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Chapter 12. Vibrations and Waves. Section 12-1 Simple Harmonic Motion. Force is maximum at maximum displacement. Force is zero at equilibrium. Spring. Simple Harmonic Motion con’t.

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Chapter 12

Vibrations and Waves


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Section 12-1Simple Harmonic Motion

  • Force is maximum at maximum displacement.

  • Force is zero at equilibrium

Spring


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Simple Harmonic Motion con’t

  • Simple Harmonic Motion–vibration about an equilibrium position in which a restoring force is proportional to the displacement from equilibrium.

  • Hooke’s Law–Discovered in 1678 by Robert Hooke. The relationship between the force and displacement in a mass-spring system.


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Some problems

1. A 76 N crate is attached to a spring (k = 450 N/m). How much displacement is caused by the weight of the crate?

Given:

F = 76 N

k = 450 N/m


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Some Problems con’t

2. A spring of k = 1962 N/m loses its elasticity if stretched more than 50.0 cm. What is the mass of the heaviest object the spring can support without being damaged?

Given:

k = 1962 N/m

x = 50.0 cm = 0.50 m


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Simple Pendulum

  • A simple pendulum consists of a mass called a bob, which is attached to a fixed string.

Pendulum


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Foucault Pendulum

  • This type of pendulum was first used by the French physicist Jean Foucault to verify the Earth’s rotation experimentally. As the pendulum swings, the vertical plane in which it oscillates appears to rotate as the bob successively knocks over the indicators arranged in a circle on the floor.

© Bob Emott, Photographer


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Section 12-2Measuring Simple Harmonic Motion

  • Amplitude–maximum displacement from equilibrium.

  • Period–the time it takes to execute a complete cycle of motion.

  • Frequency–number of cycles or vibrations per unit of time.



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A Couple of Problems

1. What is the period of a 3.98 m long pendulum?


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Problems con’t

2. What is the free-fall acceleration at a location where a 6.00 m long pendulum swings through exactly 100 cycles in 492 s?


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Spring Problem

3. A 1.0 kg mass attached to one end of a spring completes one oscillation every 2.0 s. Find the spring constant.


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Pendulum Problem

4. A man needs to know the height of a tower, but darkness obscures the ceiling. He knows, however, that a long pendulum extends from the ceiling almost to the floor and that its period is 12.0 s. How tall is the tower?


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Section 12-3Properties of Waves

  • Medium–material through which a disturbance travels.


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Types of Waves

  • Mechanical Wave–a wave whose propagation requires the existence of a medium.

  • Electromagnetic Waves–a wave consisting of oscillating electric and magnetic fields at right angles to each other, no medium is required.

  • Matter Waves–Electrons show wave-like behavior.


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Kinds of Waves

  • Transverse wave–A wave in which the vibration is at right angles (perpendicular) to the direction in which the wave is traveling.

  • Longitudinal Wave–A wave in which the vibration is in the same direction (parallel) as that in which the wave is traveling.

  • Surface Wave–Surface disturbance with characteristics of both transverse and longitudinal waves.




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Single or Multiple

  • Wave Pulse–A single disturbance traveling through a medium.

  • Periodic Wave–A wave whose source is some form of periodic motion.


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Parts of Waves

  • Crests–One of the places in a wave where the wave is highest or the disturbance is greatest.

  • Troughs–One of the places in a wave where the wave is lowest or the disturbance is greatest in the opposite direction from a crest.

  • Wavelength–The distance from the top of crest of a wave to the top of the following crest, or equivalently, the distance between successive identical parts of the wave.



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Wave Speed

Velocity

For waves

Speed of a Wave


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Section 12-4Wave Interactions

  • Principle of Superposition–The displacement of a medium caused by two or more waves is the algebraic sum of the displacements caused by the individual waves.

  • Interference–Combining of two waves arriving at the same point at the same time.


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Constructive Interference

  • Constructive Interference–The superposition of two equal wave pulses resulting in a combined wave of larger amplitude than its components.


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Destructive Interference

  • Destructive Interference–The superposition of two wave pulses with equal but opposite amplitude.



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Standing Waves

  • Standing Waves–Wave pattern that results when two waves of the same frequency, wavelength and amplitude travel in opposite directions and interfere.

  • Node–point in a standing wave that always undergoes complete destructive interference.

  • Antinode–point in a standing wave, halfway between two nodes, at which the largest amplitude occurs




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