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**In a wave pool, the waves carry energy across the pool**

In a wave pool, the waves carry energy across the pool. You can see the effects of a wave's energy when the wave lifts people in the water.

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**What Are Mechanical Waves?**

What causes mechanical waves? A mechanical wave is created when a source of energy causes a vibration to travel through a medium.

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**What Are Mechanical Waves?**

A mechanical wave is a disturbance in matter that carries energy from one place to another. The material through which a wave travels is called a medium. Mechanical waves require a medium to travel through. Solids, liquids, and gases all can act as mediums. A vibration is a repeating back-and-forth motion.

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**Types of Mechanical Waves**

What are the three main types of mechanical waves? The three main types of mechanical waves are transverse waves, longitudinal waves, and surface waves. Mechanical waves are classified by the way they move through a medium.

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**Types of Mechanical Waves**

Transverse Waves When you shake one end of a rope up and down, the vibration causes a wave. The highest point of the wave is the crest. The lowest point of the wave is the trough. A single point on the rope vibrates up and down between a crest and trough.

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**Types of Mechanical Waves**

A transverse wave causes the medium to vibrate in a direction perpendicular to the direction in which the wave travels. Direction of wave Crest Rest position Direction of vibration Trough

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**Types of Mechanical Waves**

A transverse wave is a wave that causes the medium to vibrate at right angles to the direction in which the wave travels. The wave carries energy from left to right, in a direction perpendicular to the up-and-down motion of the rope.

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**Types of Mechanical Waves**

Longitudinal Waves In a spring toy, the wave carries energy along the spring. An area where the particles in a medium are spaced close together is called a compression. An area where the particles in a medium are spread out is called a rarefaction.

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**Types of Mechanical Waves**

A compression starts to move along the spring. A rarefaction follows the compression along the spring. Push Compression Pull Direction of wave Rarefaction Compression Rest position

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**Types of Mechanical Waves**

As compressions and rarefactions travel along the spring, each coil vibrates back and forth around its rest position. A longitudinal wave is a wave in which the vibration of the medium is parallel to the direction the wave travels.

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**Types of Mechanical Waves**

Surface Waves Ocean waves are the most familiar kind of surface waves. A surface wave is a wave that travels along a surface separating two media.

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**Types of Mechanical Waves**

Wave direction As the ocean wave moves to the right, the bobber moves in a circle, returning to its original position.

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**Types of Mechanical Waves**

The bobber helps to visualize the motion of the medium. When a crest passes the bobber, the bobber moves up. When a trough passes, the bobber moves down. The bobber also is pushed back and forth by the surface wave, like the motion of a longitudinal wave. The two motions combine, and the bobber moves in a circle.

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**A surfer needs to understand the properties of waves.**

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Frequency and Period What determines the frequency of a wave? Any periodic motion has a frequency, which is the number of complete cycles in a given time. A wave’s frequency equals the frequency of the vibrating source producing the wave.

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Frequency and Period Any motion that repeats at regular time intervals is called periodic motion. The time required for one cycle is called the period. Frequency is the number of complete cycles in a given time. Frequency is measured in cycles per second, or hertz (Hz).

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**A wave vibrating at one cycle per second has a frequency of 1.0 Hz.**

Frequency and Period Frequency = 1.0 hertz One cycle per second A wave vibrating at one cycle per second has a frequency of 1.0 Hz. A wave vibrating at two cycles per second has a frequency of 2.0 Hz. Frequency = 2.0 hertz Two cycles per second

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Wavelength How are frequency and wavelength related? Wavelength is the distance between a point on one wave and the same point on the next cycle of the wave. Increasing the frequency of a wave decreases its wavelength.

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Wavelength For a transverse wave, wavelength is measured between adjacent crests or between adjacent troughs. For a longitudinal wave, wavelength is the distance between adjacent compressions or rarefactions.

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Wavelength Wavelength can be measured from any point on a wave to the same point on the next cycle of the wave. Long wavelength Short wavelength Rest position

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Wave Speed How are frequency, wavelength, and speed related? If you assume that waves are traveling at a constant speed, then wavelength is inversely proportional to frequency.

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Wave Speed When the wavelength is in meters, and the frequency is in hertz, the units for speed are meters per second. The speed of a wave is also calculated by dividing its wavelength by its period.

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Wave Speed Speed of Mechanical Waves One end of a rope is vibrated to produce a wave with a wavelength of 0.25 meter. The frequency of the wave is 3.0 hertz. What is the speed of the wave?

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Wave Speed Read and Understand What information are you given?

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Wave Speed Read and Understand What information are you given?

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Wave Speed Plan and Solve What unknown are you trying to calculate? What formula contains the given quantities and the unknown?

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Wave Speed Plan and Solve What unknown are you trying to calculate? What formula contains the given quantities and the unknown?

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Wave Speed Plan and Solve Replace each variable with its known value.

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Wave Speed Plan and Solve Replace each variable with its known value.

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Wave Speed Look Back and Check Is your answer reasonable?

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**Wave Speed Look Back and Check**

Is your answer reasonable? Because the frequency is 3.0 hertz, the wave should travel a distance of 3 wavelengths in 1 second. This distance is 0.75 meters, which agrees with the calculated speed of 0.75 m/s.

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Wave Speed 1. A wave on a rope has a wavelength of 2.0 m and a frequency of 2.0 Hz. What is the speed of the wave? Answer:

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Wave Speed 1. A wave on a rope has a wavelength of 2.0 m and a frequency of 2.0 Hz. What is the speed of the wave? Answer: The speed is 2.0 m × 2.0 Hz = 4.0 m/s

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Wave Speed 2. A motorboat is tied to a dock with its motor running. The spinning propeller makes a surface wave in the water with a frequency of 4 Hz and a wavelength of 0.1 m. What is the speed of the wave? Answer:

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Wave Speed 2. A motorboat is tied to a dock with its motor running. The spinning propeller makes a surface wave in the water with a frequency of 4 Hz and a wavelength of 0.1 m. What is the speed of the wave? Answer: The speed is 0.1 m × 4 Hz = 0.4 m/s

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Wave Speed 3. What is the speed of a wave in a spring if it has a wavelength of 10 cm and a period of 0.2 s? Answer:

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Wave Speed 3. What is the speed of a wave in a spring if it has a wavelength of 10 cm and a period of 0.2 s? Answer: The speed is 10 cm/0.2 s = 50 cm/s

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Wave Speed 4. What is the wavelength of an earthquake wave if it has a speed of 5 km/s and a frequency of 10 Hz? Answer:

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Wave Speed 4. What is the wavelength of an earthquake wave if it has a speed of 5 km/s and a frequency of 10 Hz? Answer: The wavelength is (5 km/s)/10 Hz = 0.5 km.

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Wave Speed The speed of a wave can change if it enters a new medium, or if variables such as pressure and temperature change. For many kinds of waves, the speed of the waves is roughly constant for a range of different frequencies. The wave with the lower frequency has a longer wavelength.

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Amplitude How is the amplitude of a wave related to the wave’s energy? The amplitude of a wave is the maximum displacement of the medium from its rest position. The more energy a wave has, the greater is its amplitude.

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Amplitude The amplitude of a transverse wave is the distance from the rest position to a crest or a trough. It takes more energy to produce a wave with higher crests and deeper troughs.

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**The more energy a wave has, the greater is its amplitude.**

High amplitude Low amplitude Rest position

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Amplitude The amplitude of a longitudinal wave is the maximum displacement of a point from its rest position. The more energy the wave has, the more the medium will be compressed or displaced.

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