Page 173 - Physics

Linear motion
Distance and displacement
To defi ne the position of an object, you fi rst
choose a reference point. For example,
to measure the position of a book in the
classroom shown in Figure 8.2 you could
choose a point either on the walls, fl oor
or the ceiling of the room as the reference
point. You would then measure how far
the book is from the front wall and side
wall, and how far it is above the fl oor.
These three measurements defi ne the
unique position of the book. If the book is
moved to another position, at least one of
these measurements will change.
Figure 8.2: Books on the table
Distance and displacement are measures
of how much the position of an object
changes with respect to a reference.
Distance is the length of the path
taken by an object while in motion.
It has no specifi c direction.
However, displacement involves the
length of the line drawn from the starting
(initial) position to the end (fi nal)
position in a specifi c direction shown in
Figure 8.3.
Displacement is the distance
covered in a particular direction.
Physics Form 1 Final.indd 167
The SI unit of both distance and
displacement is metre (m). Distance and
displacement can also be expressed in
centimetre (cm), millimetre (mm) and
kilometre (km). Though their SI units
are the same, distance and displacement
are not used interchangeably. Distance is
a scalar quantity while displacement is
a vector quantity. A scalar quantity has
magnitude only, but a vector quantity has
both magnitude and direction.
Distance
Initial position
Displacement Final position
Figure 8.3: Distance and displacement
Suppose your home is 1.5 km due south
of your school. In the morning, you
walk along a straight path to school.
The distance travelled is 1.5 km, but the
displacement is 1.5 km towards the north.
In the afternoon, you follow the same
path to go back home. For that day, you
have covered a total distance of 3 km, but
your displacement is 0 km because at the
end you are in the same position you were
in the morning (Final position and initial
position are the same).
Distance has magnitude only, but
displacement has both magnitude and
direction. So, when you walk to school
you travel a distance of 1.5 km, but your
displacement is 1.5 km due north (your
home is south of the school). When you
walk home in the afternoon, you again
cover a distance of 1.5 km, but your
displacement is 1.5 km due south.
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