Introduction Section 0 0.000000002 angstrom Lecture 1 Slide 7 “odd” unit 2 · 10-8 angstrom INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 7 Dimensions of Motion Distance: Dimensions; Scalar Symbol: Units: (L) d m, cm, km, in, ft, light years, furlongs Time: Dimensions; Scalar Symbol: Units: Speed: (T) t s (or sec), min, hr, day, year, ms, ns, fs, fortnights A distance divided by time (LT-1) Dimensions; Scalar Symbol: Units: Introduction Acceleration: s m/s, mi/hr, mm/yr, furlongs/fortnight Section 0 Lecture 1 Slide 8 A distance divided by time squared A speed divided by time Dimensions; (LT-2) INTRODUCTION TO Modern Physics PHYX 2710 Scalar Fall 2004 Symbol: a Units: m/s2, mi/hr2, mm/yr2, furlongs/fortnight2 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 8 Physics of Technology PHYS 1800 Lecture 3 Motion Introduction Section 0 Lecture 1 Slide 9 Speed and Velocity INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 9 What Is Speed? Speed is how fast something is moving. – Speed is always some distance divided by some time. – The units of speed may be miles per hour, or meters per second, or kilometers per hour, or inches per minute, etc. Rate is one quantity divided by another quantity. – For example: gallons per minute, pesos per dollar, points per game. – So average speed is the rate at which distance is Introduction Section 0 Lecture 1 Slide 10 covered over time. INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 10 What Is Speed? Average speed is total distance divided by total time. distance traveled average speed = time of travel Instantaneous speed is the speed at that precise instant in time. is the rate at which distance is being covered at a – Itgiven instant in time. Introduction Lecture 1 Slide the 11 – It is foundSection by 0calculating average speed, over a short enough time that the speed does not change much. INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 11 Average Speed Kingman to Flagstaff: 120 mi 2.4 hr = 50 mph Flagstaff to Phoenix: 140 mi 2.6 hr = 54 mph Total trip: 120 mi + 140 mi = 260 mi 2.4 hr + 2.6 hr = 5.0 hr Introduction Section 0 Lecture 260 mi 5.0 hr = 52 mph 1 Slide 12 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 12 Instantaneous Velocity Instantaneous velocity is a vector quantity having: a size (magnitude) equal to the instantaneous speed at a given instant in time, and a direction equal to the direction of motion at that instant. Introduction Section 0 Lecture 1 Slide 13 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 13 Instantaneous Speed: a car traveling on a local highway A steep slope indicates a rapid change in velocity (or speed), and thus a large acceleration. A horizontal line has zero slope and represents zero acceleration. Introduction Section 0 Lecture 1 Slide 14 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 14 What Does a Speedometer Measure? The speedometer tells us how fast we are going at a given instant in time. A speedometer measures instantaneous speed. Introduction we’ll Section 0 see Lecture 1 (In a moment, why a speedometer doesn’t measure velocity.) Slide 15 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 15 Which quantity is the highway patrol more interested in? Average speed? Instantaneous speed? The speed limit indicates the maximum legal instantaneous speed. To estimate the time a trip may take, you want to Introduction Section 0 Lecture 1 Slide 16 use average speed. INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 16 Velocity Velocity involves direction of motion as well as how fast the object is going. – Velocity is a vector quantity. – Vectors have both magnitude and direction. – Velocity has a magnitude (the speed) and also a direction (which way the object is moving). A change in velocity can be a change in the object’s speed or direction of motion. A speedometer doesn’t indicate direction, so it indicates instantaneous speed but not velocity. Introduction Section 0 Lecture 1 Slide 17 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 17 A car goes around a curve at constant speed. Is the car’s velocity changing? Yes No At position A, the car has the velocity indicated by the arrow (vector) v1. At position B, the car has the 0 Lecture 1 velocityIntroduction indicatedSection by the arrow (vector) v2, with the same magnitude (speed) but a different direction. Slide 18 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 18 Changing Velocity A force is required to produce a change in either the magnitude (speed) or direction of velocity. For the car to round the curve, friction between the wheels and the road exerts a force to change the car’s direction. For a ball bouncing from a wall, the wall exerts a force on the ball, causing the ball toSection change direction. Introduction 0 Lecture 1 Slide 19 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 19 Why Velocity Is So Useful Velocity is a vector and represents a bodies speed and direction. • A force must act on a body to change its velocity (i.e. its speed, direction or both). • The force causes the body to accelerate resulting in a change in its velocity. • Acceleration is a vector and represents the rate of change of velocity with time. Introduction Section 0 Lecture 1 Slide 20 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 20 Physics of Technology PHYS 1800 Lecture 3 Motion Introduction Section 0 Lecture 1 Slide 21 Acceleration INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 21 Acceleration Acceleration is the rate at which velocity changes. – Our bodies don’t feel velocity, if the velocity is constant. – Our