Notes: Work, Power, & Energy
•Energy is the ability to do work.
•When you do work on an object, you transfer energy
to that object.
•Whenever work is done, energy is transformed or
transferred to another system.
•Work - the application of a FORCE that causes the body
to MOVE in the DIRECTION of the force.
•Force - a push or a pull
• Work
Equation
work = force (N)  distance (m) (W = F  d)
• Joule (J) - the scientific unit of energy (work)
– = 1 kg•m2/s2
– amount of energy required to raise 1 Newton 1 meter
above the Earth.
• 1 Newton = .2248 lbs or 1 lb = 4.448 Newtons
• 1 kg = 9.8 N
– amount of energy required to raise 1 gram of water 1
degree Celsius.
– Practical examples:
• the energy required to lift a small apple one meter
straight up.
• the energy released when that same apple falls one
meter to the ground.
• Calories
– 1 calorie = 4.184 joules
– 1 dietary calorie (Calorie or Kcal) = 1000 calories
– 1 dietary calorie = 4,184 joules.
• Snickers = 271 Calories (kcal)
– 271 kcal = 271,000 calories
– 271,000 calories = 1,133,864 joules
• 1,133,864 joules would raise an apple 1,133,864 meters
above the Earth ( 704 miles).
• 1,133,864 joules would raise an average car (3,300
pounds = 1500 kg = 15,000 Newtons) 75 meters above
the Earth.
• Power - the rate at which work is done or energy is
transformed.
• Power Equation
work
power 
time
W
P
t
• Power is measured in watts.
• watt = joule per second (1 J/s).
Two Main Categories of Energy
Kinetic Energy
Potential Energy
Both are forms of
Mechanical Energy
• Kinetic Energy - energy of a moving object due
to the object’s motion.
– Kinetic energy depends on mass and velocity
(speed).
– Kinetic Energy Equation
1
kinetic energy = ´ mass ´ speed squared
2
1
KE = mv 2
2
• Potential Energy - “Stored Energy” - energy that
an object has because of the position, shape, or
condition of the object .
• Gravitational potential energy - energy stored in
the gravitational field which exists between any two
or more objects.
• GPE = mass  gravity  height
• Elastic potential energy
• Ex. spring or a rubber band.
• The Law of Conservation of Energy
– The law of conservation of energy states
that energy cannot be created or destroyed,
it can only be transformed (converted) from
one form to another.
• Pendulum Demonstration
Pendulum Demonstration
(to be continued tomorrow with
bowling ball pendulum)
CHECK FOR UNDERSTANDING:
You give yourself and your sled gravitational
potential energy as you pull your sled to the top
of a snowy hill. You get on board your sled and
slide to the bottom of the hill, speeding up as
you go.
1. When does the sled have the most potential
energy? When does it have the least potential
energy?
2. Where does the sled have the most kinetic
energy? the least kinetic energy?
3. What happens to the relative amounts of
potential and kinetic energy as the sled slides
down the hill? What happens to the total energy?
4. After the sled reaches the bottom of the hill, it
coasts across level ground and eventually stops.
What happened to the energy the sled had?
• Forms of Energy
o Heat (Thermal) energy (KE) - energy of moving particles.
o Light energy (KE)
o Sound energy (KE)
o Electrical energy (KE) - moving charged particles.
o Chemical energy (PE) - energy of bonds between atoms.
o Gravitational Potential Energy (PE)
o Elastic energy (PE) - Ex spring, rubber band.
o Nuclear energy (PE) - Fusion in stars. Fission in reactors.
o Mechanical energy (PE +KE) – An object’s kinetic and
potential energies combined.
• Alternative Energy
– History
• Coal as an alternative to wood (16th Century)
• Petroleum as an alternative to whale oil (19th
Century)
– Renewable energy
• energy generated from natural resources.
• Sunlight, wind, rain, tides, geothermal heat.
– Other sources?
• Enthanol, wood, methane
Energy Transformations
• Energy readily changes from one form to another.
• Potential energy can become kinetic energy.
• As a car goes down a hill on a roller coaster, potential
energy changes to kinetic energy.
• Kinetic energy can become potential energy.
• The kinetic energy a car has at the bottom of a hill can do
work to carry the car up another hill.
Practice Calculation:
• Work: Imagine a father playing with his
daughter by lifting her repeatedly in the air.
How much work does he do with each lift,
assuming he lifts her 2.0 m and exerts an
average force of 190 N?
