Phys. 122: Tuesday, 01 Sept.

Written HW 2: due Thursday. Mast. Phys.: Assign. 1 due in one week. Reading: Should be finished with ch. 21 by now. Read

ch. 22 by next Tuesday (skip sections 22.5 and 22.6.).• Office Hours (tentative): Mon. 1:30-2:30, Wed. 9-11,or by appointment.

A wave on a string is traveling to the right. At this instant, the motion of the piece of string marked with a dot is

QuickCheck 20.5

A. Up.

B. Down.

C. Right.

D. Left.

E. Zero. Instantaneously at rest.

Slide 20-45

The constant, k, introduced in Section 20.3 on Sinusoidal Waves is

A. The Boltzman’s constant, with units: J/K.B. The Coulomb constant, with units: N m2/c2.C. The force constant, with units: n/m.D. The wave number, with units: rad/m.E. The wavelength, with units: m.

Reading Question 20.4

Slide 20-16

The wavelength measures (nearest) distancebetween two identical points of the wave. It'sthe equivalent of period (T) for distance insteadof for time.

Chapter 20: Wave Motion

Waves come in two basic types, dependingupon the direction of the wiggles of the wave

compared to the direction that the wavetravels.

A transverse wave has its wiggles at90 degrees (perpendicular) to the wave

travel direction.A longitudinal wave has its wiggles in thesame direction that the wave is moving.

... but, what is this the solution of???It is the solution of the wave equation:

Harmonic wave motion: mathematical form

∂2 y∂ x2

=( 1v2) ∂2 y

∂ t2

(F = m a slightly disguised.)k is called the wavenumber, given by k = 2 π/λ.It converts distance into angle (just like ω=2π/Tdoes with time instead of distance).

Properties: v = wave speed dependsupon the properties of the materialwhich carries the wave. For instance,v² = T / μ for transverse string waves,where T= tension and μ= mass/length.

Direction of wave is to the RIGHT withthe – sign, and to the LEFT for +.

(Also called D(x,t) sometimes in your book.)

A. 2 s.

B. 4 s.

C. 1 s.

D. Not enough information to tell.

The period of this wave is

QuickCheck 20.6

Slide 20-49

Note: wave speed is alsogiven by ω/k!

Be careful: wave speed is NOT the same asspeed of the particles doing the wiggling! Thelatter quantity is vy = dy/dt and wiggles withtime; the wave speed is CONSTANT, anddetermined by properties of the thing that'swiggling.

Wave Motion on a String

Shown is a snapshot graph of a wave on a string with vectors showing the velocity of the string at various points.

As the wave moves along x, the velocity of a particle on the string is in the y-direction.

Slide 20-53

Sinusoidal Waves

A wave source at x = 0 that oscillates with simple harmonic motion (SHM) generates a sinusoidal wave.

Slide 20-42

Example: ABQ radio station“94 Rock”broadcasts at a frequency f =94.1 MHz.What is the wavelength of the radio waves?

Clickers: A mass on a spring has a fixedfrequency and period. What about waves on

a string?

a) The wave f and T are also fixed for a given thingsupporting the waves.

b) The wave f is fixed, but T is variable.

c) The wavelength λ is fixed, but any value of f ispossible.

d) Any wavelength and f can be made for a givenwave, independently.

e) Any wavelength or f are possible, but only oneof these quantities can be chosen by the source; theyare not independent.

Clickers: when a wave in one medium movesto another medium with different wave speed,what determines the new f and λ?

• a) λ remains the same; f changes accordingly• b) f remains the same; λ changes accordingly• c) Both f and λ will be different• d) Both f and λ will remain the same• e) There's just no way of knowing.

Clickers: why are waves important?

• a) They're not.• b) All things have wavelike behavior.• c) Waves which obey quantum mechanics look like particles.• d) Conservation of energy and momentum is secretly just conservation of wave phase. • e) All of (b)-(d) are true.

Clickers: A sound wave is a longitudinalwave. Which of the following accurately

describes what the air molecules are doing?

a) The air molecules move from the source of thesound to the listener. b) The molecules move at right angles compared with the wave direction (from sound to listener). c) The molecules exchange places with eachother as the wave moves, transporting some of theair from the sound to the listener. d) The molecules bump into each other whilethey are wiggling parallel to the wave direction butthey don't travel with the wave itself. e) The air moves from your ear to the source of the sound

Slide 20-89

The Doppler Effect: Moving Source

Doppler shift: general formula

f L=vw±vL

v w∓vS

f S

This gives the frequency shift. L stands for“Listener” and S stands for “Source.” Use theupper signs for motion toward one another, andthe lower signs for motion apart from one another.