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Work and Energy
Come in and
get out your
notes Journals
and a paper
off the blue
bookshelf
!
Energy is the ability of a physical
system to cause change
All Energy is measured in Joules
(J)
Energy
Energy is anything that can be con-verted into
work; i.e., anything that can exert a force
through a distance.
Energy is the capability for doing work.
Types of Energy
* Kinetic energy – the energy of motion
* Potential Energy – the energy of position
* Chemical Potential Energy – energy stored in molecules
* Thermal Energy – heat energy
* Nuclear Energy – energy related to matter itself
* Radiant Energy – light, microwaves, radio waves, x rays
Work
Work energy required to make something move.
WORK is ENERGY!
Force (N)
Work = F d
Distance (m)
Work Energy (J)
Work is measured in Joules
What is a Joule?
Work
Force Distance
1 Nm = 1 Joule (J)
Mass is NOT Force!!
Many times the Force that an object exerts is not
known. We can find that Force by using the
Formula F = mg (g =9.8)
Example:
W =
m = 8 kg
d=2m
W = (mg) * d
W=?
F
* d
W = (8* 9.8) * 2
W = 156.8 J
Three things are necessary for the
performance of work
1. Force “F”.
2. distance “d”.
3. The force and distance need to be parallel
If a force does not affect displacement, it
does no work.
F
W
The force F exerted on the
pot by the man does work.
The earth exerts a force W on
the pot, but does no work even
though there is displacement.
3 things to have Work!
1.
Force
2.
Distance
3.
A component of force
in the direction of
distance
F
W
When you lift something your
force is in the upward direction
and your distance is going up
F
d
WORK DONE
When you carry something, you
move to the side, but your force
is up
NO WORK
d
F
Power
Power is defined as the rate at which work
is done:
Work (J)
Force (N)
Distance (m)
P = W = Fd
Power (watts)
t
t
Time (sec)
Power is measured in Watts
Power
1.
Two physics students, Bob and Sam are in the
weightlifting room. Bob lifts the 50-kg barbell a distance
of .5 meters one minute; Sam lifts the 50-kg barbell the
same distance in 10 seconds.
A). Which student does the most work?
B). Which student delivers the most power?
Work of Bob = Fd
Wb = mgd
Wb = (50 kg) (9.8 m/s2) (.5m)
Wb = 245 J
Work of Sam = mgd
Ws = (50 kg) (9.8 m/s2) (.5m)
Ws = 245 J
Power
1.
Two physics students, Bob and Sam are in the
weightlifting room. Bob lifts the 50-kg barbell a distance
of .5 meters one minute; Sam lifts the 50-kg barbell the
same distance in 10 seconds.
A). Which student does the most work? SAME FOR BOTH!
B). Which student delivers the most power?
Power of Bob =W/t
P of bob= 245 J / 60 sec
Power = 4.08 WATT
Power of Sam = W/t
P of sam = 245 J/ 10 sec
Power = 24.5 WATT
Potential Energy: Energy of Position.
PE = mgh
Potential Energy
(J)
Mass (Kg)
Height of object (m)
Gravity (9.8 m/s2)
Potential Energy measured in Joules
If h = 0 , then PE = 0
Kinetic Energy: Energy in motion
KE = ½ mv2
Kinetic Energy (J)
Mass (kg)
Velocity (m/s)
Kinetic Energy measured in Joules
If v = 0 , then KE = 0
Kinetic and Potential Energy are EQUAL!!
Total PE = Total KE
There is
height
but no
velocity
There is
height
and
velocity
There is
velocity
Height is
0
P.E. = max
K.E. = 0
P.E. = K.E.
K.E. = max
P.E. = 0
Energy is converted but not lost!
Total Energy in this system = 100 J
P.E. = 100 J
P.E. = 50 J
K.E. = 100 J
K.E. = 0
K.E. = 50 J
P.E. = 0
Examples:
What is the Potential energy of a 1000-kg
car resting at the top of a 20m hill?
P = mgh = (1000 kg)(9.8)(20 m)
P = 196,000 J
What is the kinetic energy of a 1000-kg
car traveling at 14.1 m/s?
K  mv  (1000 kg)(14.1 m/s)
1
2
2
1
2
K = 99,405 J
2
Beware of the evil force!
Sometimes the problem will tell you
the force of the object. This refers to
the WEIGHT.
F = W = mg
PE = F h
You can also use the Weight to find
the mass.
m = W/g or m = F/g
How are P.E. and K.E. related?
There is
height
but no
velocity
P.E. = max
K.E. = 0
There is
height
and
velocity
There is
velocity
Height is
0
P.E. = K.E.
K.E. = max
P.E. = 0
Law of Conservation of Energy
@ highest point
P.E. = max
K.E. = 0
@ midpoint
P.E. = K.E.