Work is an activity involving a force and movement in the direction of the force.

1. Work, Energy and Power
by Prof. Liwayway Memije-Cruz

2.  Work isan
activity involving
aforce and
movement in the
direction of the
force.
 A forceof 20
Newtonspushing
an object 5 meters
in thedirection of
theforcedoes100
joulesof work.

3. Work is the transfer of
energy. In physics we say
that work is done on an
object when you transfer
energy to that object. If one
object transfers energy to a
second object, then the first
object does work on the
second object. Work is the
application of a force over a
distance. Lifting a weight
from the ground and putting
it on a shelf is a good
example of work. The force
is equal to the weight of the
object, and the distance is
equal to the height of the
shelf (W= Fxd).

4. Energy
is the capacity fordoing work
To do 100 joules of work, you must expend 100 joules of
energy.

5. Typesof Energy

6. The change in the kinetic energy of an object is equal to the net work
doneon theobject.

7. Work-kinetic energy theorem
Work on an object can transform into kinetic energy.
– When you pushing on an object, it will move: Fd
= ½mv2
– When gravity doeswork on an object, it will
move: Fweighth = mgh = ½mv2
Kinetic energy of an object can do work.
– A moving object can slideup an inclined plane
beforecoming to astop: ½mv2
= mgh
– A moving object can slideagainst friction for a
whilebeforecoming to astop: ½mv2
= Ffrictiond

8. Poweris the
rate of doing
work orthe rate
of using energy,
which are
numerically the
same.
If you do 100
joules of work in
one second
(using 100
joules of
energy), the
poweris 100

9. W = Fdcosθ
F is force, d is the distance over which the force is applied, and θ is the angle
between the force and distance.
Derived units
 Work is energy, and the unit is the Joule.
 Joule = N·m = kg·m/s2
·m = kg·m2
/s2
 If the force and the distance applied is in the same direction, work is
positive.
 For example, pushing a crate across a rough terrain involves you
doing positive work (you are pushing forward and the crate is moving
forward).
 If the force and the distance applied is in opposite directions, work is
negative.
 For a non-rotating system, friction always does negative work
because it acts against the direction of motion.
 If the force is acting in one direction, but the object moves in a
perpendicular direction, then no work is done.
 The classic example is that no work is done by your arms when you
carry a bucket of water for a mile. Because you are lifting the bucket
vertically while its motion is horizontal.
 Cosine of 90 is zero; cosine of anything below 90 is positive and
between 90-180 is negative.

10. Units of Work
Since work done W = F X D, its units are force
timeslength.
Newton-meter(Nm). (SIunit)
Joule (J) - work done when a point of
application of a force of one Newton moves 1
meter in thedirection of theforce
 1 Nm = 1 J
Dyne cm or erg –(cgs) – work done by a force
of 1 dyne when it acts through a distance of 1 cm
in thedirection of theforce.
 1 J= 107
ergs.
Foot-pound – work done by a force of 1 pound
over a displacement of 1 foot in the direction of
theforce.
Kilowatthour- used in electrical measurements.

11. Power istherateof work, or work over time: P= W/t
Theunit for power istheWatt, or W (don't confuse
thisW with theshorthand of work).
Watt = Joule/ second

12. Work?

13. Law of Conservation of Energy - the total amount of energy before = the total amount
of energy after.
Gravitational potential energy is converted to kinetic energy as an object falls, but the
total amount of energy stays the same.
Kinetic energy is converted to heat and sound energy as a crate slides to a stop on a
rough surface.