Physics 41: Waves, Optics, Thermo

Transcription

1 Physics 41: Waves, Optics, Thermo

2 Particles & Waves Localized in Space: LOCAL Have Mass & Momentum No Superposition: Two particles cannot occupy the same space at the same time! Particles have energy. Particles can have any energy. Spread Out in Space: NONLOCAL Superposition: Waves add in space and show interference. Do not have mass or Momentum Waves transmit energy. Bound waves have discreet energy states they are quantized.

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4 is a What you Hear The ear converts sound energy to mechanical energy to a nerve impulse which is transmitted to the brain. The Pressure Wave sets the Ear Drum into

5 electroencephalogram

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7 v S v = Mach #

8 Electron Waves Probability Waves in an Ocean of Uncertainty A wave packet in a square well (an electron in a box) changing with time.

9 Superposition: Waves ADD in Space Interference: Waves interfere with other waves and with themselves without any permanent damage!

10 Contour Map of Interference Pattern

11 Importance of Simple Harmonic Oscillators Simple harmonic oscillators are good models of a wide variety of physical phenomena Molecular example If the atoms in the molecule do not move too far, the forces between them can be modeled as if there were springs between the atoms The potential energy acts similar to that of the SHM oscillator

12 Natural Frequency & Resonance All objects have a natural frequency of vibration or oscillation. Bells, tuning forks, bridges, swings and atoms all have a natural frequency that is related to their size, shape and composition. A system being driven at its natural frequency will resonate and produce maximum amplitude and energy.

13 Some Sytems have only ONE natural frequency: springs, pendulums, tuning forks, satellites orbits

14 Some Systems have more than one frequency they osciallate with: Harmonics.

15 Natural Frequency & Resonance When the driving vibration matches the natural frequency of an object, it produces a Sympathetic Vibration - it Resonates!

16 Sound Waves: Mechanical Vibrations The Ear: An Acoustic Tuner Cilia: Acoustic Tuning Forks

17 Eyes: Optical Tuner Optical Antennae: Rods & Cones Rods: Intensity Cones: Color

18 Light Waves: EM Vibrations E = Emax cos (kx ωt) B = Bmax cos (kx ωt) E c B 1 v c / n 0 0 Speed of Light in a vacuum: 186,000 miles per second 300,000 kilometers per second 3 x 10^8 m/s

19 Coupled Oscillators Molecules, atoms and particles are modeled as coupled oscillators. Waves Transmit Energy through coupled oscillators. Forces are transmitted between the oscillators like springs Coupled oscillators make the medium.

20 Atoms are EM Tuning Forks They are tuned to particular frequencies of light energy.

21 Strings & Atoms are Quantized The possible frequency and energy states of an electron in an atomic orbit or of a wave on a string are quantized. f v n 2 l En nhf, n= 0,1,2,3, h 6.626x10 Js

22 String Idea: Particles are string vibrations in resonance

23 DARK ENERGY The Vibration of Nothing

24 Chapter 14 Simple Harmonic Motion

25 Some Sytems have only ONE natural frequency: springs, pendulums, tuning forks, satellites orbits

26 Our Task: Springs and Pendulums Obey Hooke s Law and exhibit Oscillatory Motion. Find the equations of motion: Position vs Time: Sinusoidal

27 THE GAME We want to describe the motion of oscillating systems and find the natural frequency of objects and systems. If you know the natural frequency of an object, the frequency it can oscillate or vibrate with, then you know everything about it, most importantly it s ENERGY and the MUSIC it makes! Use Hooke s Law!

28 Review: Hooke s Law Ut tensio, sic vis - as the extension, so is the force Hooke s Law describes the elastic response to an applied force. Elasticity is the property of an object or material which causes it to be restored to its original shape after distortion. An elastic system displaced from equilibrium oscillates in a simple way about its equilibrium position with Simple Harmonic Motion.

29 Hooke s Law It takes twice as much force to stretch a spring twice as far. The linear dependence of displacement upon stretching force: Fapplied kx

30 Hooke s Law Stress is directly proportional to strain. F ( stress) kx( strain) applied

31 Hooke s Law: F = - k x +

32 Hooke s Law: F = - k x +

33 Hooke s Law: F = - k x +

34 Hooke s Law: F = - k x +

35 Hooke s Law: F = - k x +

36 Review: Energy in a Mass-Spring K mv U kx E=K+U

37 Review: Circular Motion x( t) A cos ( t 0 ) v( t) A sin( t ) 0 a t A t 2 ( ) cos( 0 ) 2 R v ( ),,, R 2 2 t t vt R at R ac R

38 Simple Harmonic Motion Equations x( t) A cos ( t ) dx v Asin( t 0) dt 2 dx 2 a Acos( t 2 0) dt 0 Notice: a 2 x In SHM, the acceleration is proportional to the negative of the displacement.

39 Review Terms: Amplitude : [ A] m Period : T time / cycle, [ T] sec 1 Frequency : f (# cycles / sec), [ f ] Hz 2 Angular Frequency : 2 f, [ ] rad / s Displacement of Mass

40 THE GAME We want to describe the motion of oscillating systems and find the natural frequency of objects and systems. If you know the natural frequency of an object, the frequency it can oscillate or vibrate with, then you know everything about it, most importantly it s ENERGY and the MUSIC it makes! Finding the energy states of systems is pretty much the goal of most of physics!

