== Lecture 6: Chapters 15, 16 ==

== Homework return folder ==

  1. Pick up yours only.
  1. Two homeworks (homework 1 in the 2nd partition within each folder.
  1. Can't find yours?  Let me know (no name).
  
== Announcement (Reading quiz) ==

  1. There will be no announcements about reading quiz, any more.
  1. From now on, it should be done in the ''auto-pilot'' mode.
    a. Must know that the quiz will be up 24 hours before class.
    a. Must do it.  But, more importantly, '''must read'''.
  1. If you miss one, no big deal—you can always make up for it by showing me how good you are at figuring out correct answers!

== Q: intrinsic property of a medium ==

Which of the following quantities is the instrinsic property of a wave medium?

    A. $\omega$ (angular frequency)
    A. $v$ (wave speed)
    A. $k$ (wave number)

Ans: B

== Q: intrinsic property of a wave ==

A sinusoidal wave is propagating through multiple media.  Which of the following quantities does not change as the wave propagates from one medium to another?

    A. $\omega$ (angular frequency)
    A. $v$ (wave speed)
    A. $k$ (wave number)

Ans: A

== Q: light ==

Microwaves travel at the speed of light, $c = 3 \times 10^8$ m/s.  You are heating a burrito in a microwave.  You know that water molecules in the microwave are vibrating.  You find out that the frequency of the microwave is $10$ GHz.  What is the wave length of the microwave?

  A. 0.3 mm
  A. 3 cm
  A. 30 cm
  A. 300 m

Ans: B

== Q: $D(x,t)$ for a string wave ==

A string wave is moving to the right as shown below (a snapshot).

<<c>>
<<lia(String-wave-fig1.png, align = center)>> 
<<c(1)>>

What is the direction of the velocity of a particle at the point labeled '''A'''?

  A. Right
  A. Up
  A. Down
  A. Undefined (not moving)

Ans: D (extremum of a SHM)

== Q: $D(x,t)$ for a string wave ==

A string wave is moving to the right as shown below (a snapshot).

<<c>>
<<lia(String-wave-fig2.png, align = center)>>
<<c(1)>>

What is the direction of the velocity of a particle at the point labeled '''B'''?

  A. Right
  A. Up
  A. Down
  A. Undefined (not moving)

Ans: B (non-extremum of a SHM&mdash;so finite $v$; the direction can be figured out by considering a second snapshot after a small time interval $\Delta t$ after, and considering the sign of displacement, $\Delta D(x, t) = D (x, t + \Delta t) - D (x, t)$, at the ''fixed'' $x$ value, as indicated)
 
== Q: Wave, wave, standing wave... ==

A string is clamped at both ends and plucked so it vibrates in a standing mode between two extreme positions a and b.   Let upward motion correspond to positive velocities.   When the string is in position b, the instantaneous velocity of points on the string:

<<c>>
<<lia(Standing-wave-fig1.png, align = center)>>
<<c(1)>>

  A. is zero everywhere.
  A. is positive everywhere.
  A. is negative everywhere.
  A. depends on the position along the string.

Ans: A

== Q: Wave, wave, standing wave... ==

A string is clamped at both ends and plucked so it vibrates in a standing mode between two extreme positions a and b.   Let upward motion correspond to positive velocities.   When the string is in position c, the instantaneous velocity of points on the string:

<<c>>
<<lia(Standing-wave-fig2.png, align = center)>>
<<c(1)>>

  A. is zero everywhere.
  A. is positive everywhere.
  A. is negative everywhere.
  A. depends on the position along the string.

Ans: D