4 Step Method:
Step 1: List variables
W=?
f = 190 N
Step 2: Write equation/s
W=f d
Step 3: Plug & Chug
W = 190 N  2.0 m
Step 4: Answer
W = 380 N-m = 380 J
d = 2.0 m
Practice Calculation:
• Kinetic Energy: What is the kinetic energy of a
40 kg cheetah running at 30 m/s?
4 Step Method:
Step 1: List variables
m = 40 kg v = 30 m/s
KE = ?
Step 2: Write equation/s
KE = 1/2 mv2
Step 3: Plug & Chug
KE = 1/2 x 40 kg x (30m/s)2
Step 4: Answer
KE = 18,000 kg-m2/s2 = 18,000 J
Practice Calculation:
• Gravitational Potential Energy A 65 kg rock
climber ascends a cliff. What is the climber’s
gravitational potential energy at a point 35 m
above the base of the cliff?
4 Step Method:
Step 1: List variables
m = 65 kg
GPE = ?
g = 10 m/s2 h = 35 m
Step 2: Write equation/s
GPE = mgh
Step 3: Plug & Chug
GPE = 65 kg x 10m/s2 x 35 m
Step 4: Answer
GPE = 22,750 kg-m2/s2 = 22,750 J
Practice Calculation:
• Power It takes 100 kJ of work to lift an
elevator 18 m. If this is done in 20 s, what is
the average power of the elevator during the
process?
4 Step Method:
Step 1: List variables
W = 100,000 Jt = 20 s d=18 m P = ?
Step 2: Write equation/s
P = W/t
Step 3: Plug & Chug
P = 100,000 Joules / 20 seconds
Step 4: Answer
P = 5,000 J/s = 5,000 W = 5 kW
Machines and Mechanical Advantage
• Machines multiply and redirect forces.
• Machines help people by redistributing the work
put into them.
• They can change either the size or the direction of
the input force.
<Clicker
• If you push for an
hour on a
stationary wall, you
do no work
A) on the wall
B) at all
C) both of these
D) none of these
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<Clicker
• How much PE does the
diver have at point Y?
• A) 0
• B) 250
• C) 500
• D) 750
• E) 1000
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V
W
X
Y
Z
<Clicker
• When an object is lifted 10 m, it gains a certain
amount of potential energy. If the same object is
lifted 20 m, its potential energy gain is
• A) less
• B) the same
• C) twice as much
• D) four times as much
• E) more than four times as much
• E
• D)
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Turn & Talk
<Clicker
• Which scenario below requires the most
work to raise the boulder to the top of
the cliff?
A
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B
C
A) Scenario A
B) Scenario B
C) Scenario C
D) All are the same
E) Not enough information
<Clicker
• An object that has kinetic energy must be
•
•
•
•
•
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A) moving
B) falling
C) at an elevated position
D) at rest
E) none of these
<Clicker
• At which point does the
pendulum have the highest
KE?
•
•
•
•
•
•
A) 1
B) 2
C) 3
D) 4
E) 5
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<Clicker
• If you push an object twice as far
while applying the same force, you
do
• A) twice as much work
• B) four times as much work
• C) the same amount of work
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<Clicker
• A job is done slowly, while an identical
job is done quickly. Both jobs require the
same amount of work, but different
amount of
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•
•
•
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A) energy
B) power
C) both of these
D) neither of these
<Clicker
• If you do work on an object in half the
time, your power output is
• A) half the usual output
• B) the same output
• C) twice the usual output
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<Clicker
• An object that has potential energy may
have this energy because of its
•
•
•
•
•
A) speed
B) acceleration
C) momentum
D) location
E) none of these
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<Clicker
• A bow is drawn so that it has 40 J of
potential energy. When fired, the arrow
will ideally have a kinetic energy of
• A) less than 40 J
• B) more than 40 J
• C) 40 J
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<Clicker
• When a car is braked to a stop, its kinetic
energy is transformed to
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•
•
•
•
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A) stopping energy
B) potential energy
C) energy of motion
D) energy of rest
E) heat
<Clicker
• Which requires more work: lifting a 50kg sack vertically 2 m or lifting a 25-kg
sack vertically 4 m?
• A) lifting the 50-kg sack
• B) lifting the 25-kg sack
• C) both require the same amount of work
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<Clicker
• A car moves 4 times as fast as another
identical car. Compared to the slower
car, the faster car has
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•
•
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•
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A) 4 times the KE
B) 8 times the KE
C) 12 times the KE
D) 16 times the KE