Energy
and
its Nature
Energy
is the capacity
to do
.
ork.
work
What is work ?
A force does work when it acts on a body so that the
body is displaced in the direction of the force.
Staff Sgt. Joshua
Miles, Marine Drum
& Bugle Corps
marketing chief,
thrusts a weighted
bar over his head
during the first part
of the Civilian
Military Combine at
Bryce Resort, Va.,
April 21, 2012.
Hawkeye
does work
on his bow
by drawing
it.
W  F s
Constant force in
direction of straightline displacement
The unit for Work is the joule.
1 joule = 1 N●m
Analyze Work
Force
Force
Motion
Motion
Doing Work?
Analyze Work
Forward Force
Downward
Force
Total
Force
Motion
Sample Problems
Sample Problems
Seatwork
1. The painter lifted a load for a distance of 75
cm and the work done by the painter is 38 J.
What is the mass of the load?
2. A load of 5-kg mass was lifted by the man
and the total work done is equal to 40 J. By
what distance was the load lifted?
Energy
is the capacity
to do
.
ork.
work
Energy Transfer
LAW OF CONSERVATION OF ENERGY
Energy is neither
created nor destroyed,
it is just transformed.
The only thing that
changes is the form in
which it appears.
Types of Energy
Kinetic Energy
– energy of motion
– increases when object moves
faster
– increases as mass of object
increases
Potential energy
– energy of position
– stored energy
– increases as mass of
object increases
Types of Energy
Potential energy
Kinetic Energy
K . E .  mv
1
2
2
P . E .  mgh
(gravitational)
P . E .  kx
1
2
2
(elastic)
Work and Potential Energy
Work and Potential Energy
Gravitational Potential Energy
Work and Potential Energy
Gravitational Potential Energy (10 N Ball)
3m
5m
(a)
(b)
(c)
Sample Problem
A person’s heart and head are 1.3m and 1.8m
above the feet, respectively. Determine the
potential energy associated with 0.50 kg of
blood in the heart relative to
a.) the feet,
b.) the head.
Work and Kinetic Energy
Work and Kinetic Energy
Sample Problem
1.) A 7.00-kg bowling ball moves at 3.00 m/s. How
fast must a 2.45-g Ping-Pong ball move so that
the two balls have the same kinetic energy?
LAW OF CONSERVATION OF ENERGY
Energy is neither
created nor destroyed,
it is just transformed.
The only thing that
changes is the form in
which it appears.
E  P .E .  K .E
for an isolated system
The Work-Energy Theorem
The work done on an object by a net force equals the
change in kinetic energy of the object
 K .E .  W  F  d
Why do you run out of breath when running up stairs
but not when walking up, when in fact the work
done in both running and climbing up the stairs is the
same?
What is power?
Power is the measure of how fast the work is done. It is
the rate at which energy is changed from one form to
another.
Power 
work done
time int erval

joule
sec ond
The unit for Power is the Watt.
1 Watt = 1 joule/second
1 hp = 745.70 watts
 Watt
Sample Problems
1.An electric lift can raise a 500.0 kg mass
a distance of 10.0 m in 5.0 s. What is the
power of the lift?
Seatwork # 6
1(a) How many horsepower is a 1400 W blow
dryer? (b) How many watts is a 3.5 hp
lawnmower?
2.An electric hoist does 196,000 J of work
in raising a 250.0 kg load. How high was
the load lifted?
end
Homework
15 May 2014
Answer the problems. Show your solutions.
●
Present your answers on ½ of a Letter or A4
sized sheet of unlined paper.
1.
A circus diver at the top of a pole
has a potential energy of 8820 J.
If the diver has a mass of 45 kg,
what is the height of the pole?
Fill in the kinetic and potential
energies asked for in the
illustration on the right.
What is the diver’s speed just as
he hits the water?
2.
A pendulum is displaced so that it has a potential
energy of 10 J. Once the bob is released, the
potential energy is converted into kinetic energy.
Fill in the kinetic and potential energies asked for
in the illustration.
3.
A mass hangs above the floor on a copper wire. When a
lit candle is moved along the wire and warms it, what
happens to the height of the wire from the floor? Why?
If a cube of ice was used to cool the wire, how would it
affect the height of the mass from the floor? Why?