Whiteboard Work

1. A child on a playground swing makes a complete to-and-fro swing every 2 seconds.

a. What is the frequency of the swinging?

b. What is the period of the swinging?

Don’t forget the units!

Waves

Part 1: Basics

Objectives

• Identify the parts of a wave.

• Relate wave speed, frequency, wavelength, and period.

• Trace the movement of the medium in a string wave, in a sound wave, and in a compression wave in a spring.

What’s the Point?

• How are waves described?

• Why do waves occur?

Week Outline

• Waves in One Dimension– Features of Waves– Types of Waves

• Combinations of Waves– Interference– Standing Waves

• Waves in Two dimensions– Ocean waves

Waves and Vibrations

• Vibrations: Repeat in time– no net displacement of disturbance– no net displacement of medium

• Waves: Repeat in time and space– wave travels– no net displacement of medium

• Wavelength: crest-crest distance• Trough: low point

• Period: crest-crest-timing

Features of a Wave

• Crest: high point

crest

trough

Features of a Wave• Amplitude A: (crest height – trough height) / 2

A

u

• Frequency f: repeats in a given time (= 1/T)

• Velocity u: speed of crest motion

Relations between Features

• Period T = 1 / f; Frequency f = 1 / T

• Velocity u = / T

• Wavelength = uT = u / f

• Frequency f = u / ; Period T = / u

= f

Group Poll Question

Doubling the frequency of a wave while keeping its speed constant will cause its wavelength to

A. increase.B. decrease.C. stay the same.

Group Poll Question

Doubling the speed of a wave while keeping its wavelength constant will cause its frequency to

A. increase.B. decrease.C. stay the same.

Group Poll Question

Doubling the wavelength of a wave while keeping its frequency constant will cause its speed to

A. increase.B. decrease.C. stay the same.

Group Whiteboard Work

2. A wave generator produces 10 pulses each second. The pulses travel at 300 cm/s.

a. What is the period of the waves?

b. What is the wavelength of the waves?

Wave Pulse in a Rope

• Why does the pulse move?

• What determines its speed?

• What is happening inside the rope?

Prediction Poll

Increasing the tension on a rope will make waves travel along the rope

A. faster.B. slower.C. At the same speed (no effect).

Prediction Poll

Increasing the length density (mass per meter) of a rope will make waves travel along the rope

A. faster.B. slower.C. At the same speed (no effect).

Points to Ponder

The particles of the string change their motion as the wave travels. What force accelerates them?

a cb

d

What are the velocity and acceleration of the string particles at the following positions? Why?

a. middle (leading edge)b. crestc. middle (trailing edge)d. trough

Types of Waves

• Motion of the medium is perpendicular to the direction the wave travels: transverse wave (example: string wave)

• Motion of the medium is parallel to the direction the wave travels: longitudinal wave (examples: sound wave, slinky wave)

• Animation

Poll Question

“The Wave” performed by sports fans is an example ofA. a longitudinal wave.B. a transverse wave.C. an unnatural wave.

Combining Waves

Group Work

0

–3

3

3. Sketch the wave resulting from the addition of the two waves shown at one instant.

Group Work

3. Sketch the wave resulting from the addition of the two waves shown at one instant.

result0

–3

3

Interference

• Constructive: Sum of waves has increased amplitude

• Destructive: Sum of waves has decreased amplitude

• Two-wave simulation

Beats

• Waves of similar frequency combine to give alternating times of constructive and destructive interference

• Distinctive “waa-waa” sound with beat frequency equal to the difference in frequency of the component waves (Why?)

• (sound files)

Reading for Next Time

• Sound

• Big ideas:– What sound is– How sound is created– What affects the way sound travels