Vibrations, Waves and Sound

Unit 7: Vibrations, Waves & Sound

Chapter 19: Waves19.1 Waves

19.2 The Motion of Waves

19.3 Wave Interference and Energy

19.1 Investigation: Waves in Motion

Key Question:

How do waves move?

Objectives:

Explain how waves move.

Compare and contrast transverse and longitudinal waves.

Use knowledge of longitudinal and transverse waves to describe water waves.

WavesA wave is an oscillation that travels from

one place to another.

If you poke a floating ball, it oscillates up and down.

The oscillation spreads outward from where it started.

Why learn about waves?

Waves carry useful information and energy.

Waves are all around us:— light from the stoplight— ripples in a puddle of— electricity flowing in wires— radio and television and

cell phone transmissions

Recognizing waves around you

Waves are present:— when you see a vibration that

moves.— when something makes or

responds to sound.— when something makes or

responds to light.— when technology allows us to “see

through” objects.— when information travels through

the air (or space) without wires.

WavesWaves are a traveling

form of energy because they can change motion.

Waves also carry information, such as sound, pictures, or even numbers.

Wave pulses

A wave pulse is a short ‘burst’ of a traveling wave.

It is sometimes easier to see the motion of wave pulses than it is to see long waves with many oscillations.

Transverse wavesA transverse wave has its oscillations

perpendicular to the direction the wave moves. A

A wave pulse along a rope attached to a wall moves left to right, while the boys hand moves up and down.

Longitudinal waves

The oscillations of a longitudinal wave are in the same direction that the wave moves.

Longitudinal waves A sharp push-pull on the

end of the spring results in a traveling wave pulse as portions of the spring compress, then relax.

Sound waves are longitudinal waves.

Like a wave pulse on a spring, air molecules oscillate back and forth as sound travels.

Frequency, amplitude, and wavelengthYou can think of a wave as a moving series

of high points and low points.

A crest is the high point of the wave.

A trough is the low point.

Frequency

The frequency of a wave is the rate at which every point on the wave moves up and down.

Frequency means “how often”.

Amplitude

The amplitude of a water wave is the maximum height the wave rises above the level surface.

WavelengthWavelength is the distance from any point

on a wave to the same point on the next cycle of the wave.

The distance between one crest and the next crest is a wavelength.

The speed of wavesThe speed of a water wave is how fast the

wave spreads, NOT how fast the water surface moves up and down or how fast the dropped ball moves in the water.

How do we measure the wave speed?

The speed of wavesA wave moves one

wavelength in each cycle.

Since a cycle takes one period, the speed of the wave is the wavelength divided by the period.

The speed of wavesThe speed is the distance traveled (one

wavelength) divided by the time it takes (one period).

We usually calculate the speed of a wave by multiplying wavelength by frequency.

A wave has a wavelength of 0.5 meters, and its frequency is 40 hertz. What is the speed of the wave?

Calculating wave speed

1. Looking for: …speed of the wave.

2. Given: …wavelength (0.5 m) and frequency (40 Hz).

3. Relationships: Use formula: speed = ƒ x

4. Solution: …speed = 40 Hz × 0.5 m = 40 1/s × 0.5 m

speed = 20 m/s

Cooking with waves A microwave heats food by

transferring wave energy to the food.

The magnetron is a device in a microwave oven that creates a wave with electricity.

The wave vibrates inside the cooking space at 2.5 gigahertz- the frequency best absorbed by water.

Standing waves on a stringA wave that is confined between boundaries is

called a standing wave.With all waves, resonance and natural

frequency are dependent on reflections from boundaries of the system containing the wave.

Standing Waves and Harmonics

The standing wave with the longest wavelength is called the fundamental.

The fundamental has the lowest frequency in a series of standing waves called harmonics.

The first five standing wave patterns of a vibrating string shows that patterns occur at multiples of the fundamental frequency.

Standing wavesStanding waves have nodes and antinodes.

A node is a point where the string stays at its equilibrium position.

An antinode is a point where the wave is as far as it gets from equilibrium.

Standing waves

It is easy to measure the wavelength of a standing wave on a string.

Two harmonics equals one wave!

Unit 7: Vibrations, Waves & Sound

Chapter 19: Waves19.1 Waves

19.2 The Motion of Waves

19.3 Wave Interference and Energy

19.2 Investigation: Resonance and Standing WavesKey Question:

How do we make and control waves?

Objectives: Describe how frequency, wavelength, and speed are

related. Measure the wavelength and frequency of a vibrating

string. Recognize and apply the concept of harmonics in

resonant systems. Define natural frequency and apply methods for changing

the natural frequency of a system.

Waves propagationWaves propagate, which means they spread

out from where they begin.

When you drop a ball into water, some of the water is pushed aside and raised by the ball.

Wave motionA wave front is the leading

edge of a moving wave which is considered to be the crest for purposes of modeling.

The crests of a plane wave look like parallel lines.

The crests of a circular wave are circles.

Four wave interactions

1. reflection,

2. refraction,

3. diffraction, or

4. absorption.

When a wave encounters a surface, four interactions can occur:

Diffraction

Diffraction usually changes the direction and shape of the wave.

When a plane wave passes through a small hole diffraction turns it into a circular wave.

Unit 7: Vibrations, Waves & Sound

Chapter 19: Waves19.1 Waves

19.2 The Motion of Waves

19.3 Wave Interference and Energy

19.3 Investigation: Exploring Standing Wave PropertiesKey Question:

How does changing the tension affect a vibrating string?

Objectives: Apply an understanding of inertia and restoring force to

describe the effects of increasing the tension of a vibrating string.

Explain how a string’s tension and mass affects its frequency and amplitude.

Describe how wave properties are applicable to musical instruments.

Superposition principle Interference happens when two or more

waves mix together.When more than one wave is present, the

total oscillation of any point is the sum of the oscillations from each individual wave.

Noise canceling headphones Specialized headphones can

create “anti-noise.”

A microphone in the headphone samples the noise and generates anti-noise, or sound that is 180 degrees out of phase with the noise.

The anti-noise uses superposition to reduce or muffle noise.

Constructive interferenceConstructive interference happens when

waves add up to make a larger amplitude.

Suppose you make two wave pulses on a stretched string.

One comes from the left and the other comes from the right.

When the waves meet, they combine to make a single large pulse.

Destructive interference

What happens when one pulse is on top of the string and the other is on the bottom?

When the pulses meet in the middle, they cancel each other out.

During destructive interference, waves add up to make a wave with smaller or zero amplitude.

Resonance and lightA wave has to be caught in a system with

boundaries to show resonance.

Catch light between two perfect mirrors and we can get resonance of light waves.

This is exactly how a laser works!

Resonance and elastic string

Resonance elastic strings is created by adding new pulses so that each adds to the reflected pulse in constructive interference.

Waves and energy

The energy of a wave is proportional to its frequency.

Higher frequency means higher energy.

Waves and energyThe energy of a wave

is also proportional to its amplitude.

Given two standing waves of the same frequency, the wave with the larger amplitude has more energy.

Waves that Shake the Ground

On January 12, 2010, a 7.0 magnitude earthquake struck the Caribbean nation of Haiti.

Its capital, Port-au-Prince, was nearly destroyed.

Many government buildings, schools, hospitals, and businesses collapsed.

The powerful earthquake was caused by waves traveling through Earth.