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Reading Quiz

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Reading Quiz

Most of the power released by a hydrogen bomb comes from which of the following?

nuclear fusion

nuclear fission

TNT

cold fusion

Relativity: how things appear from different frames of reference.

Principle of Relativity

The laws of physics are the same in any inertial frame of reference

The forward motion of an adjacent bus can give you the impression that your own bus is moving backward.

Inertial Frame of Reference

For Newtonian Mechanics to work you must be in an inertial frame

How do you know if you are in an inertial frame?

Not when in a non-inertial frame you feel forces or accelerations

Must add fictitious forces for “physics” to work. (centifugal forces)

Newton choose “at rest respect to the distance stars”

A motorboat and a piece of wood move downstream. The boat moves a distance dbe relative to the earth while the wood moves a distance dwe.

A motorboat moving upstream loses ground to the stream’s current if it cannot travel fast enough relative to the water (vbw).

A

B

C

v

This truck platform serves as the “rest” frame for Observers A and B, but it moves with a speed v towards Observer C, who stands on the roadside.

Question 1

Observer A throws a baseball to Observer B at a speed of 30 m/s; B catches the ball. The truck platform moves in the same direction with a speed of 20 m/s.

A. For Observers B & C, the ball has the same speed.

B. For Observers B & C, the ball travels the same distance.

C. For Observers B & C, the ball is in the air for the same amount of time.

D. Observers A & B both say the ball has a speed of 50 m/s.

E. Observer C says the ball had a speed of 10 m/s.

A

B

C

v

A

B

C

v

Question 2

Observer A blows a whistle. The speed of sound is 340 m/s in air; no wind is blowing. The truck has a speed of 20 m/s.

For the sound wave moving in the direction of C

A. B & C agree that the whistle’s sound travels at 340 m/s.

B. B&C agree that the whistle’s sound travels at 360 m/s.

C. B says the speed is 360 m/s, while C says 340 m/s.

D. B says the speed is 340 m/s, while C says 320 m/s.

E. B says the speed is 320 m/s, while C says 340 m/s.

Faster than Sound

If the observer moves faster than sounds then the sounds wave never catches up to the observer.

But note that something physical happens when one goes faster than sounds. A shock wave is formed.

A

B

C

v

What if Observer A aims a light beam in the forward direction? Will observers B and C see different speeds of light, or the same?

- Einstein’s postulates of special relativity
- The laws of physics are the same in any inertial frame of reference
- The speed of light is the same in any inertial frame of reference.

Luminiferous Ether

Just as air is the medium which allows sounds wave to exist. The ether allows electromagnetic wave to exist.

Why is the speed of light

299,792,458 m/s

and not some other value?

There must be a media controlling it’s value

Maxwell used the concept of this media to predict the existence of electromagnetic wave and their speed.

Observers B and C will measure the same speed of light, no matter what the speed the truck travels.

Instead, Observers B and C will find that time itself, and distances, will be different!

Michelson-Morley Experiment

Is the speed of light the same in all directions in a moving reference frame?

In contrast to sound waves or a ball, the velocity of the airplane does not add to the velocity of a flashlight beam to yield the velocity of light relative to the earth. The speed of light is the same for all observers. (Velocity vectors are not drawn to scale.)

- Special Relativity: The speed of light looks the same to all observers! (Light does not act like a wave in a medium, unlike sound traveling through air.)
- It doesn’t matter if…
- The source of light is moving.
- The source is at rest.
- You are moving.
- You are at rest.

- This must be true for the laws of physics to be the same for everybody.

In a light clock, the time taken for light to travel the distance 2d to the overhead mirror and back becomes the basic measure of time to.

To an observer standing on earth, the light takes a diagonal path to the mirror and back to the source. This yields a longer time interval as measured by the light clock.

With the help of assistants, the observer on earth can mark the position of the spaceship when the light pulse is emitted and when it returns. The distance between these positions L0 can be easily measured.

Consequences:

Time Dilation

Length Contraction

E=mc2

Note: g is larger than 1.

If v=0.6c, then g=1.25.

Time dilation:

T(moving) = T(rest)e.g. muons

Length contraction:

L(moving) = (1/ L(rest)

Apply to Newton’s Laws, find that:

E=mc2

Time dilation: Time slows down for moving objects. In principle, the half-life of Radon would be greater than 3.8 days in our frame of reference if it were moving. This effect is actually measurable for particles called muons.

Time dilation is an experimental FACT, in agreement with the theory of relativity.