41 Some Sytems have only ONE natural frequency and energy state: springs, pendulums, tuning forks, satellites orbits

42 QuickCheck System 1: Ideal Mass-Spring System. Is there any difference in the resulting motion between a horizontal and vertical system? Slide 14-71

43 Vertical Oscillations In the absence of any retarding forces the motion for a mass hanging from a spring is the same as for horizontal SHM, but the equilibrium position is affected. The role of gravity is to determine whwere the equilibrium position is but it doesn t affect the SHM around the equilibrium position Pearson Education, Inc. Slide 14-67

44 Mass-Spring Systems that obey Hooke s Law exhibit Simple Harmonic Motion Position vs Time: Sinusoidal a 2 x F ma kx 2 m( x) kx k m 2 k m angular frequency

45 Simple Harmonic Motion k m T 2 T 2 Does the period depend on the displacement, x? m k Both the angular frequency and period depend only on how stiff the spring is and how much inertia there is.

46 In Sum: Ideal Mass Spring System obeys Hooke s Law exhibit Simple Harmonic Motion Position vs Time: Sinusoidal Frequency and Period: k T 2 m m k NOTE: There is only ONE frequency for mass-spring and the equation is specific to the system and only depends on m and k. Equations of motion for ANY System Oscillating in SHM: x( t) A cos ( t 0 ) 2 a x

47 Position Equation for SHM x( t) A cos ( t 0 ) A is the amplitude of the motion The phase is: ( t 0 ) is called the angular frequency (rad/s) 0 is the phase constant or the initial phase angle at t=0 A,, are all constants

48 Phase and Phase Difference The quantity ( t + 0 ) is called the phase of the wave, denoted. The phase constant 0 is the initial phase angle at t=0 The phase difference between two points on a wave depends on only the ratio of their separation x to the wavelength. PHASE

49 The Phase Constant What if an object in SHM is not initially at rest at x = A when t = 0? Then we may still use the cosine function, but with a phase constant measured in radians. In this case, the two primary kinematic equations of SHM are: 2013 Pearson Education, Inc. Slide 14-34

50 Simple Harmonic Motion and Circular Motion Figure (a) shows a shadow movie of a ball made by projecting a light past the ball and onto a screen. As the ball moves in uniform circular motion, the shadow moves with simple harmonic motion. The block on a spring in figure (b) moves with the same motion Pearson Education, Inc. Slide 14-33

51 Pearson Education, Inc.

52 The Phase Constant 2013 Pearson Education, Inc. Slide 14-35

53 The Phase Constant: CONVENTIONS All values of 0 between 0 and correspond to particle moving to the LEFT. Velocity is NEGATIVE but the phase constant is POSTIIVE. All values of 0 between and 2 correspond to particle moving to the RIGHT. The convention is to use values between - and 0. The velocity is POSITIVE but the phase constant is NEGATIVE Pearson Education, Inc. Slide 14-35

54 QuickCheck 14.4 The figure shows four oscillators at t = 0. For which is the phase constant 0 / 4? Slide 14-44

55 QuickCheck 14.4 The figure shows four oscillators at t = 0. For which is the phase constant 0 / 4? Initial conditions: x = 0.71A v x > 0 Slide 14-45

56 QuickCheck 14.2 This is the position graph of a mass oscillating on a horizontal spring. What is the phase constant 0? A. /2 rad. B. 0 rad. C. /2 rad. D. rad. E. None of these. Slide 14-40

57 QuickCheck 14.2 This is the position graph of a mass oscillating on a horizontal spring. What is the phase constant 0? A. /2 rad. B. 0 rad. C. /2 rad. D. rad. E. None of these. Initial conditions: x = 0 v x > 0 Slide 14-41

58 QuickCheck 14.3 This is the position graph of a mass oscillating on a horizontal spring. What is the phase constant 0? A. /2 rad. B. 0 rad. C. /2 rad. D. rad. E. None of these. Slide 14-42

59 QuickCheck 14.3 This is the position graph of a mass oscillating on a horizontal spring. What is the phase constant 0? A. /2 rad. B. 0 rad. C. /2 rad. D. rad. E. None of these. Initial conditions: x = A v x = 0 Slide 14-43

60 QuickCheck 14.9 A mass oscillates on a horizontal spring. It s velocity is v x and the spring exerts force F x. At the time indicated by the arrow, A. v x is and F x is. B. v x is and F x is. C. v x is and F x is 0. D. v x is 0 and F x is. E. v x is 0 and F x is Pearson Education, Inc. Slide 14-63

61 QuickCheck 14.9 A mass oscillates on a horizontal spring. It s velocity is v x and the spring exerts force F x. At the time indicated by the arrow, A. v x is and F x is. B. v x is and F x is. C. v x is and F x is 0. D. v x is 0 and F x is. E. v x is 0 and F x is Pearson Education, Inc. Slide 14-64

62 QuickCheck A mass oscillates on a horizontal spring. It s velocity is v x and the spring exerts force F x. At the time indicated by the arrow, A. v x is and F x is. B. v x is and F x is. C. v x is and F x is 0. D. v x is 0 and F x is. E. v x is 0 and F x is Pearson Education, Inc. Slide 14-65

63 QuickCheck A mass oscillates on a horizontal spring. It s velocity is v x and the spring exerts force F x. At the time indicated by the arrow, A. v x is and F x is. B. v x is and F x is. C. v x is and F x is 0. D. v x is 0 and F x is. E. v x is 0 and F x is Pearson Education, Inc. Slide 14-66