Bodies in a state of rest (motion wise)

BODIES IN A STATE OF REST (MOTION WISE)Which of the following are bodies in a state of rest?

• A carrot in a fridge• A pendulum hanging at equilibrium position• Jens• A geostationary satellite• Venus orbiting the sun• The sun

IT’S ALL RELATIVE!

1. To begin to understand what is meant by the term special relativity

2. To be able to give examples of and compare different frames of reference

3. Be able to describe the Michelson-Morley experiment and explain its significance

An introduction to special relativity

WHAT IS SPECIAL RELATIVITY?What do you think the term special relativity means?

Answers on a post-it

FRAMES OF REFERENCE• The motion of a body can only be described relative to something

else - other bodies, observers, or a set of space-time coordinates. These are called frames of reference.

• Look at the following pictures, what are the possible frames of reference?

To analyse the motion of train A:

Identify the roles in the frames

Possible Observers Relative motion

The motion of train A:

Possible Observers Relative motion

The motion of one of the books:

Possible Observers Relative motion

The motion of one of the balls:

Key Question: What is the importance of choosing a suitable observer when analysing motion in different frames of reference?

QUESTIONS1. What is a "frame of reference"?

2. Why is it important to define frames of reference when analysing motion?

3. Imagine a ball that is dropped from the top of a ladder on a boat floating down a river. If you wanted to study the motion of the ball, from where would you choose to observe it? The top of the ladder? The deck of the boat? The riverbank? Explain your choice.

4. Extension – try to come up with a more complex situation of relative motion and explain what observation you would make

Mr Furneaux’s baguette

Maxwell’s hypothesis•After Maxwell’s theory of EM waves, many believed the waves were vibrations in the ‘ether’.

•It should be possible to detect the Earth’s motion through the ether using experiments involving interference of light.

•Roughly what length should we be able to find information about using light interference?

Light travel parallel to the motion

Light travel perpendicular to the motion

Michelson-Morley interferometer

Why will interference fringes appear?

http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/mmexpt6.htm

•Align the apparatus so that one beam is parallel and one perpendicular to the motion of the Earth through the ether.

•Turn the apparatus through 90 degrees.

•Fringes should shift by 0.4 of a fringe width. Apparatus capable of detecting 0.05 fringe shift.

•Absolutely no shift detected – a ‘null result’

Exam question

(a) (i) beam splitter [or semi-silvered mirror] (1)

(ii) a compensator [or a glass block] (1)allows for the thickness of the (semi-silvered) mirrorto obtain equal optical path lengths in thetwo branches of the apparatus) (1) 3

(b) (i) concentric rings (1)an interference pattern (1)[alt: whole view shows one shade (1) because there isa constant phase difference(1)]

(ii) fringes [or rings] shift (1)05l extra for l1 gives one complete fringe shift[or fraction of wavelength extra causes noticeable fringe shiftor noticeable change of intensity (if uniform)] (1) 4

(c) (i) rotate apparatus through 90° (1)observe the fringes at the same time (1)observed fringes did not change [or shift] (1)

(ii) speed of light in free space is invariant[or does not depend on motion of source or observeror no evidence for absolute motion] (1) max 3

[10]

SPECIAL RELATIVITY IS BASED ON TWO POSTULATES:1. The laws of physics, expressed in equations, have the same

form in all inertial frames of reference.

2. The speed of light in free space is invariant (ie the same for all observers regardless of their state of motion and of the speed of the light source).

THINKING POINT!

• How could you test your theory experimentallyHow could you test your theory experimentally?

• Come up with an experiment to do in the lab that could test this theory• Hints are available if you find this tricky• If you already know of an experiment for this, move on to the calculations

section

•Consider a convertible approaching you at a speed of 20 m/s. The passenger pulls out a slingshot and shoots a rock at you with a speed of 8 m/s .

•How fast is the rock going?

•Assume the same convertible is driving towards you at 20 m/s and it switches its lights on.

•How fast is the light going when you see it?

The orbital speed of the Earth is 29.8km/s

Some of the available equipment: Some of the available equipment: Monochromatic light source, mirrors, glass blocks, semi

silvered glass blocks, viewing telescope, screen, accurate length measuring equipment, mercury

When two sources of light of a constant phase difference meet they interfere and produce an

interference pattern

If a wave has an increased velocity it will have a shorter wavelength

What affects the spacing between fringes in an interference pattern?

What sort of effect couldcould the relative motion of the Earth have on the speed of light?

The Significance of the Michelson-Morley?

• How did scientists ideas about the speed of light change after this experiment had been verified?

• How is this important in developing a theory of special relativity?

http://webphysics.davidson.edu/physlet_resources/special_relativity/

INERTIAL FRAMES OF REFERENCEThere are several ways to describe an inertial frame. Here are a few descriptions:

An inertial frame of reference is a frame of reference with constant velocity.

An inertial frame of reference is a non-accelerating frame of reference.

An inertial frame of reference is a frame of reference in which the law of inertia holds.

An inertial frame of reference is a frame of reference in which Newton's laws of motion hold.

In an inertial frame of reference no fictitious forces arise.