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Vibrations and Waves Honors Physics

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**Biblical Reference Ephesians 4:14**

Then we will no longer be infants, tossed back and forth by the waves. Ephesians 4:14

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**Simple Harmonic Motion**

Back and forth motion that is caused by a force that is directly proportional to the displacement. The displacement centers around an equilibrium position.

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Springs – Hooke’s Law One of the simplest type of simple harmonic motion is called Hooke's Law. This is primarily in reference to springs.

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Springs – Hooke’s Law The negative sign tells us that “F” is a restoring force; it works in the opposite direction of the displacement.

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Hooke’s Law Common formulas which are set equal to Hooke's law are Newton’s Second Law and weight.

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Example A 0.55 kg mass is attached to a vertical spring, which stretches 36 cm from it’s original equilibrium position. What is the spring constant?

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Example A load of 50 N attached to a spring hanging vertically stretches the spring 5.0 cm. The spring is now placed horizontally on a table and stretched 11.0 cm. What force is required to stretch the spring this amount?

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**Conservation of Energy in Springs**

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**Springs are like Waves and Circles**

The amplitude, A, of a wave is the same as the displacement ,x, of a spring. Both are in meters. Crest Equilibrium Line Trough

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CREST Period Equilibrium Line Period (T): the time for one revolution or one complete oscillation (one crest and trough). Oscillations could also be called vibrations and cycles. Ts = sec/cycle Trough In the wave above we have 1.75 cycles or waves (vibrations or oscillations). Assume that the wave crosses the equilibrium line in one second intervals. T = 3.5 seconds/1.75 cycles. T = 2 sec.

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**Frequency The Frequency of a wave is the inverse of Period.**

That means that the frequency is cycles/sec. The commonly used unit is Hertz (HZ).

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**Measuring SHM for a Spring**

The period of a Spring-Mass System is: Proportional to 2 Inversely proportional to the square root of the spring constant Proportional to the square root of the mass on the spring The greater the mass, the larger the period The greater the spring constant (more stiff), the smaller the period

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Example A 125 N object vibrates with a period of 3.56 seconds when hanging from a spring. Find the spring constant.

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**Remember the Pendulum…**

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**Measuring SHM for a Pendulum**

The period of a pendulum is: Proportional to 2 (it’s sweeping out an arc of a circle) Inversely proportional to the square root of gravity Proportional to the square root of the length of the pendulum

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Example The height of a tower is unknown, but a pendulum, extending from the ceiling almost touches the floor. If the period of the pendulum is 12 s, what is the approximate height of the tower?

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**What is a wave A Wave is a vibration or disturbance in space.**

A Medium is the substance that all sound waves travel through and need to have in order to move.

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**Longitudinal Waves 2 areas:**

Longitudinal Wave - A fixed point will move parallel with the wave motion 2 areas: Compression - an area of high molecular density and pressure Rarefaction - an area of low molecular density and pressure

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Transverse Waves Transverse Wave - A fixed point will move perpendicular with the wave motion. Wave parts: Crest, Trough, Wavelength, Amplitude, Frequency, Period

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**All waves have 4 basic properties:**

Properties of Waves All waves have 4 basic properties: Amplitude Wavelength λ lambda Frequency f Speed c

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Properties of Waves Amplitude – the maximum distance the wave moves up and down. The more energy a wave has the greater the amplitude.

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Properties of Waves Wavelength – the distance between two corresponding parts of a wave Short Waves can complete more cycles than Long Waves in the same amount of time.

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Properties of Waves Frequency – the number of complete waves that pass a given point Frequency is measured in the unit called Hertz (Hz). A wave that occurs every second has a frequency of 1 Hz.

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Properties of Waves Speed – the distance a wave travels in a given amount of time. The speed of sound through air is 331 m/s.

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Wave Speed You can find the speed of a wave by multiplying the wave’s wavelength in meters by the frequency (cycles per second). Since a “cycle” is not a standard unit this gives you m/s.

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Example A harmonic wave is traveling along a rope. It is observed that the oscillator that generates the wave completes vibrations in 30.0 s. Also, a given maximum travels 425 cm along a rope in 10.0 s . What is the wavelength?

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**Wave Behavior Superposition - The combination of two overlapping waves**

Interference - The result of superposition

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**Standing Waves Two main parts of standing waves:**

A standing wave is produced when a wave that is traveling is reflected back upon itself. Two main parts of standing waves: Antinodes – Areas of maximum amplitude Nodes – Areas of zero amplitude

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**Interference Interference is the interaction between waves that meet**

There are two types of interference: Constructive and Destructive

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Reflection When an object hits a surface it can not pass, it bounces back. This is called reflection.

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Refraction The bending of waves due to a change in speed is called refraction.

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Diffraction When a wave moves around a barrier or through an opening in a barrier, it bends and spreads out. This is known as diffraction.

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