work.
In science the term work represents a definite concept. A gateman guards a house
all
day long sitting at a place and can say he has done his work. A boat was
floating with
the
current of a river or canal and Mr. Karim was pulling it back. He might say he
has
done
work to hold the boat otherwise the current of the river could pull it away.
These
are
recognized as work in our daily life but these are not work in the point of
view of
science.
Rather had the gateman guarded the house walking instead of sitting or had the
boat
floated with the current of the river, work could have been done. The concept
of
work
in science is different from that of daily life. In fact in science work is
done when
displacement
is associated with force. So, if a force acts on a body and causes its
displacement
only then work is said to be done. We see many examples of work around
us
in our daily life. For example, bull pulls the plough; a laborer pushes forward
a push
cart,
someone throws iron sphere is sports competition etc.
Let
us consider the following examples:
a)
Ratan is standing still with a packet of book in his hand.
b)
Mita is pushing her physics book from one end to another of a table.
c)
Niru is lifting a heavy bag through stairs.
d)
Rimi is pushing the wall strongly.
As
work is said to be done only when a force is acted on a body and displaces it,
so in
the
above examples (b) and (c) work is done but in the example (a) and (d) no work
is
done.
We can apply force to shift a body from one place to another. We can change the
shape
of any body by applying force. In these cases work is done.
If
a construction laborer wants to get to the second floor of a building with ten
bricks, he
has
to do more work than that of lifting a single brick to the same place as he has
to use
more
force. He has to do more work if he wants to lift those brick on the third floor.
Therefore,
the amount of work depends on the applied force and the distance. Work is
measured
by the product of applied force on a body and its displacement along the
direction
of force. Therefore,
Work
= Force × Distance travelled along the direction of force
If
a force F is applied on a body and the body travels a distance s along the direction of
force
(fig: 4.1) then the work done (W) will be,
W=
Fs… … … (4.1)
Work
has no direction. It is a scalar quantity.
Dimension of Work:
Dimension
of work will be the dimension of force × dimension of displacement
Figure:
4.1
70
Physics
or,
Work = Force × displacement
=
mass × acceleration × displacement
=
mass × × displacement
=
mass ×
or,
W =
∴ [W] = [ML2T-2]
Unit of work: The unit of work is obtained by multiplying the unit of
force with unit of
distance.
Since the unit of force is Newton (N) and the unit of distance is meter (m)
then
the
unit of work will be newton-meter (Nm) which is called joule. Joule is
expressed by
J.
If a force of 1N is applied on a body and the body gets a displacement of 1m
along the
direction
of force then the work done is said to be 1joule (1J), i.e. 1J = 1Nm.
If
the displacement takes place along the direction of force then work done is
said to be
work
done by the force.
If
a duster falls on a floor from a table the work is done here by the force of
gravity.
If
the displacement of a body takes place opposite to the direction of force then
the work
done
is called work done against force.
If
a duster is lifted on the top of a table from the floor then the work is done
against the
force
of gravity. This is because the displacement takes place opposite to the
direction of
the force
of gravity.
