PH 103 Dr. Cecilia Vogel Lecture 10. Review Outline Interference 2-slit Diffraction grating ...
-
date post
21-Dec-2015 -
Category
Documents
-
view
213 -
download
0
Transcript of PH 103 Dr. Cecilia Vogel Lecture 10. Review Outline Interference 2-slit Diffraction grating ...
PH 103
Dr. Cecilia VogelLecture 10
Review
Outline
Interference 2-slit Diffraction grating spectra
Relativity classical relativity
constants velocity addition when is it a good approx
Relativity means comparing physical
quantities measured by observers in different states of motion (aka reference frames). maybe the values are the same maybe the values are different
if different, look for patterns, relationships between the values of
the same thing measured by different observer
What is your reference frame? Doesn’t matter where you are Just how you are moving
Classical Relativity Historical Common experience Applicable ONLY when all speeds are much
less than the speed of light in vacuum. The following classical relativity ideas hold
when v<<c: Different observers measure same time intervals Different observers measure same lengths Different observers measure different velocities...
of each other. Pattern: vAB = -vBA
of another object. Pattern: v13 = v12 + v23
Relative Velocities Earth is a convenient reference frame
but it is not special Anyone moving relative to the Earth
will observe that the Earth is moving! If you want to know the velocity of
something relative to some observer, Consider that observer to be at rest,
(pretend you are them) and ask how does the position of that
thing change relative to them?
Relative Velocities What direction is
the water moving in photo?
The water is moving South – relative to Earth.
However, relative to the boy, S
the edge of the water is North of himand it is getting farther North of himSO…. it is moving North relative to the boy.
Vector Addition of Velocities
1, 2, & 3 stand for reference frames (NOT velocities!)
So if v13= velocity of Fred relative to Earth, then 1 is Fred and 3 is Earth
Pay attention to the sign: v has direction Pay attention to order of subscripts:
If car goes North relative to cows, then cows go South past car
231213 vvv
vAB = -vBA
Using Vector Addition Step 1: Let v13= answer you seek. Step 2: Identify frames 1 and 3 with person
or object. Step 3: Identify frame 2 -- what’s left? Step 4: Determine value of v12 and v23
If you have v21 or v32 :
CHANGE THE SIGN when you trade subscripts
Step 5: Plug v12 and v23 into eqn to get v13 Step 6: Check that your answer makes
sense!
Postulate of Classical Relativity
Laws of Mechanics same in all inertial reference frames
What is an inertial frame?One in which Newton’s first law holds
When doesn’t it?! Accelerating frame
Do objects at rest remain at rest when you stop, start, turn corner in your car?
In practice, inertial frame moves at constant velocity.
Different but the SameLaws of Mechanics same in all
inertial reference frames Means: Same mechanics experiment
repeated in two different reference frames will yield the same outcome.
Example: Throw a pretzel up and catch it on Earth on smoothly flying airplane same result
Why smooth? -- no acceleration
Different but the Same Laws of Mechanics same in all inertial
frames Means: Same mechanical process observed by
observers in different reference frames will not look the same but will follow the same laws
Example: Throw a pretzel up and catch it on an airplane in smooth flight as viewed on plane as viewed on Earth
SAME law of gravity applies to both
Postulate If all frames yield same laws, then
How do you tell whether or not you are moving? You don’t!
There is NO preferred frame No frame can claim to be at absolute rest. All frames at rest relative to themselves. Relative to the trees, the cars are
moving, but relative to the cars, the trees are moving.(Earth is a convenient reference frame for us,
but it’s not special in the laws of physics)
Tempted to extend that rule
If there really is no preferred reference frame, thenALL laws of physics should be same for all inertial observers
That’s Einstein’s first postulate of special relativity.