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.