PHYS 20 LESSONS

Unit 6: Simple Harmonic

Motion

Mechanical Waves

Lesson 6: Mechanical Waves

Reading Segment:

Mechanical Waves

To prepare for this section, please read:

Unit 6: pp.18 - 19

MECHANICAL WAVES

There are two types of mechanical waves:

1. Transverse Waves

- wave motion is perpendicular to particle

vibration

2. Longitudinal Waves

- wave motion is parallel to particle vibration

Transverse Waves

Consider a vertical vibration at the end of a spring:

spring (medium)

Vibration

wave motion

Vibration

The vibration is the source of energy for the wave.

It creates a disturbance in the medium, moving away from the

source.

wave motion

Vibration

All particles are vibrating perpendicular to the wave motion.

They never move left or right.

Thus, it is called a transverse wave.

Crests

Vibration

Troughs

The top of the transverse wave is called the crest.

The bottom of the wave is called the trough.

Equilibrium

Vibration

The rest position of the medium is called the equilibrium

position. This is where the particles would be if there was

no wave going through it.

A

A

Vibration

The maximum amplitude (A) is the energy of a wave.

It is the distance from equilibrium to the crest

(or from equilibrium to the trough).

Vibration

The wavelength () is the length of 1 cycle or repetition

of the wave.

Vibration

The wavelength could be the length from crest to crest.

Vibration

The wavelength could also be the length from trough

to trough.

Vibration

Note: If two particles are one wavelength apart, they will move

identically as the wave train goes through the medium.

We say that they are in phase.

Animations

Transverse Waves

http://surendranath.tripod.com/Applets.html

Go to: "Waves"

"Transverse Waves"

"Transverse Waves"

Animations

Anatomy of Transverse Waves

1. http://id.mind.net/~zona/mstm/physics/waves/partsOfAWave/waveParts.htm

2. http://id.mind.net/~zona/mstm/physics/waves/introduction/intr

oductionWaves.html

Longitudinal Waves

Consider a horizontal vibration at the end of a spring:

Vibration

spring (medium)

Direction of wave

Vibration

The vibration is the source of energy, and it causes

a horizontal disturbance in the medium.

The wave moves away from the source.

Direction of wave

Vibration

The particles are always vibrating parallel to the wave motion.

Thus, this is called a longitudinal wave.

C C C C

Vibration R R R R

The regions of high density are called compression (C) regions.

The regions of low density are called rarefaction (R) regions.

Vibration C C

The wavelength of a longitudinal wave is the distance

between two successive compression regions

i.e.

"from C to C"

Animations

Longitudinal Waves

http://surendranath.tripod.com/Applets.html

Go to: "Waves"

"Longitudinal Waves"

"Transverse Waves"

Note:

Due to friction, waves will lose their energy

- transverse waves lose their amplitude

- longitudinal waves will lose their density

But, the frequency will never change.

Surface waves (e.g. water waves) are circular waves

Surface waves (e.g. water waves) are circular waves

Upper particles move

in circles

Lower particles move

in ellipses

Deep particles move back

and forth

Earthquakes create all 3 kinds of waves

Primary (P) waves

- the first to arrive

- longitudinal waves

Secondary (S) waves

- second to arrive

- transverse waves

Surface waves

- last to arrive

- these do the most damage

Animation

Earthquakes

1. http://www.pbs.org/wnet/savageearth/animations/earthquakes/

main.html

2. http://schools.matter.org.uk/Content/Seismology/index.html

3. http://encyclopedia.thefreedictionary.com/Seismic%20wave