2. Work
Work is a term that everybody uses
everyday to emphasize the
accomplishment of a certain task.
In physics, the term work is referred to
the amount related to an applied force
that moves an object a certain distance.
Work is not a form of energy but a way to transfer or
transform energy.
In physics we say that we do work on an
object when we transfer energy to that
object.
3. Whenever a force is exerted on a moving object,
we say that work is done by the force or by the
agent exerting the force.
The work done on an object depends directly
proportional to the force applied
The work done is proportional to the distance
the object moves.
The work done depends on the angle between
the direction of the force and the direction of
motion
We measure the amount of work that is done by
multiplying the force exerted, and the distance
the object moves in the direction of the force.
4. Definition of work
The work done by a force is a scalar
quantity defined as the product between
the magnitude of the force acting on the
direction of motion and the displacement.
W = F d cos Θ
The measuring unit in International
System is newton x meter = joule (J)
5. Case 1
θ=90°
The angle between the direction of motion and
the direction of the force is 90 degrees.
Example:
a) a person is holding an object that is moved
forward.
b) a cart is lifted with a force F but still moves
forward.
The work done by the force is zero.
cos 90 = 0 therefore W=0 J.
6. Case 2
Δ d= 0 m
The displacement is zero.
Example: a person stands
next to a wall and pushes the
buttons on a wall with a
force, but the wall is not
displaced.
The work done is zero.
7. Case 3
θ = 180°
The angle between the direction of motion
and the direction of the force is 180
degrees.
Example: the friction force is always
opposing motion, but objects are still
moving for certain distances.
cos 180 = -1
the work done by the force is negative.
8. Case 4
θ = 0 °
The angle between the direction of motion and
the direction of the force is 0
Example: a person is pulling horizontally a
wagon.
Because cosine of 0 degrees has the value 1
cos 0 °= 1
The magnitude of the work done by the force is
equal with the product between the force and
the displacement. W = F d.
9. Graphic representation
of the work done by a force
The work done by a force is often represented
on a graph.
The variables to study are the value of the
force and the displacement.
Thus the coordinates to represent the work
done will be force-displacement or F-d.
There are two types of graphs:
1. the work done by a constant force
2. the work done by a variable force.
10. The work done by a constant
force
Force
[N]
d [m]
20 0
20 5
20 10
20 15
20 20
20 25
The work done is calculated as
W = F d.
11. The work done by a
variable force
Force [N] d [m]
0 0
5 0.02
10 0.04
15 0.06
20 0.08
The formula to calculate
the work done by a
variable force is:
12. Mechanical energy is the movement of machine
parts. Mechanical energy is also the total amount
of kinetic and potential energy in a system. Wind-
up toys, grandfather clocks, and pogo sticks are
examples of mechanical energy. Wind power uses
mechanical energy to help create electricity.
Potential energy + Kinetic energy = Mechanical energy
13. Potential energy + Kinetic energy = Mechanical energy
Example of energy
changes in a swing or
pendulum.
14. Kinetic energy exists whenever an object which has mass is in motion with some velocity.
Everything you see moving about has kinetic energy. The kinetic energy of an object in this
case is given by the relation:
KE = (1/2)mv2
m=mass of the object
V=velocity of the object
The greater the mass or velocity of a moving object, the more kinetic energy it has.
15. The greater the mass or velocity of a moving object, the more kinetic energy it has.
16. Potential energy exists whenever an object which has mass has a position within a force field.
The most everyday example of this is the position of objects in the earth's gravitational field. The
potential energy of an object in this case is given by the relation:
PE = mgh
PE = Energy (in Joules)
m = mass (in kilograms)
g = gravitational acceleration of the earth (9.8 m/sec2)
h = height above earth's surface (in meters)
17. The efficiency of a system is defined as the ratio of required form of energy from a
system as output to the total energy given in it as input. The efficiency of a system can
be calculated by the following relation.
An ideal system is that which gives an output equal to the total energy used by it. In
other words, its efficiency is 100 %.
18. “Power is defined as the rate of doing work.‟
Mathematically;
Two persons have done equal work, one took one hour to complete it and the other
completed it in five hours. No doubt, both of them have done equal work but they differ
in the rate at which work is done. One has done it faster than the other. The quantity
that tells us the rate of doing work is called power.
Since work is a scalar quantity, therefore, power is also a scalar quantity. SI unit of
power is watt (W). It is defined as;
“The power of a body is one watt if it does work at the rate of 1 joule per second “.
