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AP Physics B Chapter 13

AP Physics B Chapter 13. Vibrations and Waves. Periodic Motion. Periodic Motion is said to occur when an object vibrates or oscillates over the same path again and again. Examples: springs and pendulums

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AP Physics B Chapter 13

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  1. AP Physics BChapter 13 Vibrations and Waves

  2. Periodic Motion • Periodic Motionis said to occur when an object vibrates or oscillates over the same path again and again. • Examples: springs and pendulums • Simple Harmonic Motionis a special form of periodic motion in which the restoring force is proportional to the displacement of the object. It occurs when the net force along the direction of the motion is a Hooke’s law type of force • Examples: springs

  3. Definitions • Amplitude – maximum displacement of an object from it’s equilibrium point. • Cycle – one complete “to-and-fro” motion. • Period– time required to complete one cycle. • Frequency– number of cycles completed each second. • Recall:

  4. Hooke’s Law` • Hooke's law is a principle of physics that states that the force  needed to extend or compress a spring by some distance  is proportional to that distance. That is: where  is a constant factor characteristic of the spring, its stiffness. • Hooke's law: the force is • proportional to the • extension.

  5. Equations that are associated with Hooke’s Law

  6. Elastic Potential Energy

  7. Elastic Potential Energy

  8. Motion of a Pendulum • A simple pendulum is another mechanical system that exhibits periodic motion • Ft= -mgsinФ

  9. Finding Period • To find the period of an pendulum in SHM, you can simply use the following equation: • To find the period of an ideal spring in SHM, you can simply use the following equation:

  10. Equations of SHM

  11. Wave Motion • Mechanical Wave – wave that travels through a medium. • There are two types of mechanical waves. • Transverse – object particles move perpendicular to the wave’s motion. • Longitudinal – object particles move parallel to the wave’s motion. • Link to: The Institute of Sound and Vibration Research.

  12. l A l A l Dissecting a Wave crest (peak) trough Link to ISVR

  13. Wave Reflection Free Point When the wave strikes a free point, Fixed Point When the wave strikes a fixed point, the wave becomes inverted. the wave remains erect.

  14. Wave Transmission • When transmitting to a less dense material: • Frequency remains the same. • Velocity increases. • Wavelength increases. • Reflected wave is small. • Transmitted wave is big. • When transmitting to a more dense material: • More of the wave will be reflected back along the lighter cord. • Frequency still remains the same. • Velocity and wavelength decrease.

  15. Reflection of Wave Fronts • Two- or three-dimensional waves travel along wave fronts. • The motion of these waves can be seen using rays drawn perpendicular to each wave front. Law of reflection – angle of incidence equals angle of reflection.

  16. Link to ISVR Interference • Principle of Superposition – when two waves overlap, the resultant displacement will be the algebraic sum of their separate displacements. Constructive Interference – waves are in phase Destructive Interference – waves are out of phase

  17. Standing Waves • Standing wave – when reflected waves are in phase with incident waves and the wave appears to stand still. • Fundamental Mode or Frequency – frequency at which a standing wave of the lowest resonance can be produced. • ResonantFrequencies – frequencies at which standing waves are produced.

  18. Anti-node Node L Resonant Frequencies Link to ISVR

  19. Sample Problem • A mass of 4 kg can stretch a spring by 0.5 m. • Determine the spring constant of the spring. • A slingshot consists of a light leather cup containing a stone that is pulled back against two rubber bands. It takes 30 N to stretch the bands 1.0 cm • A) What is the potential energy stored in the bands when a 50 g stone is placed in the cup and pulled back 0.20 m from the equilibrium position • B.) What speed does the stone leave the slingshot.

  20. Sample Problem • A device used to test guitar strings hangs a mass off the end of a string, and then sends waves throughout the string at a rate of 120 Hertz (cycles/second) to determine if the string has the correct mass to length ratio. For a particular string, a mass is hung over a pulley that is 0.6 meters away, and hanging a mass of 4 kg creates a standing wave with 6 loops. Determine the m/L ratio of the string.

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