PHY138 – Waves, Lecture 3“..imagine a stadium full of people doing "the wave"

with a giant sheet covering them so that none of the spectators can be seen. To someone watching such an event it would appear that something was moving around the stadium under this sheet. It would look as if some giant creature was running in circles. In reality no one moves around the stadium; each spectator simply stands up and sits back down in the same seat. There is energy moving around the stadium while the people remain stationary.”

- thinkquest.org

Reading Assignment Please read the following from Serway

and Jewett before class on Wednesday: Chapter 13, Sections 13.7 and 13.8, and Chapter 14, Sections 14.1 and 14.2

A Web-CT quiz is due Wednesday morning, which tests your basic familiarity with Chapter 14 assigned reading.

McDermott Tutorials book, page 149 regarding Electromagnetic Waves

Written Homework

Please complete Problems 8, 22, 34 and 41 from Chapter 13. They are due in your tutorial drop-box by Monday, November 22 at 5:00 PM.

Note from David Harrison: Regarding Test 1: Problem involving two

horse teams pulling a set of connected hemispheres:

Both the correct answer and “Answer C” were marked as correct. Students who interpreted the question to be about net-force were correct to reason that the net-force on a non-accelerating object is always zero.

Harrison will not answer emails about this question

Last day’s lecture: some comments The simple pendulum was modeled as a

point-mass on a massless string. Moment of inertia was therefore I=mr2=mL2.

Objects such as bridges, trumpets, radio antennae, etc, all have natural oscillation frequencies. Resonance occurs when an outside driving force is close to one of these frequencies. At resonance, the amplitude of oscillations is very high.

First mini-quizA person swings on a swing. When the

person sits still, the swing oscillates back and forth at its natural frequency. If, instead, two people sit on the swing, the natural frequency of the swing is

1. greater

2. the same

3. smaller

Second mini-quizA person swings on a swing. When the

person sits still, the swing oscillates back and forth at its natural frequency. If, instead, the person stands on the swing, the natural frequency of the swing is

1. greater

2. the same

3. smaller

PHY138 – Waves, Lecture 3Today’s overview

Mathematical Description of a Wave Speed of Waves on a String Reflection and Transmission of Waves Sound, Ultrasound

Sinusoidal Wave snapshot

k=2π/λ

Sinusoidal Wave at a point in space

ω=2π/T

Reflection of Transverse Wave Pulse

A pulse traveling to the right on a heavy string attached to a lighter string

Speed suddenly increases

Reflection of Transverse Wave Pulse

A pulse traveling to the right on a light string attached to a heavier string

Speed suddenly decreases

Reflection of Transverse Wave Pulse

Reflection of a traveling pulse at the free end of a stretched string.

Reflection of Transverse Wave Pulse

Reflection of a traveling pulse at the fixed end of a stretched string.

Physics of Ultrasound Speed of sound in bone, flesh and blood

are all different When the speed of any wave suddenly

changes, there is a reflection and transmission

Ultrasound images are formed from reflected high frequency sound

Image resolution is set by wavelength, λ λ=v/f, so higher frequency yields smaller λ,

and better resolution

Speed of sound in humans

Tissue Sound Speed (m/s)

Air 350

Fat 1450

Brain 1540

Blood 1570

Bone 4080

Muscle 1585