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Einstein's principle
of relativity states that the same physical laws are observed
from any inertial reference system (the same forces are observed from
all of them).
In the following visual you have two reference systems: the one of the
observer who is outside the train and the one of the observer who is
inside, both observing a box which is in the compartment.
The moving observer inside the train will
be an inertial reference system or not, depending on whether the
train's movement is uniform or uniformly accelerated.
The outside observer constitutes an
inertial reference system in all cases.
While the train travels at a URM both see different things, the
velocity of the box is 0 for the traveller on the train and v for the
outside observer. The acceleration and the force which act on the box
are identical = 0 for the two observers. These are inertial reference
systems.
When the train accelerates, the moving
observer on the train needs a fictitious
force, or inertial one, to explain the movement of the box towards him;
in this case the observer is in a non-inertial reference system. The outside observer
just needs the frictional force to explain the movement of the box.
The three activities will guide you through the process of
understanding
this different behaviour.
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