{rofessor Dave here, let's talk about
relative motion.
By now we understand that physics is concerned
with the motion of objects, and the
quantitative analysis of that motion, but
this can be tricky business if you
really think about the fact that
everything is in constant motion. When we
say that a car is moving at 100
kilometers per hour on the freeway
we are ignoring the fact that the
surface of the earth is rotating around
the center at about 1,700 km/hr at the
equator. Earth is also moving around the
Sun at around 30 km/s. The entire solar
system moves around the galactic center
at 20 kilometers per second and the
Milky Way galaxy is moving through the
universe at about 230 kilometers a
second, so it's not completely wrong to
say that the car is actually moving with
this incredible speed as well, meaning
that in a certain context, there's
practically no difference between flying
in a plane and sitting still in terms of
absolute velocity. But that's why we tend
to discuss relative motion. We don't
notice the movement of the earth and the
Sun and the galaxy in our everyday lives.
To us, the earth seems to sit still, and a
few dozen kilometers an hour may be
insignificant in the grand scheme of the
universe, but it's a big deal to the cop
with a radar gun ready to pull you over.
So we often discuss motion that is
relative to the surface of the earth.
When we do this we are assigning
something called an inertial reference
frame. This is the object or person or
location that we pretend is completely
still and we assess the motion of other
objects relative to this inertial
reference frame. For example if we treat
the earth
as an inertial reference frame and we
ignore its rotation and movement through
space, it becomes meaningful to say that
this car is moving directly north at 100
km/hr relative to the ground. But if we
are inside the car moving with constant
velocity it seems as though we are not
moving but rather that the surroundings
are rushing past us at 100 km/hr. The car
has become the inertial reference frame
that is standing still and everything is
in motion around us. It may seem strange
to pretend that the car isn't moving, but
remember it's just as incorrect to
pretend that the earth isn't moving, so
it really is all relative.
Imagine you are sitting in a moving
train. You toss a ball into the air and
catch it. To you, the ball went straight
up and down
but to an observer on the ground the
ball followed a parabolic path as the
train went past. This is because when the
ball leaves your hand it still has all
of the forward velocity that the train
does, but if you are inside the inertial
reference frame of the train, the train
is not moving at all, it is the
surroundings that are moving, so the ball
no longer has any horizontal velocity,
only the vertical velocity you impart to
it by moving your arm to throw the ball.
Assigning frames of reference and
understanding what they mean for the
motion of objects is a big part of
physics, and although some aspects of
this kind of thinking seem obvious, they
were not always well understood. Galileo
was the first to rigorously describe
relative motion after doing experiments
in a moving ship. He found that if he
dropped a ball while the ship was moving
with constant velocity it fell straight
down just like it did when the ship with
stationary in the port. He then
contemplated how a fish nearby would view
the motion, with the ball moving forward
as it falls, and how this differs from
his own experience. He used these
experiments to prove that velocity
measurements depend on which reference
frame you adopt, and he published his
findings in a book called "Dialogue
Concerning the Two Chief World Systems"
in 1632. These ideas were extended to
describe the motion of the earth around
the Sun instead of the other way around
by proving that the earth could be
moving even if we don't feel it.
This got Galileo and other scientists
into hot water with the Catholic Church,
but humanity eventually caught up to his
revolutionary thinking. We refer to this
portion of his work as Galilean
relativity and these concepts will be
built upon to enter very strange territory
when we cover Einstein's special
relativity in the modern physics course.
But for right now, we won't go flying
through space, we have much more work to
do on earth, which despite its rotation
and motion is actually quite a good
approximation of an inertial reference
frame, one that will be used frequently
throughout our study of physics. With
each topic we go through, whenever we
examine an object's velocity, just remember
it's all relative.
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