1. Work is defined as the product of the applied force and the displacement produced in the direction of the force. 2. For work to be done, there must be a displacement of the object in the direction of the applied force. If there is no displacement, then no work is done. 3. Work can be positive, negative or zero depending on whether the force and displacement are in the same direction, opposite directions or perpendicular to each other.

1. WORK

2.  In every day language , the word work is due
to describe any activity in which muscular or
mental effort is exerted.
 In physics the word work has a special
meaning – work only done when the force
acting on an object produces displacement in
the direction of force or in direction of
component of the force
 W=F.D (direction of force)
 F=Force, D=Displacement
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3.  Therefore, for work to be done by a force on an
object, the following two conditions must be met:
 1. There must be displacement of the object
 2. There must be a component of force in the direction of
the displacement
 Example – suppose you are pushing the wall very
hard but the wall does not move. In this case, you
are doing zero work on the wall because its
displacement is zero. You may feel tired after
pressing hard against the wall but from physics
point of view, work done is zero
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4. Work done by a constant force
 The work done on an object by the
constant force is defined as the
product of the component of the force
in the direction of the displacement
and the magnitude of the
displacement
 Generally the constant force acting on
a body does not act in the direction in
which the body moves
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5.  IN SAME DIRECTION FORCE & DISPLACEMENT
 Hold your book– lift of the book–
displacement of book upward– Due to
upward force
 FORCE=10 N, Displacement=40 cm
 W = F. D =10 N. 40 CM= 10 N. 0.4 m (SI unit)=
4Nm=4J
 Because SI Unit of Work= joule
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6.  FORCE IN ANGLE & DISPLACEMENT ALONG WITH STRAIGHT
LINE
 W=( F cos θ . Displacement(s) ) = F s cos θ
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7. Nature of work done
 1. POSITIVE WORK DONE : If θ < 90, cos θ is positive so
that work done is positive, under this condition, there
will be a component of force in the direction of
objects displacement
 If Cos θ = 00  W=F s
E.g:
1.when body falls freely under gravity, θ = 0 so that cos θ
= 1. there for work done by the gravitational force on the
body is positive
2. When spring stretched, the displacement and force
are in the same direction so that θ=0, therefore, the work
done by the stretching force is positive
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8.  2. NEGATIVE WORK DONE : If θ> 90, cos θ is negative so that
work done is negative. Under this condition, there is a
component of force opposite to the direction of object’s
displacement
 If Cos θ = 1800  W = -F s
E.g:
1. When a body is moved over a rough horizontal surface, the
work done by the frictional force is negative.
 It is because frictional force is negative. Because frictional
force opposes motion so that θ=180 & Cos θ= -1. so work
done by the applied negative force
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9.  3. ZERO WORK DONE: if θ=90 , cos θ= 0 and no work is
done by the force on the body. Also work done is zero
when either F Or d Or both are zero.
 If Cos θ = 900  W= 0
E.g:
1. when man carrying some load on his head moves on
a level road, work done by the man is zero. In this case,
the load force is vertically downward and displacement
is horizontal so that If θ=90 & cos θ=0.
2. A man pushing the wall is doing no work. It is because
there is no displacement of the wall.
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12. POWER

13.  The rate at which an agent (electric
motor) can do work is called its power
 P=WORK DONE/TIME TAKEN=W/t
 The power of agent indicates how
fast it can do work. The faster a given
amount of work is done by an agent,
the greater is its power and vice versa
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14.  We can also obtain another useful relation for power.
Suppose force “F” acts on a body so that it moves with
velocity “V”
 Now, P = WORK / TIME
= Force × displacement / t
= F × V
 Here θ is the angle between F and v
 If θ = 0, P= Fv cos 00 = Fv
 If θ = 90, P = Fv cos 900 = 0
 So power is the dot product of F and v , it is scalar
quantity : power has magnitude but no direction
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15.  SI Unit of POWER = WATT
 POWER = WORK / TIME = JOULE/SECOND=WATT
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16. ENERGY

