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1. Frames of Reference
In order to describe the motion of moving bodies, we need
to state where
the object is at any given time.
But to state where
an object is, we need to measure its position relative
to something else, right? So we need a
reference point
from which to define the position
of objects. Once we have chosen such a point, which is called the
origin,
we can specify the position of the object by saying,
for instance, that the object is
distance x to the east,
distance y to the north,
and distance z up from the
origin.
We also need a
clock
so that we can specify at what
time t the
object was at the given position.
When we have the origin and the directions in which to
measure the distance from the origin set up,
and a clock to measure the time, we say that we have a
frame of reference or simply a
frame.
