Transcript Einstein`s Laws

Einstein’s Laws
First Law
• Newton’s first law says that with no net force there is no
acceleration.
– Objects at rest
– Objects at constant velocity
• If there is no observed acceleration on an object with no
net force, the observer is in an inertial reference frame.
– Newton’s laws of mechanics apply equally
– No absolute motion
Inertial Frame
• An observer on the table
sees two ball fall.
– First straight down
– Second in a curve
• An moving with the
second ball sees the
reverse.
– Second straight down
– First in a backwards
parabola
• Both frames are inertial.
– Motion consistent with
Newton
Accelerated Frame
• A rotating observer throws a
ball across a merry-goround.
– Ball veers to the side
– No external force
• This is a non-inertial frame.
– Observed motion
inconsistent with Newton’s
laws
– Fictitious forces
Galilean Relativity
• Galileo described the
conversion between inertial
frames in 1638.
y
y’
S
x
Event P
x’
– Galilean relativity
– Basis for Newton
• The Galilean transformation
describes velocities as sums.
v
S’
x’
x
x  x  vt
y  y
ux  u x  v
uy  u y
z  z
uz  uz
t  t
Moving Light
• Light as a wave should have
a medium for transmission.
medium
observed
source
light
u  c 2  v 2
– Velocity like a plane in the
wind
– Speed c without motion
• The earth’s velocity and the
light velocity must add to
get the result from the earth.
Michelson-Morley
• An interferometer can
split and recombine light.
– Interference shifts for
different paths
mirror 2
semisilvered
mirror
mirror 1
• In 1887 an experiment
sowed the direction didn’t
matter.
• The speed c must be a
universal constant.
Einstein’s Principles
•
In 1905 Einstein described
two basic principles.
I.
The laws of physics are the
same in all inertial frames.
II. The speed of light in a
vacuum is the same for all
inertial frames, from all
sources.
Equivalence Principle
• The 1905 principles
became known as special
relativity.
• In 1915 Einstein added
another principle to form
general relativity.
III. Gravitational mass is the
same as inertial mass.
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