Introduction. How does sound move from one place to another? Is there sound in outer space? If...

Introduction

How does sound move from one place to another?

Is there sound in outer space?If waves continuously crash on the beach, why doesn’t the ocean pile up on the coast?

Waves we can see (Mechanical Waves)◦ Waves on water◦ Waves on a string◦ Waves in springs◦ Traffic◦ Others?

Waves we can’t see (Electromagnetic & other)◦ Radio waves, microwaves◦ X-rays◦ Sound waves◦ Earthquakes?◦ Light waves

Energy causes something to vibrate Vibration is a repeating back and forth or up and down motion◦A vibration is a wiggle in time◦Think of a bell ringing

A wave is a wiggle that moves in space◦A wave moving across the ocean◦Sound coming from a bell

Observe the mass bouncing on a spring◦ Describe as many things as you can about what

you see.

Click here to see animation

Spring with a massPendulum at low swing angles

Bicycle pedal

Mechanical waves require a medium (mass)◦Particles are in motion◦There is a restoring force◦Due to inertia, the mass passes through the resting point to the other side

◦Can be described with a sine wave

Do the wave!

http://www.walter-fendt.de/ph14e/stwaverefl.htm

TOC

Transverse: Wave moves medium moves

Longitudinal Wave moves medium moves

•The wave crests are moving through the water at 1 m/s•The wave crests are 1 meter apart

•The bird sees a wave crest A right now. How long before it sees:•Wave crest B?•Wave crest C?•Wave trough D?

ABC

D

•The PERIOD (T) of the wave is the time between two points on the wave that repeat the pattern

•What is the period of this wave?•The FREQUENCY (f) of a wave is how often the wave repeats itself in one second

•What is the frequency of this wave?

•Relationship between frequency and period: f = 1/T

ABC

D

The SPEED (v) of a wave is equal to wavelength x frequency

v = f

Frequency: f = 1/T

Period: T = 1/f ) Velocity: v = f

Speed v m/sWavelength metersPeriod T secondsFrequency f Cycles/sec (Hertz)

The spring completes one cycle in 0.5 s.◦What is its period, T?◦What is its frequency, f?

T = 0.5 sf = 1/T = 1/0.5s = 2 Hertz (Hz)

A surfer feels a wave every 10 s. If they are 50 m apart, how fast are the waves moving?

v = f , so we need to find fF = 1/T = 1/10sSo, v = 50m/10s = 5 m/s

Waves transfer energy without transferring matter

Mechanical waves require a medium

A continuous wave is a regularly repeating sequence of wave pulses