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In this film, a small cart moving
at constant velocity and spraying paint dots passes in front
of a wheel rotating at constant angular velocity. The paint
dots on the white rectangle placed to the left of the wheel
record the motion of the cart as seen by an observer in the
laboratory; the dots on the wheel describe the motion of the
cart as seen by an observer at the center of the wheel and
rotating with it. Standing on the ground in our laboratory
reference frame, we see that the cart is subject only to
forces which we can ascribe to specific objects in the
environment, i.e. the drive, friction, etc. We call the
laboratory an inertial reference frame. To an observer on
the wheel, however, the cart must be subject to some
additional force since both the direction and the magnitude
of its velocity change. We call the rotating frame a
non-inertial reference frame because we must postulate
pseudo forces (sometimes called inertial forces) to account
for the motion of the cart in this system.
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