Transcript Relativity, Inertia, and Equivalence Principle

Relativity, Inertia, and the
Equivalence Principle
Law of Inertia (Newton’s 1st Law)
Formulated first by Galileo and
then used by Newton as his 1st law
of motion
 Objects at rest remain at rest…
 Objects in motion remain moving
at constant velocity (straight line
and constant speed), UNLESS
 …a net (unbalanced) force acts

Inertia and Mass
The mass of an object (in kilograms) is
a direct measure of its resistance to
change in motion (its “laziness”)
 The greater the mass, the harder it is to
start it moving, or to change its motion
(speed or direction) once it is moving

Mass and the Large Hadron
Collider
One of the greatest mysteries in physics
is why fundamental particles have
specific masses
 In analogy with electromagnetic theory,
the “Higgs field” was hypothesized to
explain mass

Higgs mechanism*

Imagine a
room full
of
physicists
chatting
quietly is
like
“empty”
space
filled with
the Higgs
field
*Thanks to CERN
A well-known scientist walks in, creating a
disturbance as he moves across the room –
attracting a cluster of admirers with each step…
…this increases his resistance to movement, his
“inertia” or his “mass” - just like a particle
moving through the Higgs field
…if a rumor crosses the room…
…it creates the same kind of clustering, but this
time among the scientists themselves (the
clusters are like the Higgs particles)
Inertial Frame of Reference
An “inertial frame of reference” is one
that is motionless or one that is moving
at a constant velocity
 An accelerating reference frame is
“non-inertial”

First Postulate of Relativity



The laws of physics are the same for all observers
in constant relative motion (an inertial reference
frame)
Any accelerating system is non-inertial, there would
be break in symmetry (a “special” direction would
be established)
If motion in one dimension is not acceleration, then
we can consider an inertial frame along that
direction – consider plane flying at constant
speed… you could do experiments there as well as
in this room
Law of Falling Bodies

All objects in a
uniform
gravitational field
fall with the same
acceleration,
regardless of
mass
Einstein’s Principle of
Equivalence

The effect of a uniform gravitational
field is indistinguishable from a
reference frame that is accelerating at
constant rate
Sample problems
Monkey shoot
 Fuzzy dice and
helium balloon in
car
 Light beam

Implications of Equivalence
Principle
Light must be affected by gravitational
field, even though light doesn’t have
mass
 Gravity is an effect of the curvature of
space and time around mass-energy
