1 - Waves…Review from Overview

You are standing on a dock at your favorite lake. You have a rock in your hand, what do you do with it?

Throw it in, and what do you see?

Waves

Waves are not just what happens in water or a way to say good bye to someone

Foucault Pendulum

Types of Waves

• Longitudinal Waves

– Particles vibrate in the same direction the wave travels

– Ex. Sound

Types of Waves

Transverse Waves-

– Particles vibrate perpendicular (up and down) when the wave moves left or right

– Ex. Waves on a rope, light waves

Parts of a Wave• Crest- highest point• Trough- lowest point• Amplitude- maximum height from middle to

crest or trough

amplitude

crest

trough

Parts of a Wave

• Wavelength- distance between two consecutive crests or troughs

Wavelength

Parts of a Wave

• Period- time taken for one complete cycle

Frequency - how often something happens

- number of cycles / unit time

- Hertz = cycles / second

How many waves pass the white line?

2

If that took 1 second, what is the frequency of the wave?

frequency = cycles / second

= 2 / 1 s

= 2 Hz

How many waves pass the white line?

6

If that took 3 second, what is the frequency of the wave?

frequency = cycles / second

= 6 / 3 s

= 2 Hz

Example Problems

1) Billy takes his pulse and counts 22 heartbeats in 30 seconds, what is the frequency in hertz?

frequency = cycles / second

= 22 / 30 s

= 0.73 Hz

2) Ian takes his pulse and counts 54 heartbeats in 1 minute. What is the frequency in hertz?

frequency = cycles / second

= 54 / 60 s

= 0.90 Hz

T/1f

f/1T

period (s) frequency (Hz)

More Example Problems3) If something has a frequency of 4 Hz, what is its period? T=1 / f T=1 / 4 Hz T= 0.25 s

4) If something has a period of 3 seconds, what is its frequency? f =1 / T f =1 / 3 s f = 0.3 Hz

5) At an amusement park, a giant ship swings back and forth with a frequency of 0.125 Hz. How long does it take the ship to swing back and forth once?

T=1 / f T=1 / 0.125 Hz T=8 s

Wave Speed - distance per unit time

tdv /Speed (m/s) distance (m)

time (s)

fv Speed (m/s) frequency (1/s)

wavelength (m)

More Example Problems6) Calculate the speed of ocean waves that are 0.4 m apart and have a

frequency of 2 Hz. v=fλ v= 2 Hz · 0.4 m v= 0.8 m/s

7) If a wave traveling at the speed of light (3x108 m/s) has a frequency of 95.5 MHz, what is the wavelength? (1 megahertz = 1x106 hertz )

f=95.5 MHz = 9.55x107 Hz

v = f λ 3x108 m/s = 9.55x107 Hz · λ

λ = 3.14 m

smx /8103

Hzx

Hzx

Hzx

smx7

7

7

8

1055.9

1055.9

1055.9

/103

Interferences

Constructive interferences - Crests overlap crests increasing the amplitude

Interferences

Destructive Interferences - Crests overlap troughs decreasing the amplitude

Physics Lab Safety

Think before you act.

…so in lab the #1 rule in safety is’ “THINK before you ACT!”