Mr. BaldwinPHYSICS

1

• Mr. Baldwin PHYSICS
• 5/23/2014 Relativity
• Aim What does it mean to travel at light speed?
• Do Now
• What happens when an objects length (dimensions)
  as it approaches the speed of light?

2
Relativity Theory

• The days come and go like muffled veiled figures
  from a distant friendly party, but they say
  nothing, and if we do not use gifts they bring,
  they carry them as silently away.
• Ralph Waldo Emerson

3
Its only relative.

• Einsteins theory of relativity is actually
  fairly simple. It is based on two very simple
  ideas.
• Principle of relativity
• Principle of the constancy of lightspeed
• To understand this theory one needs the mental
  flexibility to follow an argument wherever it
  might lead.

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Constancy of Lightspeed Strange but true.

• No matter how fast you move, a light beam always
  passes you at a speed c, relative to you.
• Einstein concluded that every observer ought to
  observe every light beam to move at speed c,
  regardless of the observers motion.
• The Principle of the Constancy of Lightspeed
• The speed of light in empty space is the same for
  all nonaccelerated observers, regardless of the
  motion of the light source or of the observer.
• The principle is valid only for nonaccelerated
  observers.

5
Newtons Laws and Mass-Energy Equivalence

• Accepting Einsteins postulates requires some
  major changes in how we think about space and
  time.
• For example, does Newtons second law of motion
  still apply when objects are moving at large
  velocities?
• F ma ?p / ?t
• In order to maintain conservation of momentum,
  Einstein redefined momentum as
• p ?mv

6

• Mechanical energy also must take on a new
  meaning
• KE ?mc2 - mc2
• The quantity mc2 E0 is called the rest
  energy.
• Accelerating an object increases its energy above
  its rest energy.
• Therefore its total energy would be E ?mc2.

7

• The rest energy term E0 mc2 indicates a
  startling equivalence between mass and energy.
• Einsteins famous equation
• correctly predicts the
• amount of energy released
• in nuclear reactions.
• Rest-mass energy is
• transformed into kinetic
• energy in nuclear fission
• and fusion.

8
A Bunsen burner adds 1000 J of heat energy to a
beaker of water. What is the increase in the
mass of the water?

1. 1000 kg
2. 1110 kg
3. 1.11 x 10-14 kg
4. 1.11 x 10-18 kg

9
The Relativity of Time

• Time is the measure of events that goes through
  periodic or identical repetitions.
• A light clock two parallel facing mirrors with a
  light beam placed between them.
• Distance between them 150,000 km
• Time from one mirror to the next and back is 1
  sec
• The relativity of time is called time dilation
• Time is relative to the observer
• Objects that travel near the speed of light
  experiences an increase in time as observed by an
  outside observer.

10
Time Travel You Cant Go Home Again

• The twin paradox in the accelerated reference
  frame relativity theory does not apply. Twin that
  leaves and returns ages slower.
• A light year (l.y.) is the distance light travels
  in one year
• 1 l.y. 9.45 x 1015 m

11

• Mr. Baldwin PHYSICS
• 2/26/09 Relativity
• Aim What happens to objects when they travel at
  light speed?
• Do Now
• What happens to the length of an object when it
  travels near the speed of light?
• What happens to the mass of an object when it
  travels at the speed of light?
• What happens to a clock on a spaceship traveling
  at a fraction of light speed when viewed from
  Earth?

12
Time Dilation
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The Relativity of Space

• Space what is measured by rulers
• At high speeds space-time becomes distorted
  (non-relative) for observers
• Time space are tangled up with each other!
• Measurements parallel to the direction of motion
  becomes contracted (shortened) while
  measurements perpendicular to the direction of
  motion remain unchanged.
• This distortion of space is called length
  contraction.

14
Length Contraction

• Problem Maias spaceship is 100 m long and 10 m
  high as measured by Maia. She passes Marshall at
  60 of lightspeed (0.6C). As observed by
  Marshall, what are the dimensions of Maias space
  craft?