Natural Science Forum / Physics / Relativity / August 2005

Is there a point to posting this? How many here weren't taught this in what,
grade 9 physics?

(a) according to the principle of relativity, the laws of physics (not just
the laws of classical mechanics, as stated below) are valid in all inertial
reference frames (enertial reference frames, as stated below, are those in
which all observers would  measure the same acceleration for a moving body).

(b) also, according to the principle of the constancy of the speed of light,
the speed of light in free space has the same value c in all inertial
reference frames (as defined below).

with respect to principle (b), let A, B, and C be three observer, each at
rest in their corresponding inertial reference frame. a flash of ligfht is
emitted by A, who observes the light to travel at speed c. the frame of B is
moving away from A at a speed of c/4. galilean kinematics predicts that B
measures the value c - c/4 = 3c/4. C is in a frame that is moving towards A
with  speed of c/4. according to galileo, C measures a speed of c + c/4 =
5c/4 for the speed of the light emitted by A. but according to principle
(b), all three observers measure the same speed c for the light pulse.

let A be a passenger in a car. A is tightly held in the seat by the seat
belt. B is standing along the side of the road and sees A, the car, and a
book that is on the seat next to A all moving together at 22 m/s. if A and
the car suddenly slow to 20 m/s, in the absence of friction with the seat
the book continues to move. relative to A, there is no apparent force on
the book that is pushing it forward, but it appears to start moving, in
violation of newton's first law. but relative to B, the book continues to
move at 22 m/s and detects no violation of newton's first law.

each observer, A in the car, and B standing on the ground, defines a
reference frame. a reference frame requires a coordinate system and a set of
clocks, which enable A or B to measure positions, velocities, and
accelarations in A's or B's particular reference frame.

A and B reach different conclusion about the acceleration of the book. A
concluded that it accelerates foward, while B concluded that its
acceleration is zero.

in general, the acceleration of a body depends on the reference frame to
which it is measured. however, the laws of classical mechanics are valid
only in a certain set of reference frames in which all observers would
measure the same acceleration for a moving body. this set of reference
frames is called inertial frames.

the tendency of a body to remain at rest or in uniform linear motion is
called inertia, and newton's first law is often called the law of inertia
because it allows us to choose this special family of reference frames if we
express the first law as follows:

if the net force acting on a body is zero, then it is possible to find a
set of reference frames in which that body has no acceleration. from physics
by resnick, halliday, and krane