Announcements 11/28/12






Prayer
Exam 3 ongoing…
BYU A Cappella Jam: Friday, 7:30 pm (doors open at 7 pm). Room
151, Tanner building. Tickets are available at the door for $7 or in
advance for $5 (Wilk Center info desk). Come see Noteworthy, Vocal
Point, and the other BYU a cappella groups!
Project Show & Tell. Email me by tonight (midnight) if you are
interested in sharing your project with class. Reminder: +5 bonus pts
a. I’ve only had two groups volunteer so far
Term project final report – due Wed Dec 5, midnight
All late homework & extra credit papers due Sat, Dec 8 (2nd reading
day). Earlier if possible!
Frank &
Ernest
From warmup

Extra time on?
a. Figure 39.13 is difficult for me to understand.

Other comments?
a. Why do physicists often use "unity" rather than "one"?
Is there any other reason other than to sound cooler?
From warmup
The Lorentz transformation equations relate the space and time
coordinates of "events". What is an "event"? Give three
examples.
Event: Something that happens at a particular point in space at a
particular moment of time.
a. Slapping a table
b. I click my mouse
c. I stomp my foot on the ground
d. A firecracker going off
e. Cracking an egg
f.
a car hitting a wall
g. lightning striking one side of a railway car
h. getting out of bed
i.
A book closes
j.
A glass shattering on the ground
k. An LED going on.
l.
A bunny exploding. (What???)
m. Note: You could also probably just call an object's motion as the
event, but I suppose that's what we'll learn Friday. --NOPE

From warmup
The Lorentz transformation equations relate the
coordinates of events occurring in different "frames of
reference". What is a "frame of reference"? Give three
examples.
Reference frame: any observation point with constant
velocity.
a. A boxcar moving close to the speed of light
b. An observer on a constant velocity space-ship
c. Henry Lorentz on a spaceship to the planet Zog.
d. An observer standing still
e. Doctor Colton running with a ladder through a
garage

Space-Time Diagrams
t vs. ct

Examples:
a. measuring the time between book slams
b. measuring the length of the book

“World lines” for Dr. Colton, book, & for space
traveler passing by earth
a. When/where do these events occur?
Quick Writing

Back to Dr. Colton’s trip to Zyzyx: Dr. Colton travels at
0.9 c to Zyzyx, 1 light year away. Draw, as accurately
as you can, these four world-lines:
a. The planet Earth
b. The planet Zyzyx
c. Dr Colton, as he travels to Zyzyx
d. Leaving from the Earth at the same time/place, a
light beam that travels to Zyzyx

Draw, as accurately as possible, the same four worldlines, from Dr. Colton’s point of view.
Light cones





Present
Possible futures
Possible pasts
“Elsewhere”
Terminology: “timelike” vs “spacelike”
Example

Dr. Colton, on a fast train, turns on a
flashlight at x=0, ct=1. People on the ground
watch. Draw worldlines for Dr. Colton and
the people on the ground, as well as the light
from the flashlight. Mark the “flashlight
turned on” event.

Do this for both frames of reference.
From warmup

The Lorentz transformation equations are given as Eqn
39.11 and Eqn 39.12 (reverse transformation) (8th
edition). Why are the equations for y and z so trivial?
a. The motion is only in the x direction, so we don't
need to worry about y and z.
b. The equation for x needs to account for length
contraction; y and z do not.
Lorentz Transformations, derived

Not in this class! :-)

Can be done by using properties we’ve
discussed: length contraction, time dilation,
gamma factor, basic worldline transformations
The Lorentz Transformations

As regular equations:
xframe2   xframe1   (ct )frame1
(ct )frame2   xframe1   (ct )frame1

In matrix form:
x
 

 
 ct  frame 2  
  x 
 
  ct 
frame1
Differences between my equations & book’s equations
Disclaimer: I often reverse order of x and ct in matrix eqn. That’s OK.
Lorentz transformations, graphically



Lee’s program
Similarity to rotations
How to choose + vs –
Note: for HW problems, you
can use Lee’s program to
check your answers—but not to
DO your problems.