PHY361 Principles of Modern Physics

About myself:Kwok-Wai NgOffice: CP 385Telephone: 7-1782E-mail: kwng@uky.edu

Office hour: Tue 9:00-10:00

About this course

Time: Monday, Wednesday, and Friday 11:00-11:50 a.m.

Place: CP 183

Text book: Concepts of Modern Physics, 6th edition, by Arthur Beiser. (Publisher: McGraw-Hill Higher Education)

Grading policy

Homework 30%Test I 20%Test II 20%Final Examination 30%Total 100%

Examination and homework

• All tests and final examination are close book and close notes. Final examination is comprehensive.

• Homework assigned every Friday, and it will be due next Friday. Problems will be randomly selected for grading.

• One or two problems in the tests will be selected from the homework (but not exactly the same).

Syllabus

• Read syllabus carefully at home. Ask if you have any question.

• Please sign the class roll when it is passed to you.

• Evaluation window for this semester:Monday, April 9, 2007 - Monday, April 23,2007

Two Postulates of Special Relativity (Text 1.1)

Postulate I:The laws of physics are the same in all inertial frames of reference.

1. An inertial frame of reference is one in which the Newton’s laws of motion holds. All inertial frames of reference are moving relative to each other with a constant velocity.

2. This is a reasonable assumption, even from the classical viewpoint. However, in classical wave mechanics, we have a very special inertial frame of reference in which the medium is at rest.

Two Postulates of Special Relativity (Text 1.1)

The speed of light in free space has the same value in all inertial frames of reference.

1. Speed of light to Norma is c (2.998×108 m/s). Speed of light to Bob is the same, c.

2. Classically, speed of light to Norma is c. Speed of light to Bob is c+v.3. Light plays a special role in relativity because of this postulate.

v

Norma

Bob

Light

More on relativity (Not in text)• In relativity, an event is marked by its position and time. In

other words, an event is a coordinates with 4 components in the four dimensional space.

• In a particular inertial frame, it is possible to have two events occurring at the same place, but at different time. The time interval between these two events can be measured by one stationary clock. The time measured by this clock is called the proper time.

• For any other moving inertial frame, these two events will not occur at the same place and the time interval between the events has to be measured by two clocks at two different places. More troublesome is, the time interval measured by these two clocks will be different from the proper time!

Time Dilation (Text 1.2)

v

Norma

Bob

Light

Now Professor Einstein has a difficulty:

Both the ruler and watch will look different to Bob and Norma. Everything gets tangle together!

To simplify the situation, let us assume the relative velocity between two inertial frames will have no effect to length perpendicular to the relative velocity.

Time Dilation (Text 1.2)

v

Norma

Bob

This is a meter stick.I agree. This is one meter long.

This is the case because Norma is holding the ruler so that it is perpendicular to v.

Time Dilation (Text 1.2)Now we can imagine an experiment:

Both Bob and Norma are holding the same kind of clock in their hands. Bob is holding the flash light perpendicular to v.

L0

Mirror

Time Dilation (Text 1.2)Why not hold the Einstein’s clock parallel to v ? What is the mirror for?

Mir

ror

Time Dilation (Text 1.2)In Bob’s eye: Mirror

L02

tc L

cL2 t

Bob0

0Bob

=⇒

=

Event 1 Event 2

Time Dilation (Text 1.2)

22Norma0

222

Norma20

2Norma

2Norma2

0

2Norma2

0

Norma

vc2

t L

)vc(2

t L

2 tc

2 tvL

c2

tvL2 t

−⎟⎠⎞

⎜⎝⎛=⇒

−⎟⎠⎞

⎜⎝⎛=⇒

⎟⎠⎞

⎜⎝⎛=⎟

⎠⎞

⎜⎝⎛+⇒

⎟⎠⎞

⎜⎝⎛+

=

In Norma’s eye:

L0

V tNorma

Mirror

Event 1Event 2

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