Concept of Work and Energy
Introduction of Work :-
In our day today life the word work is using
generally. In our life the work mean any one is reading newspaper, we said he
is working. If any child doing his/her home work we say that he/she is working.
More things are, if any labour working in our home we are saying that the man
is working. If any electrician solve the problem of light in our home, we are
saying that the electrician is working. These are the examples of word in our
daily life but in science the definition of work is totally different.
Work :- In our day today life we are generally saying, any physical
effort as ‘Work’ but in physics the definition of work is totally different. In
science if we push a book to move one point to another, it means we have to
exert a force on the book due to which that book displaced so we say work is
done, If a tractor pulls a trolly it means it exert a force on trolly to displacement
so we say work is done, If any one is pulling a trolly bag it means that person
is exerting a force on it to displace that trolly so we say work is done, If we
are pushing any table to displace one point to other it means we have to exert
a force on that table to displace it so we say work is done.
In all above discussion
we conclude that if we want to displace any object we have to exert a external
force that may be pull or push. In this any of the value is missing the work is
not done.
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| science-ncert-solution |
Definition :- Let a force exert on a object is ‘F’ to
displace that object to a distance ‘s’ and the work done in this process is ‘W’.
Then we can say that the product of force and distance is equal to work done.
So
Work = Force * Displacement
W = F * s
So the magnitude of the work is equal to the magnitude of
force product with displacement.
Work has no its own direction. It may be done in the
direction of force.
So we can say that the direction of work done is same as
force.
Unit of work = N m or Joule
If F = 1 and s = 1
Then the work done = 1
In this we say that for unit force for unit displacement,
the work done is also unit. Unit means one.
Key Points regarding ‘Work’
1). In above situation of pushing book, pulling trolly,
pulling trolly bag or pushing table one thing is common that is the
displacement takes place only when, when force is exert on it. If in all these
situation any of the value is zero (if distance = 0 or force = 0). Then the
product of F * s be zero. In this situation work done is also zero. So we can
say no work done is done.
W = F * s
If F = 0 then W = 0, If s = 0 Then W = 0
In both the situation work done is zero.
2). If the external force is applied in the direction of the
force then we can say that work done is positive. The direction of product of
force and distance is same as work done.
For example :- A boy/girl pushing any object by exerting a
force in the direction of displacement. Work done is the product of force and
displacement so we can say that work done is positive.
3). If the external force is applied in the opposite direction
of the force then we can say that work done is negative. The direction of product
of force and distance is opposite as work done.
W = - F * s
Here negative sign showing the opposite direction.
For example :- If a object moving in right side and at that
situation we applied a external force in opposite direction. ( In left side)
and if the magnitude of the external force is more than force of moving object
then the direction of moving objects will change to opposite as it moving. So
the work done is negative in this situation.
Introduction of Energy
In our daily life we are doing different works. Any time we
think, we find that all these work of our daily life is possible only with energy.
We are able to do work with energy only. We are not able to do work without
energy. This energy can be generated with eating food, drinking water and
juice, eating fruits and eating vegitables.
Definition of Energy
The capacity of doing work of any object, known as Energy.
Measurement of Energy
The burning coal converts the water to evaporation of water,
Hammering on wood gives a shape to wood, In floating water have energy because
it moves the blade of turbine, In floating wind have energy because it moves ‘Wind
turbine’. When a object doing work in this situation, energy is lost. The
measurement of energy is the work done to get a position of zero energy.
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| science-ncert-solution |
1). Kinetic Energy :- Due to the motion of moving object, the work is done in which form, is known as Kinetic Energy.
Example :- a). Kinetic energy of floating water
b). Kinetic energy of floating air
c). Kinetic energy motion of bullet
d). Kinetic energy moving car or bus
Expression for Kinetic Energy
Let a object start moving from rest with a constant force F
for time t. In this time it travelles a distance s
W = F * s …..(1)
By Newtons second law of motion
F = m a …..(2)
By Newtons third law of motion
v^2 = u^2 + 2 a s
Object start moving from rest so u = 0
So v^2 = 2 a s
S = v^2 / 2 a
By equation 1 and 2
W = ( m a v^2 ) / 2 a
= ½ * m * v^2
Kinetic Energy K = W = ½ * m * v^2
2). Potential Energy :-
It is related to stored energy. The potential energy of a
object is due to its position.
For example :- When a ball falls from a height its potential
energy converts into Kinetic energy but when ball is not thrown, the energy
stored in it is called Potential Energy.
Let m be a mass of an object, falling from a height ‘h’
It is represented by ‘V’
V = m g h
Where m = mass
g = gravitational force
h = height
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