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Prayer
Exam 3 review session: Monday 5 - 6:30 pm, room C261
Exam 3 starts Tuesday after break
Lab 10 due Tuesday after break

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From my point of view, objects which experience no force don’t accelerate. What type of reference frame am I in?
a. An “enlightened” reference frame b. An “inertial” reference frame c. A “Newtonian” reference frame d. A “normal” reference frame e. A “null” reference frame

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Toss a ball. Slam on the brakes. What happens?
Throw a ball to a friend on a merry-go-round
(as it’s spinning). What path does the ball take?
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Reference frames where Newton’s Laws apply: “inertial frames”

Credit: this slide and next one from Dr. Durfee

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I drive past a baseball field traveling north at 25 MPH. A player throws the ball south at a speed (relative to the ground) of 70
MPH. What is the speed of the ball in my inertial reference frame (to 2 sig figs)?
a. 25 MPH b. 45 MPH c. 70 MPH d. 74 MPH e. 95 MPH

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I drive past a baseball field traveling north at 25 MPH. A player throws the ball west at a speed (relative to the ground) of 70 MPH.
What is the speed of the ball in my inertial reference frame (to 2 sig figs)?
a. 25 MPH b. 45 MPH c. 70 MPH d. 74 MPH e. 95 MPH

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I’m riding my bike at 1  10 8 m/s. I turn on my
“flashlight”, front bike light (c=3  10 8 m/s). a. How fast do I see the light waves go away?
b. How fast does someone on the ground see the light waves go away?
http://stokes.byu.edu/emwave_flash.html
Changing magnetic field
 electric field
Changing electric field
 magnetic field
Nothing in equations says anything about the flashlight!!!
(the source of waves)

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Source stationary: sound waves travel at 343 m/s
(as measured by both source and observer)
Source moving at 100 m/s: sound waves still travel at 343 m/s (as measured by both source and observer). Only frequency will be changed.
Why is it a Big Deal that light waves do the same thing?

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Postulate 1: The laws of physics apply in all inertial reference frames.
Postulate 2: The speed of light is the same for all inertial observers, regardless of motion of the light source.
Michelson-Morley experiment
The “problem”: these two simple statements have some crazy implications

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If height of train car inside is h, how long did that take (to me, inside the train)?
Answer: t = 2h/c
Credit: animations from Dr. Durfee

How long did it take, really?
Why doesn’t this “problem” exist with sound waves?
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“Ether dragging” and
Michelson-Morley
If height of train car inside is h, how long did that take (to you, on the ground)?
Train is traveling at speed v
Answer: t = 2h/c
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(1-v 2 /c 2 ) -1/2

Answer 1 (measured on train): t = 2h/c
Answer 2 (measured on ground): t = 2h/c
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(1-v 2 /c 2 ) -1/2
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 D t
Time measured by you, on ground: D t 
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  v c
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1
1
  2
 v / c
 t
For v = 0.9c:
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= 0.9
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= 2.3

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Suppose I, Dr. Colton, (in the train) measure the time interval to be 1 second, presumably through lots and lots of light bounces or something along those lines. If the train is moving at 0.9c, you, the class (on the ground) measure that time interval to be 2.3 s. To you, it looks like things in the train are running in slow motion.
However, what if you on the ground are the one that is bouncing light rays back and forth. If you measure a time interval to be 1 s,
To you, my time appears to be slowed.
To me, your time appears to be slowed.
running normally Who is right?
b. (1/2.3) s – to Dr. Colton, it looks like things on the ground are sped up c.
2.3 s – to Dr. Colton, it looks like things on the ground are running in slow motion.

Jim
Light from which lightning bolt will reach Jim first?

Jim

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Jim’s friends all record the actual times in Jim’s reference frame
Or equivalently, Jim is just smart enough to factor out the time the light took while traveling.
Jim
Jim’s friends

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Speedo & Goslo…which twin is older?

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Dr. Colton on train, again
Turn the flashlights on at the same time, the photons reach the walls simultaneously. OK?

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Viewed from the ground; train moving to right.
Events which happen simultaneously in one
“reference frame” do NOT happen simultaneously in any other reference frame
Which light ray travels farther?
Which light ray hits the wall first?