Fisika Dasar I Per.20

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Fisika Dasar I Per.20

  1. 1. Fisika Dasar I Umiatin, M.Si <ul><li>Jurusan Fisika </li></ul><ul><li>Fakultas Matematika dan Ilmu Pengetahuan Alam </li></ul>
  2. 2. GELOMBANG <ul><li>Pertemuan ke-20 </li></ul>11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  3. 3. Outline <ul><li>The vector product (cross product) </li></ul><ul><li>TORKA </li></ul>11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  4. 4. Waves <ul><li>A wave is an oscillation that travels. </li></ul><ul><li>A ball floating on water can oscillate up and down in harmonic motion. </li></ul><ul><li>The surface of the water oscillates in response and the oscillation spreads outward from where it started. </li></ul>11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  5. 5. Why learn about waves? <ul><li>Waves carry useful information and energy. </li></ul><ul><li>Waves are all around us: </li></ul><ul><ul><li>light from the stoplight </li></ul></ul><ul><ul><li>ripples in a puddle of </li></ul></ul><ul><ul><li>electricity flowing in wires </li></ul></ul><ul><ul><li>radio and television and cell phone transmissions </li></ul></ul>11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  6. 6. Characteristics of waves <ul><li>Waves have cycles , frequency , and amplitude , just like oscillations. </li></ul>The frequency of a wave tells how often each point oscillates. The amplitude of a wave is the maximum movement from equilibrium. The wavelength of a wave is the length of one complete cycle. 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  7. 7. Wave pulses <ul><li>A wave pulse is a short length of wave, often just a single oscillation. </li></ul>11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  8. 8. Relationship between speed, frequency, and wavelength <ul><li>The speed of a wave equals the frequency times the wavelength. </li></ul>v = f  Frequency (cycles/sec) Wavelength (m) Speed (m/sec) 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  9. 9. <ul><li>Types of mechanical waves </li></ul><ul><li>Mechanical waves </li></ul><ul><li>are disturbances that travel through some material or substance </li></ul><ul><li>called medium for the waves. </li></ul><ul><li>travel through the medium by displacing particles in the medium </li></ul><ul><li>travel in the perpendicular to or along the movement of the </li></ul><ul><li>particles or in a combination of both </li></ul>transverse waves: waves in a string etc. longitudinal waves: sound waves etc. waves in water etc. 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  10. 10. Types of mechanical waves Longitudinal and transverse waves sound wave = longitudinal wave C = compression R = rarefaction air compressed air rarefied 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  11. 11. <ul><li>Longitudinal-transverse waves </li></ul>11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  12. 12. <ul><li>Periodic waves </li></ul><ul><li>When particles of the medium in a wave undergo periodic </li></ul><ul><li>motion as the wave propagates, the wave is called periodic. </li></ul>x=0 x t=0  A t=T/4 t=T period amplitude wavelength 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  13. 13. Mathematical description of a wave <ul><li>Wave function </li></ul><ul><li>The wave function describes the displacement of particles </li></ul><ul><li>in a wave as a function of time and their positions: </li></ul><ul><li>A sinusoidal wave is described by the wave function: </li></ul>sinusoidal wave moving in +x direction angular frequency velocity of wave, NOT of particles of the medium wavelength period sinusoidal wave moving in -x direction v->-v phase velocity 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  14. 14. <ul><li>Wave function (cont’d) </li></ul>x=0 x t=0  t=T/4 t=T period wavelength 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  15. 15. <ul><li>Wave number and phase velocity </li></ul>wave number: The speed of wave is the speed with which we have to move along a point of a given phase. So for a fixed phase, phase phase velocity 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  16. 16. <ul><li>Particle velocity and acceleration in a sinusoidal wave </li></ul>velocity acceleration Also wave equation u in textbook 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  17. 17. <ul><li>General solution to the wave equation </li></ul>Solutions: such as The most general form of the solution: wave equation 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  18. 18. Speed of a transverse wave <ul><li>Wave speed on a string </li></ul><ul><li>Consider a small segment of string whose </li></ul><ul><li>length in the equilibrium position is </li></ul><ul><li>The mass of the segment is </li></ul><ul><li>The x component of the force (tension) at both </li></ul><ul><li>ends have equal in magnitude and opposite in </li></ul><ul><li>direction because this is a transverse wave. </li></ul><ul><li>The total y component of the forces is: </li></ul>Newton’s 2 nd law mass acceleration 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  19. 19. <ul><li>Wave speed on a string (cont’d) </li></ul><ul><li>The total y component of the forces is: </li></ul>wave eq. 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  20. 20. <ul><li>Energy in wave motion </li></ul><ul><li>Total energy of a short string segment of mass </li></ul><ul><li>At point a, the force </li></ul>a does work on the string segment right of point a. <ul><li>Power is the rate of work done : </li></ul>work done P max 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  21. 21. <ul><li>Average power of a sinusoidal wave on a string </li></ul><ul><li>The average of </li></ul>over a period: <ul><li>The average power: </li></ul><ul><li>Maximum power of a sinusoidal wave on a string: </li></ul>11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  22. 22. <ul><li>Wave intensity </li></ul><ul><li>Wave intensity for a three dimensional wave from a point </li></ul><ul><li>source: </li></ul>power/unit area 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  23. 23. <ul><li>Wave interference, boundary condition, and superposition </li></ul><ul><li>The principle of superposition </li></ul><ul><li>When two waves overlap, the actual displacement of any </li></ul><ul><li>point at any time is obtained by adding the displacement </li></ul><ul><li>the point would have if only the first wave were present and </li></ul><ul><li>the displacement it would have if only the second wave were </li></ul><ul><li>present: </li></ul>11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  24. 24. <ul><li>Interference </li></ul><ul><li>Constructive interference (positive-positive or negative-negative) </li></ul><ul><li>Destructive interference (positive-negative) </li></ul>11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  25. 25. <ul><li>Reflection </li></ul><ul><li>Free end </li></ul>For x<x B At x=x B incident wave reflected wave Vertical component of the force at the boundary is zero. 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  26. 26. <ul><li>Reflection (cont’d) </li></ul><ul><li>Fixed end </li></ul>For x<x B At x=x B Displacement at the boundary is zero. 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  27. 27. <ul><li>Reflection (cont’d) </li></ul><ul><li>At high/low density </li></ul>11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  28. 28. <ul><li>Reflection (cont’d) </li></ul><ul><li>At low/high density </li></ul>11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  29. 29. REFFERENCE <ul><li>Many sources </li></ul>11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |
  30. 30. TERIMA KASIH 11/03/11 © 2010 Universitas Negeri Jakarta | www.unj.ac.id |

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