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-go-round.– 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.– Galilean relativity
– Basis for Newton
• The Galilean transformation describes velocities as sums.
vtxx
y
x
S y’
x’
S’
Event Px
x’
v
yy
zz
tt
vuu xx
yy uu
zz uu
Moving Light
• Light as a wave should have a medium for transmission.– 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.
observed source light
medium
22 vcu
Michelson-Morley
• An interferometer can split and recombine light. – Interference shifts for
different paths
• In 1887 an experiment sowed the direction didn’t matter.
• The speed c must be a universal constant.mirror 2
mirror 1semi-silvered mirror
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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