17. INTRODUCTION
 If a body can do work , means the body has
energy
 So energy of a body is defined as the ability or
capacity of the body to do work
 Whenever work is done on a body, it gains
energy.
 If 100J of work is done, the body gains 100J of
energy.
 It means that the energy of a body is the work
stores in it.
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18. TYPES FO ENERGY
 MECHANICAL ENERGY
 KINETIC ENERGY - MOTION
 POTENTIAL ENERGY – POTENTIAL / CONFIGURATION
 HEAT ENERGY – HOT/COLD
 SOUND ENERGY - HEAR
 LIGHT ENERGY – SEE
 CHEMICAL ENERGY – FUEL / FOOD
 ELECTRIC ENERGY – PLUG/ BATTERY
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19. MECHANICAL ENERGY
 The energy associated
with motion and position/
configuration of
mechanical systems is
called mechanical energy.
 Two types of mechanical
energy
1. kinetic energy
2. potential energy
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20. 1. Kinetic energy
 The energy possessed by a body for its motion
is called kinetic energy of the body
 EXAMPLES
1. The kinetic energy of a hammer drives nail
into a piece of wood
2. A bullet fired from a gun can pierce a target
due to its kinetic energy
3. The kinetic energy of air is used to run wind
mills
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22. 2. Potential energy
 The energy possessed by a body for its position or configuration
(shape/size) is called potential energy of the body
 A body may sore energy because of its position or
configuration
 This is called potential energy because in the stored state, the
body has the potential to do work.
 Two imp types of potential energy
 1. Gravitational potential energy
 2. Elastic potential energy
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24. Gravitational energy
 The work done against the
gravitational force is stored in the form
of gravitational potential energy.
 When heavy brick is lifted from the
ground to some height, work is done
against the gravitational force.
 This work is stored in the brick in the
form of gravitational potential energy.
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26. Elastic potential energy
 The potential energy associated with
the elastic materials is called elastic
potential energy.
 For example, when a spring is
compresses or stretches, work will be
done against the spring force.
 This work done is stored in spring in the
form of elastic potential energy
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28. HEAT ENERGY
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29. SOUND ENERGY
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30. LIGHT ENERGY
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31. CHEMICAL ENERGY
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32. ELECTRIC ENERGY
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33. TORQUE

34.  Torque is the turning effect of a force and its
define as under : The torque produced by a
force about the axis of rotation is equal to the
product of magnitude of force and
perpendicular distance of the line of action of
the force from the axis of rotation.
 It is denoted by Greek letter (Tau)
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35.  Torque , t = force × Perpendicular distance of
force from axis of rotation
 The perpendicular distance of the line of
action of the force from the axis of rotation
called lever arm / moment arm of the force
 Torque , t = force × lever arm
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36.  Thus in figure, r is the lever arm of the force. It is
clear that the greater the magnitudes of force
and lever arm, the greater is the torque due to
force
r
t
F
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37.  It is a common experience that we can open or close a
door easily by applying force near the edge of the door,
by doing so, we increase the lever arm of the force and
hence the torque due to the force
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38.  Similarly, to unscrew a nut fitted tightly to a bolt, we
need a wrench with a long arm
 By convention, torque tending to produce
anticlockwise rotation is assigned positive value and
that tending to produce clockwise rotation is
assigned negative value
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39.  Not that the torque is a vector quantity like it has
magnitude as well as direction
 UNIT : Torque, t = force × lever arm
 The SI unit of force is 1N and that of distance is 1m.
Therefore, SI unit of torque is 1 Nm
 Note that unit for torque is the same that for energy.
But two quantity are very different. An obvious
difference is that energy is a scalar whereas torque
is a vector
 The special name joule (1 Nm = 1 joule) is used only
for energy and for work, never for torque
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