bodies feel acceleration. • A car changing speed or direction. • An elevator speeding up or slowing down. Acceleration can be either a change in the object’s speed or direction of motion. Change in velocity V2 - V1 = Average acceleration = t Time interval Introduction Section 0 Lecture 1 Slide 22 r r a = DV m s 2 t INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 22 Instantaneous Acceleration Instantaneous acceleration is the acceleration at that precise instant in time. It is the rate at which velocity is changing at a given instant in time. It is found by calculating the average speed, over a short enough time that the speed does not change much. Introduction Section 0 Lecture 1 Slide 23 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 23 Acceleration: Vector Direction The direction of acceleration vector is given by the direction of the change in the velocity vector, DV. V2 V1 car accelerating - Acceleration V1 DV + V2 = a vector in same direction as velocity when velocity is increasing. V1 car deceleratin g Introduction Section 0 V1 Lecture 1 V2 + DV a = Slide 24 - When the velocity is decreasing the change in motion (ie. to slow car down) V2 DV is in the opposite direction to INTRODUCTION TO Modern Physics PHYX 2710 - Acceleration vector is opposite direction when velocity is decreasing. - Deceleration is negative acceleration. Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 24 Example: Negative Acceleration Jet preparing to land Initial velocity V1=200 km/hr (=55.6 m/s) Final velocity V2=120 km/hr (=33.3 m/s) Time interval t=5 sec DV V2 - V1 a = t = t 33 . 3 - 55 . 6 2 m / s = a 5 Acceleration: Introduction = - 4 . 46 m /s 2 a Section 0 Lecture 1 Slide 25 toward runway In general: - Whenever the velocity is changing we say the object is accelerating (positive or negative). INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 25 Return to car on a bend Car moved at a constant speed but its direction continuously changed – thus its velocity was changing. • But we now know that velocity changes are produced by an acceleration. • Thus when the car rounds the bend at a constant speed it is accelerating!! • Direction of acceleration is given by DV direction. V1 Acceleration Direction 0 Question:Introduction what isSection DV ? V1 + DV = Lecture 1 Slide 26 V1 V2 DV INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 V2 V2 Result: the vector acts towards the center of curvature of the bend! DV Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 26 Thus the acceleration is also directed towards the center of curvature. This is why the car does NOT change speed but you still feel a force on your body as you round the bend… (change in direction). • Force is due to friction of tires on road enabling the car to change direction. Example: For a given speed the acceleration experienced (force) depends on the curvature of the bend. shallow sharp V1 V2 DV V1 V2 V1 V1 Introduction DV Section 0 V2 a= Lecture 1 DV t Slide 27 = Large V2 a= INTRODUCTION TO Modern Physics PHYX 2710 DV t = Small Fall 2004 Skiing - sudden turns create large accelerations & large associated forces! Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 27 Physics of Technology PHYS 1800 Lecture 3 Motion Introduction Section 0 Lecture 1 Slide 28 Graphing Motion INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 28 Graphing Motion Objectives: Understand what position, speed and acceleration are Learn to graph them versus time Develop some intuition for common situations Introduction Time Section 0 Lecture 1 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Slide 29 Time Acceleration Standing still Constant speed (different magnitudes) Constant acceleration (different magnitudes) Constant deceleration (different magnitudes) Arbitrary motion Speed Distance Consider: Time Motion Lecture 3 Slide 29 Acceleration Speed Distance Standing Still Time Time Introduction Section 0 Lecture 1 Time Slide 30 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 30 Acceleration Speed Distance Constant Speed Time Time Introduction Section 0 Lecture 1 Time Slide 31 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 31 Acceleration Speed Distance Constant Speeds Time Time Introduction Section 0 Lecture 1 Time Slide 32 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 32 Acceleration Speed Distance Constant Acceleration Time Time Introduction Section 0 Lecture 1 Time Slide 33 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 33 Physics of Technology PHYS 1800 Lecture 3 Motion Introduction Section 0 Lecture 1 Slide 39 Vectors: Velocity and Acceleration INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 39 Scalars and Vectors Scalar: Measure of quantity or size Sometimes called “magnitude”. Examples: Length, volume, mass, temperature, speed… Vectors: Many measurements in physics require a knowledge of the magnitude and direction of quantity. These are termed vector quantities. Examples: Velocity, acceleration, force, electric field… Direction is an essential feature of a vector quantity. Example: Flying 1000 km/hr due North is quite different to Introduction Section 0atLecture 1 Slide 40 the same speed due East! Vectors require 2 pieces of information MAGNITUDE and DIRECTION. INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 40 Physics of Technology Next Lab/Demo: Free Fall Tuesday 1:30-2:45 ESLC 53 Ch 2 Next Class: Wednesday 10:30-11:20 Slide 44 BUS 318 room Review Ch 3 Introduction Section 0 Lecture 1 INTRODUCTION TO Modern Physics PHYX 2710 Fall 2004 Physics of Technology—PHYS 1800 Spring 2009 Motion Lecture 3 Slide 44