Work means to do something in our everyday life but in
science doing anything is not work. In science the term work represents a
definite concept. A gate-man 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. John 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 gate-man 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:
1.
John is standing still with a packet of book in his
hand.
2.
Mary is pushing her physics book from one end to
another of a table.
3.
Vicky is lifting a heavy bag through stairs.
4.
Alice 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 (2) and (3) work is done but in the
example (1) and (4) 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: Force) then the work done
(W) will be,
W=
Fs |
Fig:
Force
|
Work has no direction. It is a scalar quantity.
Dimension of Work
Dimension of work will be the dimension of force × dimension of
displacement
Or, Work = Force × displacement
= mass × acceleration × displacement
= mass × displacement / (time) 2 × displacement
= mass × displacement / (time) 2
Or, W = ML2/ T2
∴ [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 1 joule (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.
Mathematical example: A man of mass 70kg climbs on a
mountain of height 200m. How much work will he do?
Given,
Mass of man, m = 70kg
Force, F = weight of the man
= mg
= 70kg × 9.8 ms-2
= 686N
Displacement, S = 200m
Work, W =?
We know,
W = Fs
= 686N × 200m
= 1.372 × 105 J
Ans: 1.372 × 105 J
End
0 comments:
Post a Comment